[dpdk-dev] 回复: [PATCH v1 3/4] net/mlx5: fix rebuild bug for Memory Region cache

[Feifei Wang <Feifei.Wang2@arm.com>]

Hi, Slava

Please see below.

> -----邮件原件-----
> 发件人: Slava Ovsiienko <viacheslavo@nvidia.com>
> 发送时间: 2021年5月12日 19:08
> 收件人: Feifei Wang <Feifei.Wang2@arm.com>; Matan Azrad
> <matan@nvidia.com>; Shahaf Shuler <shahafs@nvidia.com>
> 抄送: dev@dpdk.org; nd <nd@arm.com>; stable@dpdk.org; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> 主题: RE: [PATCH v1 3/4] net/mlx5: fix rebuild bug for Memory Region cache
> 
> Hi, Feifei
> 
> ..ship..
> >
> > If I understand correctly, your meaning is that if without wmb, maybe
> > other agents observe changed "dev_gen", but they also observe
> > unchanged "global" cache.
> > This can be defined as  memory inconsistent state.
> >
> > 					Fig1
> > ----------------------------------------------------------------------
> > ------------------------
> > -------
> > Timeslot        	      agent_1               		               agent_2
> > 1		     take_lock
> > 2          		update dev_gen
> > 3                       					     observe changed dev_gen
> > 4						         clear local cache
> >
> > 5            		rebuild global cache		               wait_lock
> > 6		      free_lock
> > 7		         wmb			                take_lock
> > 8							get(new MR)
> > 9                           				             	   free_lock
> > ----------------------------------------------------------------------
> > ------------------------
> > -------
> 
> Yes, something like that.
> 
> 
> > 1. However, in out-of-order platform, though adding a 'wmb at last',
> > 'dev_gen' maybe updated before global cache rebuild, and then other
> > agents can observe changed 'dev_ge'
> > before rebuilding global cache.
> >
> > Thus, though add a 'wmb at last', It is still unable to prevent other
> > agents from observing some inconsistent state. As a result, 'wmb at
> > last' fails to keep consistence.
> >
> > 2. On the other hand, due to lock, agent_2 will wait to take a lock
> > until global cache rebuilt by agent_1, and this ensures agent_2 can
> > get a correct new MR and update new local cache correctly.
> >
> > In summary, 'wmb at last' cannot guarantee other agents to observe the
> > consistent state.
> > But lock can fix this error. So, the existence of wmb at last is
> > redundant and we can remove it.
> 
> If dev_gen change is committed and cache's one is not yet - the agent_2
> might see inconsistent state even inside the lock-protected section. Hence,
> we must commit all writes before leaving the locked section in agent_1.
> 
> Let’s suppose there is no wmb in agent_1 at all, and dev_gen is arbitrary
> committed by CPU and MR cache data change is not. We leave the locked
> section in agent_1, agent_2 sees dev_gen changed, takes the lock and sees
> inconsistent MR-cache state due to not all changes made in agent_1 are
> committed. With wmb we have now in the existing code - there is no issue
> like that.

I can understand you worry that if there is no 'wmb at last', when agent_1 leave
the locked section, agent_2 still may observe unchanged global cache.

However, when agent_2 take a lock and get(new MR) in time slot 7(Fig.1), it means
agent_1 has finished updating global cache and lock is freed. 
Besides, if agent_2 can take a lock, it also shows agent_2 has observed the changed
global cache.

This is because there is a store- release in rte_rwlock_read_unlock,  store-release
ensures all store operation before 'release' can be committed  if store operation after
'release' is observed by other agents.

Thus, in our case, if agent_2 can observe updated 'rwl->cnt'(lock) and take a lock, it also can
observe updated 'dev_gen' and 'global cache'.

As a result, wmb can be removed due to store-release in R/W unlock.

Best Regards
Feifei     
 
> 
> With best regards,
> Slava
> 
> >
> > Best Regards
> > Feifei
> >
> > > With best regards,
> > > Slava
> > >
> > >
> > > >
> > > > Best Regards
> > > > Feifei
> > > >
> > > > > -----邮件原件-----
> > > > > 发件人: Slava Ovsiienko <viacheslavo@nvidia.com>
> > > > > 发送时间: 2021年5月7日 18:15
> > > > > 收件人: Feifei Wang <Feifei.Wang2@arm.com>; Matan Azrad
> > > > > <matan@nvidia.com>; Shahaf Shuler <shahafs@nvidia.com>
> > > > > 抄送: dev@dpdk.org; nd <nd@arm.com>; stable@dpdk.org; Ruifeng
> > > Wang
> > > > > <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> > > > > 主题: RE: [PATCH v1 3/4] net/mlx5: fix rebuild bug for Memory
> > > > > Region cache
> > > > >
> > > > > Hi, Feifei
> > > > >
> > > > > We should consider the locks in your scenario - it is crucial
> > > > > for the complete model description:
> > > > >
> > > > > How agent_1 (in your terms) rebuilds global cache:
> > > > >
> > > > > 1a) lock()
> > > > > 1b) rebuild(global cache)
> > > > > 1c) update(dev_gen)
> > > > > 1d) wmb()
> > > > > 1e) unlock()
> > > > >
> > > > > How agent_2 checks:
> > > > >
> > > > > 2a) check(dev_gen) (assume positive - changed)
> > > > > 2b) clear(local_cache)
> > > > > 2c) miss(on empty local_cache) - > eventually it goes to
> > > > > mr_lookup_caches()
> > > > > 2d) lock()
> > > > > 2e) get(new MR)
> > > > > 2f) unlock()
> > > > > 2g) update(local cache with obtained new MR)
> > > > >
> > > > > Hence, even if 1c) becomes visible in 2a) before 1b) committed
> > > > > (say, due to out-of-order Arch) - the agent 2 would be blocked
> > > > > on
> > > > > 2d) and scenario depicted on your Fig2 would not happen (agent_2
> > > > > will wait before step 3 till agent 1 unlocks after its step 5).
> > > > >
> > > > > With best regards,
> > > > > Slava
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: Feifei Wang <Feifei.Wang2@arm.com>
> > > > > > Sent: Friday, May 7, 2021 9:36> To: Slava Ovsiienko
> > > > > > <viacheslavo@nvidia.com>; Matan Azrad <matan@nvidia.com>;
> > Shahaf
> > > > > > Shuler <shahafs@nvidia.com>
> > > > > > Cc: dev@dpdk.org; nd <nd@arm.com>; stable@dpdk.org; Ruifeng
> > > Wang
> > > > > > <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> > > > > > Subject: 回复: [PATCH v1 3/4] net/mlx5: fix rebuild bug for
> > > > > > Memory Region cache
> > > > > >
> > > > > > Hi, Slava
> > > > > >
> > > > > > Thanks very much for your reply.
> > > > > >
> > > > > > > -----邮件原件-----
> > > > > > > 发件人: Slava Ovsiienko <viacheslavo@nvidia.com>
> > > > > > > 发送时间: 2021年5月6日 19:22
> > > > > > > 收件人: Feifei Wang <Feifei.Wang2@arm.com>; Matan Azrad
> > > > > > > <matan@nvidia.com>; Shahaf Shuler <shahafs@nvidia.com>
> > > > > > > 抄送: dev@dpdk.org; nd <nd@arm.com>; stable@dpdk.org;
> > Ruifeng
> > > > > Wang
> > > > > > > <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> > > > > > > 主题: RE: [PATCH v1 3/4] net/mlx5: fix rebuild bug for Memory
> > > > > > > Region cache
> > > > > > >
> > > > > > > Hi, Feifei
> > > > > > >
> > > > > > > Sorry, I do not follow why we should get rid of the last
> > > > > > > (after dev_gen update) wmb.
> > > > > > > We've rebuilt the global cache, we should notify other
> > > > > > > agents it's happened and they should flush local caches. So,
> > > > > > > dev_gen change should be made visible to other agents to
> > > > > > > trigger this activity and the second wmb is here to ensure this.
> > > > > >
> > > > > > 1. For the first problem why we should get rid of the last wmb
> > > > > > and move it before dev_gen updated, I think our attention is
> > > > > > how the wmb implements the synchronization between multiple
> agents.
> > > > > > 					Fig1
> > > > > > --------------------------------------------------------------
> > > > > > --
> > > > > > --
> > > > > > --
> > > > > > --
> > > > > > ------------------------
> > > > > > -------
> > > > > > Timeslot        		agent_1               		   agent_2
> > > > > > 1          		rebuild global cache
> > > > > > 2                       		wmb
> > > > > > 3            		     update dev_gen ----------------------- load
> changed
> > > > > > dev_gen
> > > > > > 4                                  			        	           rebuild local
> > cache
> > > > > > --------------------------------------------------------------
> > > > > > --
> > > > > > --
> > > > > > --
> > > > > > --
> > > > > > ------------------------
> > > > > > -------
> > > > > >
> > > > > > First, wmb is only for local thread to keep the order between
> > > > > > local
> > > > > > write- write :
> > > > > > Based on the picture above, for agent_1, wmb keeps the order
> > > > > > that rebuilding global cache is always before updating dev_gen.
> > > > > >
> > > > > > Second, agent_1 communicates with agent_2 by the global
> > > > > > variable "dev_gen" :
> > > > > > If agent_1 updates dev_gen, agent_2 will load it and then it
> > > > > > knows it should rebuild local cache
> > > > > >
> > > > > > Finally, agent_2 rebuilds local cache according to whether
> > > > > > agent_1 has rebuilt global cache, and agent_2 knows this
> > > > > > information by the variable
> > > > > "dev_gen".
> > > > > > 					Fig2
> > > > > > --------------------------------------------------------------
> > > > > > --
> > > > > > --
> > > > > > --
> > > > > > --
> > > > > > ------------------------
> > > > > > -------
> > > > > > Timeslot        		agent_1               		   agent_2
> > > > > > 1		        update dev_gen
> > > > > > 2						      load changed
> > dev_gen
> > > > > > 3						          rebuild local
> > cache
> > > > > > 4        		    rebuild global cache
> > > > > > 5			 wmb
> > > > > > --------------------------------------------------------------
> > > > > > --
> > > > > > --
> > > > > > --
> > > > > > --
> > > > > > ------------------------
> > > > > > -------
> > > > > >
> > > > > > However, in arm platform, if wmb is after dev_gen updated,
> > "dev_gen"
> > > > > > may be updated before agent_1 rebuilding global cache, then
> > > > > > agent_2 maybe receive error message and rebuild its local
> > > > > > cache in
> > > advance.
> > > > > >
> > > > > > To summarize, it is not important which time other agents can
> > > > > > see the changed global variable "dev_gen".
> > > > > > (Actually, wmb after "dev_gen" cannot ensure changed "dev_gen"
> > > > > > is committed to the global).
> > > > > > It is more important that if other agents see the changed
> > > > > > "dev_gen", they also can know global cache has been updated.
> > > > > >
> > > > > > > One more point, due to registering new/destroying existing
> > > > > > > MR involves FW (via kernel) calls, it takes so many CPU
> > > > > > > cycles that we could neglect wmb overhead at all.
> > > > > >
> > > > > > We just move the last wmb into the right place, and not delete
> > > > > > it for performance.
> > > > > >
> > > > > > >
> > > > > > > Also, regarding this:
> > > > > > >
> > > > > > >  > > Another question suddenly occurred to me, in order to
> > > > > > > keep the
> > > > > > > >
> > > > > > > > order that rebuilding global cache before updating
> > > > > > > > ”dev_gen“, the
> > > > > > > > > wmb should be before updating "dev_gen" rather than after it.
> > > > > > >  > > Otherwise, in the out-of-order platforms, current order
> > > > > > > cannot be
> > > > > > kept.
> > > > > > >
> > > > > > > it is not clear why ordering is important - global cache
> > > > > > > update and dev_gen change happen under spinlock protection,
> > > > > > > so only the last wmb is meaningful.
> > > > > > >
> > > > > >
> > > > > > 2. The second function of wmb before "dev_gen" updated is for
> > > > > > performance according to our previous discussion.
> > > > > > According to Fig2, if there is no wmb between "global cache
> updated"
> > > > > > and "dev_gen updated", "dev_gen" may update before global
> > > > > > cache
> > > > > updated.
> > > > > >
> > > > > > Then agent_2 may see the changed "dev_gen" and flush entire
> > > > > > local cache in advance.
> > > > > >
> > > > > > This entire flush can degrade the performance:
> > > > > > "the local cache is getting empty and can't provide
> > > > > > translation for other valid (not being removed) MRs, and the
> > > > > > translation has to look up in the global cache, that is locked
> > > > > > now for rebuilding, this causes the delays in data path on
> > > > > > acquiring global
> > cache lock."
> > > > > >
> > > > > > Furthermore, spinlock is just for global cache, not for
> > > > > > dev_gen and local cache.
> > > > > >
> > > > > > > To summarize, in my opinion:
> > > > > > > - if you see some issue with ordering of global cache
> > > > > > > update/dev_gen signalling,
> > > > > > >   could you, please, elaborate? I'm not sure we should
> > > > > > > maintain an order (due to spinlock protection)
> > > > > > > - the last rte_smp_wmb() after dev_gen incrementing should
> > > > > > > be kept intact
> > > > > > >
> > > > > >
> > > > > > At last, for my view, there are two functions that moving wmb
> > > > > > before "dev_gen"
> > > > > > for the write-write order:
> > > > > > --------------------------------
> > > > > > a) rebuild global cache;
> > > > > > b) rte_smp_wmb();
> > > > > > c) updating dev_gen
> > > > > > -------------------------------- 1. Achieve synchronization
> > > > > > between multiple threads in the right way 2.
> > > > > > Prevent other agents from flushing local cache early to ensure
> > > > > > performance
> > > > > >
> > > > > > Best Regards
> > > > > > Feifei
> > > > > >
> > > > > > > With best regards,
> > > > > > > Slava
> > > > > > >
> > > > > > > > -----Original Message-----
> > > > > > > > From: Feifei Wang <Feifei.Wang2@arm.com>
> > > > > > > > Sent: Thursday, May 6, 2021 5:52
> > > > > > > > To: Slava Ovsiienko <viacheslavo@nvidia.com>; Matan Azrad
> > > > > > > > <matan@nvidia.com>; Shahaf Shuler <shahafs@nvidia.com>
> > > > > > > > Cc: dev@dpdk.org; nd <nd@arm.com>; stable@dpdk.org;
> > > > > > > > Ruifeng
> > > > > Wang
> > > > > > > > <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> > > > > > > > Subject: 回复: [PATCH v1 3/4] net/mlx5: fix rebuild bug for
> > > > > > > > Memory Region cache
> > > > > > > >
> > > > > > > > Hi, Slava
> > > > > > > >
> > > > > > > > Would you have more comments about this patch?
> > > > > > > > For my sight, only one wmb before "dev_gen" updating is
> > > > > > > > enough to synchronize.
> > > > > > > >
> > > > > > > > Thanks very much for your attention.
> > > > > > > >
> > > > > > > >
> > > > > > > > Best Regards
> > > > > > > > Feifei
> > > > > > > >
> > > > > > > > > -----邮件原件-----
> > > > > > > > > 发件人: Feifei Wang
> > > > > > > > > 发送时间: 2021年4月20日 16:42
> > > > > > > > > 收件人: Slava Ovsiienko <viacheslavo@nvidia.com>; Matan
> > Azrad
> > > > > > > > > <matan@nvidia.com>; Shahaf Shuler <shahafs@nvidia.com>
> > > > > > > > > 抄送: dev@dpdk.org; nd <nd@arm.com>; stable@dpdk.org;
> > > > Ruifeng
> > > > > > > Wang
> > > > > > > > > <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> > > > > > > > > 主题: 回复: [PATCH v1 3/4] net/mlx5: fix rebuild bug for
> > > > > > > > > Memory
> > > > > > Region
> > > > > > > > > cache
> > > > > > > > >
> > > > > > > > > Hi, Slava
> > > > > > > > >
> > > > > > > > > I think the second wmb can be removed.
> > > > > > > > > As I know, wmb is just a barrier to keep the order
> > > > > > > > > between write and
> > > > > > > write.
> > > > > > > > > and it cannot tell the CPU when it should commit the changes.
> > > > > > > > >
> > > > > > > > > It is usually used before guard variable to keep the
> > > > > > > > > order that updating guard variable after some changes,
> > > > > > > > > which you want to release,
> > > > > > > > have been done.
> > > > > > > > >
> > > > > > > > > For example, for the wmb  after global cache
> > > > > > > > > update/before altering dev_gen, it can ensure the order
> > > > > > > > > that updating global cache before altering
> > > > > > > > > dev_gen:
> > > > > > > > > 1)If other agent load the changed "dev_gen", it can know
> > > > > > > > > the global cache has been updated.
> > > > > > > > > 2)If other agents load the unchanged, "dev_gen", it
> > > > > > > > > means the global cache has not been updated, and the
> > > > > > > > > local cache will not be
> > > > > > flushed.
> > > > > > > > >
> > > > > > > > > As a result, we use  wmb and guard variable "dev_gen" to
> > > > > > > > > ensure the global cache updating is "visible".
> > > > > > > > > The "visible" means when updating guard variable "dev_gen"
> > > > > > > > > is known by other agents, they also can confirm global
> > > > > > > > > cache has been updated in the meanwhile. Thus, just one
> > > > > > > > > wmb before altering dev_gen can ensure
> > > > > > > > this.
> > > > > > > > >
> > > > > > > > > Best Regards
> > > > > > > > > Feifei
> > > > > > > > >
> > > > > > > > > > -----邮件原件-----
> > > > > > > > > > 发件人: Slava Ovsiienko <viacheslavo@nvidia.com>
> > > > > > > > > > 发送时间: 2021年4月20日 15:54
> > > > > > > > > > 收件人: Feifei Wang <Feifei.Wang2@arm.com>; Matan
> Azrad
> > > > > > > > > > <matan@nvidia.com>; Shahaf Shuler <shahafs@nvidia.com>
> > > > > > > > > > 抄送: dev@dpdk.org; nd <nd@arm.com>; stable@dpdk.org;
> > > > > Ruifeng
> > > > > > > > Wang
> > > > > > > > > > <Ruifeng.Wang@arm.com>; nd <nd@arm.com>; nd
> > > > > <nd@arm.com>;
> > > > > > > nd
> > > > > > > > > > <nd@arm.com>
> > > > > > > > > > 主题: RE: [PATCH v1 3/4] net/mlx5: fix rebuild bug for
> > > > > > > > > > Memory Region cache
> > > > > > > > > >
> > > > > > > > > > Hi, Feifei
> > > > > > > > > >
> > > > > > > > > > In my opinion, there should be 2 barriers:
> > > > > > > > > >  - after global cache update/before altering dev_gen,
> > > > > > > > > > to ensure the correct order
> > > > > > > > > >  - after altering dev_gen to make this change visible
> > > > > > > > > > for other agents and to trigger local cache update
> > > > > > > > > >
> > > > > > > > > > With best regards,
> > > > > > > > > > Slava
> > > > > > > > > >
> > > > > > > > > > > -----Original Message-----
> > > > > > > > > > > From: Feifei Wang <Feifei.Wang2@arm.com>
> > > > > > > > > > > Sent: Tuesday, April 20, 2021 10:30
> > > > > > > > > > > To: Slava Ovsiienko <viacheslavo@nvidia.com>; Matan
> > > > > > > > > > > Azrad <matan@nvidia.com>; Shahaf Shuler
> > > > > > > > > > > <shahafs@nvidia.com>
> > > > > > > > > > > Cc: dev@dpdk.org; nd <nd@arm.com>; stable@dpdk.org;
> > > > > > > > > > > Ruifeng
> > > > > > > > Wang
> > > > > > > > > > > <Ruifeng.Wang@arm.com>; nd <nd@arm.com>; nd
> > > > > > <nd@arm.com>;
> > > > > > > > nd
> > > > > > > > > > > <nd@arm.com>
> > > > > > > > > > > Subject: 回复: [PATCH v1 3/4] net/mlx5: fix rebuild
> > > > > > > > > > > bug for Memory Region cache
> > > > > > > > > > >
> > > > > > > > > > > Hi, Slava
> > > > > > > > > > >
> > > > > > > > > > > Another question suddenly occurred to me, in order
> > > > > > > > > > > to keep the order that rebuilding global cache
> > > > > > > > > > > before updating ”dev_gen“, the wmb should be before
> > > > > > > > > > > updating
> > > "dev_gen"
> > > > > > > > > > > rather
> > > > > than after it.
> > > > > > > > > > > Otherwise, in the out-of-order platforms, current
> > > > > > > > > > > order cannot be
> > > > > > > kept.
> > > > > > > > > > >
> > > > > > > > > > > Thus, we should change the code as:
> > > > > > > > > > > a) rebuild global cache;
> > > > > > > > > > > b) rte_smp_wmb();
> > > > > > > > > > > c) updating dev_gen
> > > > > > > > > > >
> > > > > > > > > > > Best Regards
> > > > > > > > > > > Feifei
> > > > > > > > > > > > -----邮件原件-----
> > > > > > > > > > > > 发件人: Feifei Wang
> > > > > > > > > > > > 发送时间: 2021年4月20日 13:54
> > > > > > > > > > > > 收件人: Slava Ovsiienko <viacheslavo@nvidia.com>;
> > Matan
> > > > > Azrad
> > > > > > > > > > > > <matan@nvidia.com>; Shahaf Shuler
> > > > > > > > > > > > <shahafs@nvidia.com>
> > > > > > > > > > > > 抄送: dev@dpdk.org; nd <nd@arm.com>;
> > stable@dpdk.org;
> > > > > > > Ruifeng
> > > > > > > > > > Wang
> > > > > > > > > > > > <Ruifeng.Wang@arm.com>; nd <nd@arm.com>; nd
> > > > > > <nd@arm.com>
> > > > > > > > > > > > 主题: 回复: [PATCH v1 3/4] net/mlx5: fix rebuild bug
> > > > > > > > > > > > for Memory
> > > > > > > > > Region
> > > > > > > > > > > > cache
> > > > > > > > > > > >
> > > > > > > > > > > > Hi, Slava
> > > > > > > > > > > >
> > > > > > > > > > > > Thanks very much for your explanation.
> > > > > > > > > > > >
> > > > > > > > > > > > I can understand the app can wait all mbufs are
> > > > > > > > > > > > returned to the memory pool, and then it can free
> > > > > > > > > > > > this mbufs, I agree with
> > > > > > this.
> > > > > > > > > > > >
> > > > > > > > > > > > As a result, I will remove the bug fix patch from
> > > > > > > > > > > > this series and just replace the smp barrier with
> > > > > > > > > > > > C11 thread fence. Thanks very much for your
> > > > > > > > > > > > patient explanation
> > > again.
> > > > > > > > > > > >
> > > > > > > > > > > > Best Regards
> > > > > > > > > > > > Feifei
> > > > > > > > > > > >
> > > > > > > > > > > > > -----邮件原件-----
> > > > > > > > > > > > > 发件人: Slava Ovsiienko <viacheslavo@nvidia.com>
> > > > > > > > > > > > > 发送时间: 2021年4月20日 2:51
> > > > > > > > > > > > > 收件人: Feifei Wang <Feifei.Wang2@arm.com>; Matan
> > > > Azrad
> > > > > > > > > > > > > <matan@nvidia.com>; Shahaf Shuler
> > > > > > > > > > > > > <shahafs@nvidia.com>
> > > > > > > > > > > > > 抄送: dev@dpdk.org; nd <nd@arm.com>;
> > > stable@dpdk.org;
> > > > > > > > Ruifeng
> > > > > > > > > > > Wang
> > > > > > > > > > > > > <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> > > > > > > > > > > > > 主题: RE: [PATCH v1 3/4] net/mlx5: fix rebuild bug
> > > > > > > > > > > > > for Memory Region cache
> > > > > > > > > > > > >
> > > > > > > > > > > > > Hi, Feifei
> > > > > > > > > > > > >
> > > > > > > > > > > > > Please, see below
> > > > > > > > > > > > >
> > > > > > > > > > > > > ....
> > > > > > > > > > > > >
> > > > > > > > > > > > > > > Hi, Feifei
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > Sorry, I do not follow what this patch fixes.
> > > > > > > > > > > > > > > Do we have some issue/bug with MR cache in
> > practice?
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > This patch fixes the bug which is based on
> > > > > > > > > > > > > > logical deduction, and it doesn't actually happen.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > Each Tx queue has its own dedicated "local"
> > > > > > > > > > > > > > > cache for MRs to convert buffer address in
> > > > > > > > > > > > > > > mbufs being transmitted to LKeys (HW-related
> > > > > > > > > > > > > > > entity
> > > > > > > > > > > > > > > handle) and the "global" cache for all MR
> > > > > > > > > > > > > > > registered on the
> > > > > > > > device.
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > AFAIK, how conversion happens in datapath:
> > > > > > > > > > > > > > > - check the local queue cache flush request
> > > > > > > > > > > > > > > - lookup in local cache
> > > > > > > > > > > > > > > - if not found:
> > > > > > > > > > > > > > > - acquire lock for global cache read access
> > > > > > > > > > > > > > > - lookup in global cache
> > > > > > > > > > > > > > > - release lock for global cache
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > How cache update on memory
> > > > > > > > > > > > > > > freeing/unregistering
> > > > > > happens:
> > > > > > > > > > > > > > > - acquire lock for global cache write access
> > > > > > > > > > > > > > > - [a] remove relevant MRs from the global
> > > > > > > > > > > > > > > cache
> > > > > > > > > > > > > > > - [b] set local caches flush request
> > > > > > > > > > > > > > > - free global cache lock
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > If I understand correctly, your patch swaps
> > > > > > > > > > > > > > > [a] and [b], and local caches flush is requested
> earlier.
> > > > > > > > > > > > > > > What problem does it
> > > > > > > > solve?
> > > > > > > > > > > > > > > It is not supposed there are in datapath
> > > > > > > > > > > > > > > some mbufs referencing to the memory being
> freed.
> > > > > > > > > > > > > > > Application must ensure this and must not
> > > > > > > > > > > > > > > allocate new mbufs from this memory regions
> > > > > > > > > > being freed.
> > > > > > > > > > > > > > > Hence, the lookups for these MRs in caches
> > > > > > > > > > > > > > > should not
> > > > > > occur.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > For your first point that, application can
> > > > > > > > > > > > > > take charge of preventing MR freed memory
> > > > > > > > > > > > > > being allocated to
> > > > data path.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Does it means that If there is an emergency of
> > > > > > > > > > > > > > MR fragment, such as hotplug, the application
> > > > > > > > > > > > > > must inform thedata path in advance, and this
> > > > > > > > > > > > > > memory will not be allocated, and then the
> > > > > > > > > > > > > > control path will free this memory? If
> > > > > > > > > > > > > > application  can do like this, I agree that
> > > > > > > > > > > > > > this bug
> > > > > > > > > > > > cannot happen.
> > > > > > > > > > > > >
> > > > > > > > > > > > > Actually,  this is the only correct way for
> > > > > > > > > > > > > application to
> > > > operate.
> > > > > > > > > > > > > Let's suppose we have some memory area that
> > > > > > > > > > > > > application wants to
> > > > > > > > > > free.
> > > > > > > > > > > > > ALL references to this area must be removed. If
> > > > > > > > > > > > > we have some mbufs allocated from this area, it
> > > > > > > > > > > > > means that we have memory pool created
> > > > > > > > > > > there.
> > > > > > > > > > > > >
> > > > > > > > > > > > > What application should do:
> > > > > > > > > > > > > - notify all its components/agents the memory
> > > > > > > > > > > > > area is going to be freed
> > > > > > > > > > > > > - all components/agents free the mbufs they
> > > > > > > > > > > > > might own
> > > > > > > > > > > > > - PMD might not support freeing for some mbufs
> > > > > > > > > > > > > (for example being sent and awaiting for
> > > > > > > > > > > > > completion), so app should just wait
> > > > > > > > > > > > > - wait till all mbufs are returned to the memory
> > > > > > > > > > > > > pool (by monitoring available obj == pool size)
> > > > > > > > > > > > >
> > > > > > > > > > > > > Otherwise - it is dangerous to free the memory.
> > > > > > > > > > > > > There are just some mbufs still allocated, it is
> > > > > > > > > > > > > regardless to buf address to MR translation. We
> > > > > > > > > > > > > just can't free the memory - the mapping will be
> > > > > > > > > > > > > destroyed and might cause the segmentation fault
> > > > > > > > > > > > > by SW or some HW issues on DMA access to
> > > > > > > > > > > > > unmapped memory.  It is very generic safety
> > > > > > > > > > > > > approach - do not free the memory that is still
> > > > > > > > > > > > > in
> > > > > > > > use.
> > > > > > > > > > > > > Hence, at the moment of freeing and
> > > > > > > > > > > > > unregistering the MR, there MUST BE NO any
> > > > > > > > > > > > mbufs in flight referencing to the addresses being freed.
> > > > > > > > > > > > > No translation to MR being invalidated can happen.
> > > > > > > > > > > > >
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > > For other side, the cache flush has negative
> > > > > > > > > > > > > > > effect
> > > > > > > > > > > > > > > - the local cache is getting empty and can't
> > > > > > > > > > > > > > > provide translation for other valid (not
> > > > > > > > > > > > > > > being
> > > > > > > > > > > > > > > removed) MRs, and the translation has to
> > > > > > > > > > > > > > > look up in the global cache, that is locked
> > > > > > > > > > > > > > > now for rebuilding, this causes the delays
> > > > > > > > > > > > > > > in datapatch
> > > > > > > > > > > > > > on acquiring global cache lock.
> > > > > > > > > > > > > > > So, I see some potential performance impact.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > If above assumption is true, we can go to your
> > > > > > > > > > > > > > second
> > > > point.
> > > > > > > > > > > > > > I think this is a problem of the tradeoff
> > > > > > > > > > > > > > between cache coherence and
> > > > > > > > > > > > > performance.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > I can understand your meaning that though
> > > > > > > > > > > > > > global cache has been changed, we should keep
> > > > > > > > > > > > > > the valid MR in local cache as long as
> > > > > > > > > > > > > > possible to ensure the fast
> > > > searching speed.
> > > > > > > > > > > > > > In the meanwhile, the local cache can be
> > > > > > > > > > > > > > rebuilt later to reduce its waiting time for
> > > > > > > > > > > > > > acquiring the global
> > > > cache lock.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > However,  this mechanism just ensures the
> > > > > > > > > > > > > > performance unchanged for the first few mbufs.
> > > > > > > > > > > > > > During the next mbufs lkey searching after 'dev_gen'
> > > > > > > > > > > > > > updated, it is still necessary to update the local cache.
> > > > > > > > > > > > > > And the performance can firstly reduce and
> > > > > > > > > > > > > > then
> > returns.
> > > > > > > > > > > > > > Thus, no matter whether there is this patch or
> > > > > > > > > > > > > > not, the performance will jitter in a certain
> > > > > > > > > > > > > > period of
> > > > > > > > > > > time.
> > > > > > > > > > > > >
> > > > > > > > > > > > > Local cache should be updated to remove MRs no
> > > > > > > > > > > > > longer
> > > > valid.
> > > > > > > > > > > > > But we just flush the entire cache.
> > > > > > > > > > > > > Let's suppose we have valid MR0, MR1, and not
> > > > > > > > > > > > > valid MRX in local
> > > > > > > > > cache.
> > > > > > > > > > > > > And there are traffic in the datapath for MR0
> > > > > > > > > > > > > and MR1, and no traffic for MRX anymore.
> > > > > > > > > > > > >
> > > > > > > > > > > > > 1) If we do as you propose:
> > > > > > > > > > > > > a) take a lock
> > > > > > > > > > > > > b) request flush local cache first - all MR0,
> > > > > > > > > > > > > MR1, MRX will be removed on translation in
> > > > > > > > > > > > > datapath
> > > > > > > > > > > > > c) update global cache,
> > > > > > > > > > > > > d) free lock
> > > > > > > > > > > > > All the traffic for valid MR0, MR1 ALWAYS will
> > > > > > > > > > > > > be blocked on lock taken for cache update since
> > > > > > > > > > > > > point
> > > > > > > > > > > > > b) till point
> > > > > d).
> > > > > > > > > > > > >
> > > > > > > > > > > > > 2) If we do as it is implemented now:
> > > > > > > > > > > > > a) take a lock
> > > > > > > > > > > > > b) update global cache
> > > > > > > > > > > > > c) request flush local cache
> > > > > > > > > > > > > d) free lock
> > > > > > > > > > > > > The traffic MIGHT be locked ONLY for MRs
> > > > > > > > > > > > > non-existing in local cache (not happens for MR0
> > > > > > > > > > > > > and MR1, must not happen for MRX), and
> > > > > > > > > > > > > probability should be minor. And lock might
> > > > > > > > > > > > > happen since
> > > > > > > > > > > > > c) till
> > > > > > > > > > > > > d)
> > > > > > > > > > > > > - quite short period of time
> > > > > > > > > > > > >
> > > > > > > > > > > > > Summary, the difference between 1) and 2)
> > > > > > > > > > > > >
> > > > > > > > > > > > > Lock probability:
> > > > > > > > > > > > > - 1) lock ALWAYS happen for ANY MR translation after
> b),
> > > > > > > > > > > > >   2) lock MIGHT happen, for cache miss ONLY,
> > > > > > > > > > > > > after
> > > > > > > > > > > > > c)
> > > > > > > > > > > > >
> > > > > > > > > > > > > Lock duration:
> > > > > > > > > > > > > - 1) lock since b) till d),
> > > > > > > > > > > > >   2) lock since c) till d), that seems to be  much shorter.
> > > > > > > > > > > > >
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Finally, in conclusion, I tend to think that
> > > > > > > > > > > > > > the bottom layer can do more things to ensure
> > > > > > > > > > > > > > the correct execution of the program, which
> > > > > > > > > > > > > > may have a negative impact on the performance
> > > > > > > > > > > > > > in a short time, but in the long run, the
> > > > > > > > > > > > > > performance will eventually
> > > > > > > > > > come back.
> > > > > > > > > > > > > > Furthermore, maybe we should pay attention to
> > > > > > > > > > > > > > the performance in the stable period, and try
> > > > > > > > > > > > > > our best to ensure the correctness of the
> > > > > > > > > > > > > > program in case of
> > > > > > > > > > > > > emergencies.
> > > > > > > > > > > > >
> > > > > > > > > > > > > If we have some mbufs still allocated in memory
> > > > > > > > > > > > > being freed
> > > > > > > > > > > > > - there is nothing to say about correctness, it
> > > > > > > > > > > > > is totally incorrect. In my opinion, we should
> > > > > > > > > > > > > not think how to mitigate this incorrect
> > > > > > > > > > > > > behavior, we should not encourage application
> > > > > > > > > > > > > developers to follow the wrong
> > > > > > > > > > > > approaches.
> > > > > > > > > > > > >
> > > > > > > > > > > > > With best regards, Slava
> > > > > > > > > > > > >
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Best Regards
> > > > > > > > > > > > > > Feifei
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > > With best regards, Slava