[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

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
> > >
> > > > -----Original Message-----
> > > > From: Feifei Wang <feifei.wang2@arm.com>
> > > > Sent: Thursday, March 18, 2021 9:19
> > > > To: Matan Azrad <matan@nvidia.com>; Shahaf Shuler
> > > > <shahafs@nvidia.com>; Slava Ovsiienko <viacheslavo@nvidia.com>;
> > > > Yongseok Koh <yskoh@mellanox.com>
> > > > Cc: dev@dpdk.org; nd@arm.com; Feifei Wang
> <feifei.wang2@arm.com>;
> > > > stable@dpdk.org; Ruifeng Wang <ruifeng.wang@arm.com>
> > > > Subject: [PATCH v1 3/4] net/mlx5: fix rebuild bug for Memory
> > > > Region cache
> > > >
> > > > 'dev_gen' is a variable to inform other cores to flush their local
> > > > cache when global cache is rebuilt.
> > > >
> > > > However, if 'dev_gen' is updated after global cache is rebuilt,
> > > > other cores may load a wrong memory region lkey value from old
> > > > local
> > cache.
> > > >
> > > > Timeslot        main core               worker core
> > > >   1         rebuild global cache
> > > >   2                                  load unchanged dev_gen
> > > >   3            update dev_gen
> > > >   4                                  look up old local cache
> > > >
> > > > From the example above, we can see that though global cache is
> > > > rebuilt, due to that dev_gen is not updated, the worker core may
> > > > look up old cache table and receive a wrong memory region lkey value.
> > > >
> > > > To fix this, updating 'dev_gen' should be moved before rebuilding
> > > > global cache to inform worker cores to flush their local cache
> > > > when global cache start rebuilding. And wmb can ensure the
> > > > sequence of this
> > > process.
> > > >
> > > > Fixes: 974f1e7ef146 ("net/mlx5: add new memory region support")
> > > > Cc: stable@dpdk.org
> > > >
> > > > Suggested-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > > Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
> > > > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > > ---
> > > >  drivers/net/mlx5/mlx5_mr.c | 37
> > > > +++++++++++++++++--------------------
> > > >  1 file changed, 17 insertions(+), 20 deletions(-)
> > > >
> > > > diff --git a/drivers/net/mlx5/mlx5_mr.c
> > > > b/drivers/net/mlx5/mlx5_mr.c index
> > > > da4e91fc2..7ce1d3e64 100644
> > > > --- a/drivers/net/mlx5/mlx5_mr.c
> > > > +++ b/drivers/net/mlx5/mlx5_mr.c
> > > > @@ -103,20 +103,18 @@ mlx5_mr_mem_event_free_cb(struct
> > > > mlx5_dev_ctx_shared *sh,
> > > >  		rebuild = 1;
> > > >  	}
> > > >  	if (rebuild) {
> > > > -		mlx5_mr_rebuild_cache(&sh->share_cache);
> > > > +		++sh->share_cache.dev_gen;
> > > > +		DEBUG("broadcasting local cache flush, gen=%d",
> > > > +			sh->share_cache.dev_gen);
> > > > +
> > > >  		/*
> > > >  		 * Flush local caches by propagating invalidation across cores.
> > > > -		 * rte_smp_wmb() is enough to synchronize this event. If
> > > > one of
> > > > -		 * freed memsegs is seen by other core, that means the
> > > > memseg
> > > > -		 * has been allocated by allocator, which will come after this
> > > > -		 * free call. Therefore, this store instruction (incrementing
> > > > -		 * generation below) will be guaranteed to be seen by other
> > > > core
> > > > -		 * before the core sees the newly allocated memory.
> > > > +		 * rte_smp_wmb() is to keep the order that dev_gen
> > > > updated before
> > > > +		 * rebuilding global cache. Therefore, other core can flush
> > > > their
> > > > +		 * local cache on time.
> > > >  		 */
> > > > -		++sh->share_cache.dev_gen;
> > > > -		DEBUG("broadcasting local cache flush, gen=%d",
> > > > -		      sh->share_cache.dev_gen);
> > > >  		rte_smp_wmb();
> > > > +		mlx5_mr_rebuild_cache(&sh->share_cache);
> > > >  	}
> > > >  	rte_rwlock_write_unlock(&sh->share_cache.rwlock);
> > > >  }
> > > > @@ -407,20 +405,19 @@ mlx5_dma_unmap(struct rte_pci_device
> > *pdev,
> > > void
> > > > *addr,
> > > >  	mlx5_mr_free(mr, sh->share_cache.dereg_mr_cb);
> > > >  	DEBUG("port %u remove MR(%p) from list", dev->data->port_id,
> > > >  	      (void *)mr);
> > > > -	mlx5_mr_rebuild_cache(&sh->share_cache);
> > > > +
> > > > +	++sh->share_cache.dev_gen;
> > > > +	DEBUG("broadcasting local cache flush, gen=%d",
> > > > +		sh->share_cache.dev_gen);
> > > > +
> > > >  	/*
> > > >  	 * Flush local caches by propagating invalidation across cores.
> > > > -	 * rte_smp_wmb() is enough to synchronize this event. If one of
> > > > -	 * freed memsegs is seen by other core, that means the memseg
> > > > -	 * has been allocated by allocator, which will come after this
> > > > -	 * free call. Therefore, this store instruction (incrementing
> > > > -	 * generation below) will be guaranteed to be seen by other core
> > > > -	 * before the core sees the newly allocated memory.
> > > > +	 * rte_smp_wmb() is to keep the order that dev_gen updated
> > > > before
> > > > +	 * rebuilding global cache. Therefore, other core can flush their
> > > > +	 * local cache on time.
> > > >  	 */
> > > > -	++sh->share_cache.dev_gen;
> > > > -	DEBUG("broadcasting local cache flush, gen=%d",
> > > > -	      sh->share_cache.dev_gen);
> > > >  	rte_smp_wmb();
> > > > +	mlx5_mr_rebuild_cache(&sh->share_cache);
> > > >  	rte_rwlock_read_unlock(&sh->share_cache.rwlock);
> > > >  	return 0;
> > > >  }
> > > > --
> > > > 2.25.1