Re: [dpdk-dev] [PATCH v3 6/9] eal: register non-EAL threads as lcores

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

 
> 30/06/2020 14:07, Ananyev, Konstantin:
> > > 26/06/2020 16:43, David Marchand:
> > > > On Wed, Jun 24, 2020 at 1:59 PM Ananyev, Konstantin
> > > > <konstantin.ananyev@intel.com> wrote:
> > > > > > > Do you mean - make this new dynamic-lcore API return an error if callied
> > > > > > > from secondary process?
> > > > > >
> > > > > > Yes, and prohibiting from attaching a secondary process if dynamic
> > > > > > lcore API has been used in primary.
> > > > > > I intend to squash in patch 6:
> > > > > > https://github.com/david-marchand/dpdk/commit/e5861ee734bfe2e4dc23d9b919b0db2a32a58aee
> > > > >
> > > > > But secondary process can attach before lcore_register, so we'll have some sort of inconsistency in behaviour.
> > > >
> > > > If the developer tries to use both features, he gets an ERROR log in
> > > > the two init path.
> > > > So whatever the order at runtime, we inform the developer (who did not
> > > > read/understand the rte_thread_register() documentation) that what he
> > > > is doing is unsupported.
> > >
> > > I agree.
> > > Before this patch, pinning a thread on a random core can
> > > trigger some issues.
> > > After this patch, register an external thread will
> > > take care of logging errors in case of inconsistencies.
> > > So the user will know he is doing something not supported
> > > by the app.
> >
> > I understand that, and return a meaningful error is definitely
> > better the silent crash or memory corruption.
> > The problem with that approach, as I said before, MP group
> > behaviour becomes non-deterministic.
> 
> It was already non-deterministic before these patches.
> > > It is an nice improvement.
> > >
> > > > > If we really  want to go ahead with such workaround -
> > >
> > > It is not a workaround.
> > > It is fixing some old issues and making clear what is really impossible.
> >
> > The root cause of the problem is in our MP model design decisions:
> > from one side we treat lcore_id as process local data, from other side
> > in some shared data-structures we use lcore_id as an index.
> > I think to fix it properly we need either:
> > make lcore_id data shared or stop using lcore_id as an index for shared data.
> > So from my perspective this approach is just one of possible workarounds.
> > BTW, there is nothing wrong to have a workaround for the problem
> > we are not ready to fix right now.
> 
> I think you are trying to fix multi-process handling.
> This patch is not about multi-process, it only highlight incompatibilities.

Yes, the problem has been there for a while.
David's patch just made it more visible.
We discussing different workarounds for the problem.

> > > > > probably better to introduce explicit EAL flag ( --single-process or so).
> > > > > As Thomas and  Bruce suggested, if I understood them properly.
> > >
> > > No I was thinking to maintain the tri-state information:
> > > 	- secondary is possible
> > > 	- secondary is attached
> > > 	- secondary is forbidden
> >
> > Ok, then I misunderstood you.
> >
> > > Asking the user to use an option to forbid attaching a secondary process
> > > is the same as telling him it is forbidden.
> >
> > I don't think it is the same.
> > On a live and complex system user can't always predict will the primary proc
> > use dynamic lcore and if it will at what particular moment.
> > Same for secondary process launching - user might never start it,
> > might start it straight after the primary one,
> > or might be after several hours.
> 
> I don't see the difference.
> An app which register external threads is not compatible
> with multi-process. It needs to be clear.
> If the user tries to do it anyway, there can be some error, OK.

Copying from other mail thread:
Imagine the situation - there is a primary proc (supposed to run forever)
that does  rte_thread_register/rte_thread_unregister during its lifetime.
Plus from time to time user runs some secondary process to collect stats/debug
the primary one (proc-info or so).
Now behaviour of such system will be non-deterministic:
In some runs primary proc will do rte_thread_register() first,
and then secondary proc will be never able to attach.
In other cases - secondary will win the race, and then for primary 
eal_lcore_non_eal_allocate() will always fail.
Which means different behaviour between runs, varying performance, etc.

> > > The error log is enough in my opinion.
> >
> > I think it is better than nothing, but probably not the best one.
> > Apart from possible non-consistent behaviour, it is quite restrictive:
> > dynamic lcore_id wouldn't be available on any DPDK MP deployment.
> > Which is a pity - I think it is a cool and useful feature.
> 
> So you are asking to extend the feature.

I am asking for solution that would guarantee deterministic behaviour to the user.
If dynamic lcores and MP support need to be mutually exclusive,
then there should be a clean way for the user to *always* enable
one and disable the other.
"--proc-type=standalone" will at least guarantee such consistent behaviour between runs:
secondary proc will always fail to attach and  eal_lcore_non_eal_allocate() will always succeed
(as long as there are free lcore_ids off-course).
Though I think even better would be not to make them mutually exclusive,
but instead let user to split lcore_id space accordingly.
Let me list the options currently under discussion:

a)   New EAL parameter '--lcore-allow=...'
	Explicit EAL parameter to enable dyn-lcore=Y
	Consistent behaviour between runs=Y
	DYN-lcores/MP-support are mutually exclusive=N 

b)  Extend '--proc-type' EAL parameter with new 'standalone' type
	Explicit EAL parameter to enable dyn-lcore =Y
	Consistent behaviour between runs=Y
	Dyn lcores/MP-support are mutually exclusive=Y

c) dynamic allow/forbid dynamic-lcore/MP support
	Explicit EAL parameter=N
	Consistent behaviour between runs=N
	Dyn lcores/MP-support are mutually exclusive=Y

My preference list (from top to bottom): a, b, c.

> Honestly, I'm not a fan of multi-process,
> so I would not push any feature for it.

Me too, but as we can't drop it, we probably have no
choice but to live with it. 

> 
> If we don't add any new option now, and restrict MP handling
> to error messages, it would not prevent from extending
> in future, right?

It shouldn't I think.
Though what is the urgency to push this feature without having an
agreement first?
 
> 
> > What do you guys think about different approach:
> > introduce new optional EAL parameter to restrict lcore_id
> > values available for the process.
> >
> > #let say to start primary proc that can use lcore_id=[0-99] only:
> > dpdk_primary --lcore-allow=0-99 ... --file-prefix=xz1
> >
> > #to start secondary one for it with allowed lcore_id=[100-109]:
> > dpdk_secondary --lcore-allow=100-109 ... --file-prefix=xz1 --proc-type=secondary
> >
> > It is still a workaround, but that way we don't need to
> > add any new limitations for dynamic lcores and secondary process usage.
> > Now it is up to user to decide would multiple-process use the same shared data
> > and if so - split lcore_id space properly among them
> > (same as he has to do now with static lcores).
> 
> Isn't it pushing too much to the user?

User has to do the similar thing with static lcores right now.
 
> 
> > > > A EAL flag is a stable API from the start, as there is nothing
> > > > describing how we can remove one.
> > > > So a new EAL flag for an experimental API/feature seems contradictory.
> > > >
> > > > Going with a new features status API... I think it is beyond this series.
> > > >
> > > > Thomas seems to suggest an automatic resolution when features conflict
> > > > happens.. ?
> > >
> > > I suggest allowing the maximum and raise an error when usage conflicts.
> > > It seems this is what you did in v4.
> > >
> > > > I'll send the v4, let's discuss it there if you want.
> 
>