Re: [PATCH 1/1] mm: count time in drain_all_pages during direct reclaim as memory pressure

[Suren Baghdasaryan]

On Mon, Feb 21, 2022 at 2:41 AM 'Petr Mladek' via kernel-team
<kernel-team@xxxxxxxxxxx> wrote:
>
> On Mon 2022-02-21 09:55:12, Michal Hocko wrote:
> > On Sat 19-02-22 09:49:40, Suren Baghdasaryan wrote:
> > > When page allocation in direct reclaim path fails, the system will
> > > make one attempt to shrink per-cpu page lists and free pages from
> > > high alloc reserves. Draining per-cpu pages into buddy allocator can
> > > be a very slow operation because it's done using workqueues and the
> > > task in direct reclaim waits for all of them to finish before
> > > proceeding. Currently this time is not accounted as psi memory stall.
> > >
> > > While testing mobile devices under extreme memory pressure, when
> > > allocations are failing during direct reclaim, we notices that psi
> > > events which would be expected in such conditions were not triggered.
> > > After profiling these cases it was determined that the reason for
> > > missing psi events was that a big chunk of time spent in direct
> > > reclaim is not accounted as memory stall, therefore psi would not
> > > reach the levels at which an event is generated. Further investigation
> > > revealed that the bulk of that unaccounted time was spent inside
> > > drain_all_pages call.
> >
> > It would be cool to have some numbers here.
> >
> > > Annotate drain_all_pages and unreserve_highatomic_pageblock during
> > > page allocation failure in the direct reclaim path so that delays
> > > caused by these calls are accounted as memory stall.
> >
> > If the draining is too slow and dependent on the current CPU/WQ
> > contention then we should address that. The original intention was that
> > having a dedicated WQ with WQ_MEM_RECLAIM would help to isolate the
> > operation from the rest of WQ activity. Maybe we need to fine tune
> > mm_percpu_wq. If that doesn't help then we should revise the WQ model
> > and use something else. Memory reclaim shouldn't really get stuck behind
> > other unrelated work.
>
> WQ_MEM_RECLAIM causes that one special worker (rescuer) is created for
> the workqueue. It is used _only_ when new workers could not be created
> for some, typically when there is non enough memory. It is just
> a fallback, last resort. It does _not_ speedup processing.
>
> Otherwise, "mm_percpu_wq" is a normal CPU-bound wq. It uses the shared
> per-CPU worker pools. They serialize all work items on a single
> worker. Another worker is used only when a work goes asleep and waits
> for something.
>
> It means that "drain" work is blocked by other work items that are
> using the same worker pool and were queued earlier.

Thanks for the valuable information!

>
>
> You might try to allocate "mm_percpu_wq" with WQ_HIGHPRI flag. It will
> use another shared per-CPU worker pools where the workers have nice
> -20. The "drain" work still might be blocked by another work items
> using the same pool. But it should be faster because the workers
> have higher priority.

This seems like a good first step to try. I'll make this change and
rerun the tests to see how useful this would be.

>
>
> Dedicated kthreads might be needed when the "draining" should not be
> blocked by anything. If you go this way then I suggest to use
> the kthread_worker API, see "linux/kthread.h". It is very similar
> to the workqueues API but it always creates new kthreads.
>
> Just note that kthread_worker API does not maintain per-CPU workers
> on its own. If you need per-CPU workers than you need to
> use kthread_create_worker_on_cpu() for_each_online_cpu().
> And you would need cpu hotplug callbacks to create/destroy
> ktheads. For example, see start_power_clamp_worker().

Got it. Let me try the WQ_HIGHPRI approach first. Let's see if we can
fix this with minimal changes to the current mechanisms.
Thanks,
Suren.

>
> HTH,
> Petr
>
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