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 12:55 AM Michal Hocko <mhocko@xxxxxxxx> 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.

A typical case I was able to record when drain_all_pages path gets activated:

__alloc_pages_slowpath took 44.644.613ns
    __perform_reclaim 751.668ns (1.7%)
    drain_all_pages took 43.887.167ns (98.3%)

PSI in this case records the time spent in __perform_reclaim but
ignores drain_all_pages, IOW it misses 98.3% of the time spent in
__alloc_pages_slowpath. Sure, normally it's not often that this path
is activated, but when it is, we miss reporting most of the stall.

>
> > 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.

Agree. However even after improving this I think we should record the
time spent in drain_all_pages as psi memstall. So, this patch I
believe is still relevant.
Thanks,
Suren.

> --
> Michal Hocko
> SUSE Labs