On Wed 23-02-22 11:48:12, 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.
>
> A typical captured case when drain_all_pages path gets activated:
>
> __alloc_pages_slowpath took 44.644.613ns
> __perform_reclaim took 751.668ns (1.7%)
> drain_all_pages took 43.887.167ns (98.3%)
Although the draining is done in the slow path these numbers suggest
that we should really reconsider the use of WQ both for draining and
other purposes (like vmstats).
> 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.
>
> Annotate __alloc_pages_direct_reclaim in its entirety so that delays
> from handling page allocation failure in the direct reclaim path are
> accounted as memory stall.
>
> Reported-by: Tim Murray <timmurray@xxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> ---
> changes in v3:
> - Moved psi_memstall_leave after the "out" label
>
> mm/page_alloc.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3589febc6d31..029bceb79861 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4595,13 +4595,12 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
> const struct alloc_context *ac)
> {
> unsigned int noreclaim_flag;
> - unsigned long pflags, progress;
> + unsigned long progress;
>
> cond_resched();
>
> /* We now go into synchronous reclaim */
> cpuset_memory_pressure_bump();
> - psi_memstall_enter(&pflags);
> fs_reclaim_acquire(gfp_mask);
> noreclaim_flag = memalloc_noreclaim_save();
>
> @@ -4610,7 +4609,6 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
>
> memalloc_noreclaim_restore(noreclaim_flag);
> fs_reclaim_release(gfp_mask);
> - psi_memstall_leave(&pflags);
>
> cond_resched();
>
> @@ -4624,11 +4622,13 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> unsigned long *did_some_progress)
> {
> struct page *page = NULL;
> + unsigned long pflags;
> bool drained = false;
>
> + psi_memstall_enter(&pflags);
> *did_some_progress = __perform_reclaim(gfp_mask, order, ac);
> if (unlikely(!(*did_some_progress)))
> - return NULL;
> + goto out;
>
> retry:
> page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
> @@ -4644,6 +4644,8 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> drained = true;
> goto retry;
> }
> +out:
> + psi_memstall_leave(&pflags);
>
> return page;
> }
> --
> 2.35.1.473.g83b2b277ed-goog
--
Michal Hocko
SUSE Labs
