On Wed, Feb 23, 2022 at 11:48:12AM -0800, 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%)
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>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
