501b26510a
A customer provided evidence indicating that a process was stalled in direct reclaim: - The process was trapped in throttle_direct_reclaim(). The function wait_event_killable() was called to wait condition allow_direct_reclaim(pgdat) for current node to be true. The allow_direct_reclaim(pgdat) examined the number of free pages on the node by zone_page_state() which just returns value in zone->vm_stat[NR_FREE_PAGES]. - On node #1, zone->vm_stat[NR_FREE_PAGES] was 0. However, the freelist on this node was not empty. - This inconsistent of vmstat value was caused by percpu vmstat on nohz_full cpus. Every increment/decrement of vmstat is performed on percpu vmstat counter at first, then pooled diffs are cumulated to the zone's vmstat counter in timely manner. However, on nohz_full cpus (in case of this customer's system, 48 of 52 cpus) these pooled diffs were not cumulated once the cpu had no event on it so that the cpu started sleeping infinitely. I checked percpu vmstat and found there were total 69 counts not cumulated to the zone's vmstat counter yet. - In this situation, kswapd did not help the trapped process. In pgdat_balanced(), zone_wakermark_ok_safe() examined the number of free pages on the node by zone_page_state_snapshot() which checks pending counts on percpu vmstat. Therefore kswapd could know there were 69 free pages correctly. Since zone->_watermark = {8, 20, 32}, kswapd did not work because 69 was greater than 32 as high watermark. Change allow_direct_reclaim to use zone_page_state_snapshot, which allows a more precise version of the vmstat counters to be used. allow_direct_reclaim will only be called from try_to_free_pages, which is not a hot path. Testing: Due to difficulties accessing the system, it has not been possible for the reproducer to test the patch (however its clear from available data and analysis that it should fix it). Link: https://lkml.kernel.org/r/20230530145335.677325196@redhat.com Reviewed-by: Michal Hocko <mhocko@suse.com> Reviewed-by: Aaron Tomlin <atomlin@atomlin.com> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com> Cc: Christoph Lameter <cl@linux.com> Cc: Frederic Weisbecker <frederic@kernel.org> Cc: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.