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If there is no memory allocation/freeing in the PCP (Per-CPU Pageset) of a remote zone (zone in remote NUMA node) after some time (3 seconds for now), the pages of the PCP of the remote zone will be drained to avoid memory wastage. This behavior was introduced in the commit4ae7c03943("[PATCH] Periodically drain non local pagesets") and the commit4037d45220("Move remote node draining out of slab allocators") But, after the commit7cc36bbddd("vmstat: on-demand vmstat workers V8"), the vmstat updater worker which is used to drain the PCP of remote zones may not be re-queued when we are waiting for the timeout (pcp->expire != 0) if there are no vmstat changes on this CPU, for example, when the CPU goes idle or runs user space only workloads. This may cause the pages of a remote zone be kept in PCP of this CPU for long time. So that, the page reclaiming of the remote zone may be triggered prematurely. This isn't a severe problem in practice, because the PCP of the remote zone will be drained if some memory are allocated/freed again on this CPU. And, the PCP will eventually be drained during the direct reclaiming if necessary. Anyway, the problem still deserves a fix via guaranteeing that the vmstat updater worker will always be re-queued when we are waiting for the timeout. In effect, this restores the original behavior before the commit7cc36bbddd. We can reproduce the bug via allocating/freeing pages from a remote zone then go idle as follows. And the patch can fix it. - Run some workloads, use `numactl` to bind CPU to node 0 and memory to node 1. So the PCP of the CPU on node 0 for zone on node 1 will be filled. - After workloads finish, idle for 60s - Check /proc/zoneinfo With the original kernel, the number of pages in the PCP of the CPU on node 0 for zone on node 1 is non-zero after idle. With the patched kernel, it becomes 0 after idle. That is, we avoid to keep pages in the remote PCP during idle. Link: https://lkml.kernel.org/r/20231007062356.187621-1-ying.huang@intel.com Link: https://lkml.kernel.org/r/20230811090819.60845-1-ying.huang@intel.com Fixes:7cc36bbddd("vmstat: on-demand vmstat workers V8") Signed-off-by: "Huang, Ying" <ying.huang@intel.com> Reviewed-by: Christoph Lameter <cl@linux.com> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Michal Hocko <mhocko@kernel.org> 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.