Re: [PATCH v1] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE counting

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Ruan Shiyang <ruansy.fnst@fujitsu.com> writes:

> Goto-san reported confusing pgpromote statistics where the
> pgpromote_success count significantly exceeded pgpromote_candidate.
>
> On a system with three nodes (nodes 0-1: DRAM 4GB, node 2: NVDIMM 4GB):
>  # Enable demotion only
>  echo 1 > /sys/kernel/mm/numa/demotion_enabled
>  numactl -m 0-1 memhog -r200 3500M >/dev/null &
>  pid=$!
>  sleep 2
>  numactl memhog -r100 2500M >/dev/null &
>  sleep 10
>  kill -9 $pid # terminate the 1st memhog
>  # Enable promotion
>  echo 2 > /proc/sys/kernel/numa_balancing
>
> After a few seconds, we observeed `pgpromote_candidate < pgpromote_success`
> $ grep -e pgpromote /proc/vmstat
> pgpromote_success 2579
> pgpromote_candidate 0
>
> In this scenario, after terminating the first memhog, the conditions for
> pgdat_free_space_enough() are quickly met, and triggers promotion.
> However, these migrated pages are only counted for in PGPROMOTE_SUCCESS,
> not in PGPROMOTE_CANDIDATE.
>
> To solve this confusing statistics, introduce this PGPROMOTE_CANDIDATE_NRL
> to count the missed promotion pages.  And also, not counting these pages
> into PGPROMOTE_CANDIDATE is to avoid changing the existing algorithm or
> performance of the promotion rate limit.
>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Reported-by: Yasunori Gotou (Fujitsu) <y-goto@fujitsu.com>
> Suggested-by: Huang Ying <ying.huang@linux.alibaba.com>
> Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
> Signed-off-by: Ruan Shiyang <ruansy.fnst@fujitsu.com>

LGTM, feel free to add my

Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>

in the future version.

[snip]

---
Best Regards,
Huang, Ying