On 6/5/25 14:58, Baolin Wang wrote:
> On some large machines with a high number of CPUs running a 64K pagesize
> kernel, we found that the 'RES' field is always 0 displayed by the top
> command for some processes, which will cause a lot of confusion for users.
>
> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> 875525 root 20 0 12480 0 0 R 0.3 0.0 0:00.08 top
> 1 root 20 0 172800 0 0 S 0.0 0.0 0:04.52 systemd
>
> The main reason is that the batch size of the percpu counter is quite large
> on these machines, caching a significant percpu value, since converting mm's
> rss stats into percpu_counter by commit f1a7941243c1 ("mm: convert mm's rss
> stats into percpu_counter"). Intuitively, the batch number should be optimized,
> but on some paths, performance may take precedence over statistical accuracy.
> Therefore, introducing a new interface to add the percpu statistical count
> and display it to users, which can remove the confusion. In addition, this
> change is not expected to be on a performance-critical path, so the modification
> should be acceptable.
>
> In addition, the 'mm->rss_stat' is updated by using add_mm_counter() and
> dec/inc_mm_counter(), which are all wrappers around percpu_counter_add_batch().
> In percpu_counter_add_batch(), there is percpu batch caching to avoid 'fbc->lock'
> contention. This patch changes task_mem() and task_statm() to get the accurate
> mm counters under the 'fbc->lock', but this should not exacerbate kernel
> 'mm->rss_stat' lock contention due to the percpu batch caching of the mm
> counters. The following test also confirm the theoretical analysis.
>
> I run the stress-ng that stresses anon page faults in 32 threads on my 32 cores
> machine, while simultaneously running a script that starts 32 threads to
> busy-loop pread each stress-ng thread's /proc/pid/status interface. From the
> following data, I did not observe any obvious impact of this patch on the
> stress-ng tests.
>
> w/o patch:
> stress-ng: info: [6848] 4,399,219,085,152 CPU Cycles 67.327 B/sec
> stress-ng: info: [6848] 1,616,524,844,832 Instructions 24.740 B/sec (0.367 instr. per cycle)
> stress-ng: info: [6848] 39,529,792 Page Faults Total 0.605 M/sec
> stress-ng: info: [6848] 39,529,792 Page Faults Minor 0.605 M/sec
>
> w/patch:
> stress-ng: info: [2485] 4,462,440,381,856 CPU Cycles 68.382 B/sec
> stress-ng: info: [2485] 1,615,101,503,296 Instructions 24.750 B/sec (0.362 instr. per cycle)
> stress-ng: info: [2485] 39,439,232 Page Faults Total 0.604 M/sec
> stress-ng: info: [2485] 39,439,232 Page Faults Minor 0.604 M/sec
>
> Tested-by Donet Tom <donettom@xxxxxxxxxxxxx>
> Reviewed-by: Aboorva Devarajan <aboorvad@xxxxxxxxxxxxx>
> Tested-by: Aboorva Devarajan <aboorvad@xxxxxxxxxxxxx>
> Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Acked-by: SeongJae Park <sj@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Thanks!
