Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
> On 2025/6/9 15:35, Michal Hocko wrote:
>> On Mon 09-06-25 10:57:41, Ritesh Harjani wrote:
>>> Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
>>>
>>>> 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>
>>>> ---
>>>> Changes from v1:
>>>> - Update the commit message to add some measurements.
>>>> - Add acked tag from Michal. Thanks.
>>>> - Drop the Fixes tag.
>>>
>>> Any reason why we dropped the Fixes tag? I see there were a series of
>>> discussion on v1 and it got concluded that the fix was correct, then why
>>> drop the fixes tag?
>>
>> This seems more like an improvement than a bug fix.
>
> Yes. I don't have a strong opinion on this, but we (Alibaba) will
> backport it manually,
>
> because some of user-space monitoring tools depend
> on these statistics.
That sounds like a regression then, isn't it?
-ritesh
