Re: [PATCH v2] mm: fix the inaccurate memory statistics issue for users

[IBM]

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? 

Background: Recently few folks internally reported this issue on Power
too. e.g. 

$ ps -o rss $$
  RSS
    0

So it would be nice if we had fixes tag so that it gets backported
to all stable release. Does anybody sees any concern with that?

-ritesh