On Thu 29-05-25 20:53:13, Andrew Morton wrote:
> On Sat, 24 May 2025 09:59:53 +0800 Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> 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.
> >
> > Fixes: f1a7941243c1 ("mm: convert mm's rss stats into percpu_counter")
>
> Three years ago.
>
> > 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>
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>
> Thanks, I added cc:stable to this.
I have only noticed this new posting now. I do not think this is a
stable material. I am also not convinced that the impact of the pcp lock
exposure to the userspace has been properly analyzed and documented in
the changelog. I am not nacking the patch (yet) but I would like to see
a serious analyses that this has been properly thought through.
--
Michal Hocko
SUSE Labs
