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

[Michal Hocko <mhocko@suse.com>]

On Tue 03-06-25 16:08:21, Baolin Wang wrote:
> 
> 
> On 2025/5/30 21:39, Michal Hocko wrote:
> > On Thu 29-05-25 20:53:13, Andrew Morton wrote:
> > > On Sat, 24 May 2025 09:59:53 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> 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@linux.ibm.com>
> > > > Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> > > > Tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> > > > Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> > > > Acked-by: SeongJae Park <sj@kernel.org>
> > > > Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > > 
> > > 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.
> 
> Good point. I did a quick measurement on my 32 cores Arm machine. I ran two
> workloads, one is the 'top' command: top -d 1 (updating every second).
> Another workload is kernel building (time make -j32).
> 
> From the following data, I did not see any significant impact of the patch
> changes on the execution of the kernel building workload.

I do not think this is really representative of an adverse workload. I
believe you need to have a look which potentially sensitive kernel code
paths run with the lock held how would a busy loop over affected proc
files influence those in the worst case. Maybe there are none of such
kernel code paths to really worry about. This should be a part of the
changelog though.

Thanks!
-- 
Michal Hocko
SUSE Labs