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

[Michal Hocko <mhocko@suse.com>]

On Tue 03-06-25 16:32:35, Baolin Wang wrote:
> 
> 
> On 2025/6/3 16:15, Michal Hocko wrote:
> > 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.
> 
> IMO, kernel code paths usually have batch caching to avoid lock contention,
> so I think the impact on kernel code paths is not that obvious.

This is a very generic statement. Does this refer to the existing pcp
locking usage in the kernel? Have you evaluated existing users?

> Therefore, I
> also think it's hard to find an adverse workload.
> 
> How about adding the following comments in the commit log?
> "
> 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).

This test doesn't really do much to trigger an actual lock contention as
already mentioned.

-- 
Michal Hocko
SUSE Labs