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

[Michal Hocko]

On Tue 03-06-25 22:22:46, Baolin Wang wrote:
> Let me try to clarify further.
> 
> The 'mm->rss_stat' is updated by using add_mm_counter(),
> 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. 

OK, this is exactly the line of argument I was looking for. If _all_
updates done in the kernel are using batching and therefore the lock is
only held every N (percpu_counter_batch) updates then a risk of locking
contention would be decreased. This is worth having a note in the
changelog.

> This patch changes task_mem()
> and task_statm() to get the accurate mm counters under the 'fbc->lock', but
> this will not exacerbate kernel 'mm->rss_stat' lock contention due to the
> the percpu batch caching of the mm counters.
> 
> You might argue that my test cases cannot demonstrate an actual lock
> contention, but they have already shown that there is no significant
> 'fbc->lock' contention when the kernel updates 'mm->rss_stat'.

I was arguing that `top -d 1' doesn't really represent a potential
adverse usage. These proc files are generally readable so I would be
expecting something like busy loop read while process tries to update
counters to see the worst case scenario. If that is barely visible then
we can conclude a normal use wouldn't even notice.

See my point?

-- 
Michal Hocko
SUSE Labs