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

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Message-ID: <aD8LKHfCca1wQ5pS@tiehlicka>
Date: Tue, 3 Jun 2025 16:48:08 +0200
From: Michal Hocko <mhocko@...e.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, david@...hat.com,
	shakeel.butt@...ux.dev, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
	surenb@...gle.com, donettom@...ux.ibm.com, aboorvad@...ux.ibm.com,
	sj@...nel.org, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix the inaccurate memory statistics issue for users

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