linux-kernel - Re: [PATCH] mm: convert mm's rss stats into percpu

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Date:   Mon, 24 Oct 2022 15:30:22 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: convert mm's rss stats into percpu_counter

On Mon, 24 Oct 2022 05:28:41 +0000 Shakeel Butt <shakeelb@...gle.com> wrote:

> Currently mm_struct maintains rss_stats which are updated on page fault
> and the unmapping codepaths. For page fault codepath the updates are
> cached per thread with the batch of TASK_RSS_EVENTS_THRESH which is 64.
> The reason for caching is performance for multithreaded applications
> otherwise the rss_stats updates may become hotspot for such
> applications.
> 
> However this optimization comes with the cost of error margin in the rss
> stats. The rss_stats for applications with large number of threads can
> be very skewed. At worst the error margin is (nr_threads * 64) and we
> have a lot of applications with 100s of threads, so the error margin can
> be very high. Internally we had to reduce TASK_RSS_EVENTS_THRESH to 32.
> 
> Recently we started seeing the unbounded errors for rss_stats for
> specific applications which use TCP rx0cp. It seems like
> vm_insert_pages() codepath does not sync rss_stats at all.
> 
> This patch converts the rss_stats into percpu_counter to convert the
> error margin from (nr_threads * 64) to approximately (nr_cpus ^ 2).

Confused.  The max error should be O(nr_cpus)?

> However this conversion enable us to get the accurate stats for
> situations where accuracy is more important than the cpu cost. Though
> this patch does not make such tradeoffs.

Curiousity.  Can you expand on the final sentence here?

>  8 files changed, 40 insertions(+), 107 deletions(-)

There's that, too.