linux-kernel - Re: [PATCH v2] x86,mm: only trim the mm

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Message-ID: <4360210d-0656-4328-ab67-240465ab2f9c@intel.com>
Date: Tue, 3 Dec 2024 16:46:32 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Rik van Riel <riel@...riel.com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
 lkp@...el.com, linux-kernel@...r.kernel.org, x86@...nel.org,
 Ingo Molnar <mingo@...nel.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v2] x86,mm: only trim the mm_cpumask once a second

On 12/3/24 12:07, Rik van Riel wrote:
> The tlb_flush2 threaded test does not only madvise in a
> loop, but also mmap and munmap from inside every thread.
> 
> This should create massive contention on the mmap_lock,
> resulting in threads going to sleep while waiting in
> mmap and munmap.
> 
> https://github.com/antonblanchard/will-it-scale/blob/master/tests/tlb_flush2.c

Oh, wow, it only madvise()'s a 1MB allocation before doing the
munmap()/mmap(). I somehow remembered it being a lot larger. And, yeah,
I see a ton of idle time which would be 100% explained by mmap_lock
contention.

Did the original workload that you care about have idle time?

I'm wondering if trimming mm_cpumask() on the way to idle but leaving it
alone on a context switch to another thread is a good idea.