Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

From: Michal Hocko <mhocko@suse.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
	Minchan Kim <minchan@kernel.org>,
	Christian Brauner <christian@brauner.io>,
	Christoph Hellwig <hch@infradead.org>,
	Oleg Nesterov <oleg@redhat.com>,
	David Hildenbrand <david@redhat.com>,
	Jann Horn <jannh@google.com>, Shakeel Butt <shakeelb@google.com>,
	Andy Lutomirski <luto@kernel.org>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Florian Weimer <fweimer@redhat.com>,
	Jan Engelhardt <jengelh@inai.de>,
	Linux API <linux-api@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@android.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap
Date: Tue, 2 Nov 2021 08:58:19 +0100	[thread overview]
Message-ID: <YYDvm9c/7cGtBvw6@dhcp22.suse.cz> (raw)
In-Reply-To: <CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com>

On Mon 01-11-21 08:44:58, Suren Baghdasaryan wrote:
[...]
> I'm with you on this one, that's why I wanted to measure the price we
> would pay. Below are the test results:
> 
> Test: https://lore.kernel.org/all/20170725142626.GJ26723@dhcp22.suse.cz/
> Compiled: gcc -O2 -static test.c -o test
> Test machine: 128 core / 256 thread 2x AMD EPYC 7B12 64-Core Processor
> (family 17h)
> 
> baseline (Linus master, f31531e55495ca3746fb895ffdf73586be8259fa)
> p50 (median)   87412
> p95                  168210
> p99                  190058
> average           97843.8
> stdev               29.85%
> 
> unconditional mmap_write_lock in exit_mmap (last column is the change
> from the baseline)
> p50 (median)   88312     +1.03%
> p95                  170797   +1.54%
> p99                  191813   +0.92%
> average           97659.5  -0.19%
> stdev               32.41%
> 
> unconditional mmap_write_lock in exit_mmap + Matthew's patch (last
> column is the change from the baseline)
> p50 (median)   88807      +1.60%
> p95                  167783     -0.25%
> p99                  187853     -1.16%
> average           97491.4    -0.36%
> stdev               30.61%
> 
> stdev is quite high in all cases, so the test is very noisy.
> The impact seems quite low IMHO. WDYT?

Results being very noisy is what I recall as well. Thanks!
-- 
Michal Hocko
SUSE Labs