From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>,
linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>,
Luis Chamberlain <mcgrof@kernel.org>,
Kees Cook <keescook@chromium.org>,
Iurii Zaikin <yzaikin@google.com>,
Mel Gorman <mgorman@techsingularity.net>,
Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH v2] mm/compaction: Disable compact_unevictable_allowed on RT
Date: Tue, 3 Mar 2020 18:59:10 +0100 [thread overview]
Message-ID: <20200303175910.ichnkjkgmz5y2ipb@linutronix.de> (raw)
In-Reply-To: <20200302132531.59a2c9dffe2515d78abaf909@linux-foundation.org>
On 2020-03-02 13:25:31 [-0800], Andrew Morton wrote:
> > index 64aeee1009cab..bbfa59d25eec3 100644
> > --- a/Documentation/admin-guide/sysctl/vm.rst
> > +++ b/Documentation/admin-guide/sysctl/vm.rst
> > @@ -128,6 +128,7 @@ allowed to examine the unevictable lru (mlocked pages) for pages to compact.
> > This should be used on systems where stalls for minor page faults are an
> > acceptable trade for large contiguous free memory. Set to 0 to prevent
> > compaction from moving pages that are unevictable. Default value is 1.
> > +On CONFIG_PREEMPT_RT the default value is 0.
>
> This doesn't mention that the file is unwritable on -rt, and it doesn't
> explain *why* -rt has different behaviour.
I updated this bit.
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -1483,7 +1483,11 @@ static struct ctl_table vm_table[] = {
> > .procname = "compact_unevictable_allowed",
> > .data = &sysctl_compact_unevictable_allowed,
> > .maxlen = sizeof(int),
> > +#ifdef CONFIG_PREEMPT_RT
> > + .mode = 0444,
> > +#else
> > .mode = 0644,
> > +#endif
>
> This is non-backward-compatible and introduces a possibility that
> tested-on-non-rt userspace will fail on -rt kernels. It might be
> better to accept the writes, but to ignore them. Probably with a
> pr_warn_once() to let people know what we did.
Hmm.
> But do we really need to take the option away from -rt users? Perhaps
> someone wants this feature and can accept the latency hit. How about
> switching the default and otherwise leaving the kernel behaviour as-is
> and simply emitting a warning letting -rt users know that they might
> not want to enable this?
I don't think that RT people can live with the latency spike. The
problem is that it is not deterministic in terms *when* it happens and
*how*long* does it need to complete. Also it is not visible so you end
up with additional 100us and you have no idea why.
compaction is "okay" in the setup / configuration phase when the mlock()
pages aren't around / the RT task is disabled. So it does not disturb
the RT load.
Allowing the user to change the knob and spitting a warning is probably
good. So we have a preferred default and the user is aware if it is
changed with or without his knowledge.
Let me send a patch in a bit…
Sebastian
next prev parent reply other threads:[~2020-03-03 17:59 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-15 16:10 [PATCH] mm/compaction: Disable compact_unevictable_allowed on RT Sebastian Andrzej Siewior
2020-01-15 22:04 ` Vlastimil Babka
2020-01-16 10:22 ` Sebastian Andrzej Siewior
2020-03-02 17:35 ` [PATCH v2] " Sebastian Andrzej Siewior
2020-03-02 21:25 ` Andrew Morton
2020-03-03 17:59 ` Sebastian Andrzej Siewior [this message]
2020-03-03 20:20 ` [PATCH 1/2] =?UTF-8?q?mm/compaction:=20Really=20limit=20compact?= =?UTF-8?q?=5Funevictable=5Fallowed=20to=200=E2=80=A61?= Sebastian Andrzej Siewior
2020-03-03 20:22 ` [PATCH 2/2 v3] mm/compaction: Disable compact_unevictable_allowed on RT Sebastian Andrzej Siewior
2020-03-03 23:56 ` Andrew Morton
2020-03-04 8:19 ` Vlastimil Babka
2020-03-04 9:27 ` Sebastian Andrzej Siewior
2020-03-19 16:39 ` [PATCH 2/2 v4] " Sebastian Andrzej Siewior
2020-03-19 16:49 ` Vlastimil Babka
2020-03-19 16:55 ` [PATCH 2/2 v5] " Sebastian Andrzej Siewior
2020-03-04 8:18 ` [PATCH 2/2 v3] " Vlastimil Babka
2020-03-04 9:25 ` Sebastian Andrzej Siewior
2020-03-04 9:11 ` Mel Gorman
2020-03-04 8:12 ` [PATCH 1/2] mm/compaction: Really limit compact_unevictable_allowed to 0…1 Vlastimil Babka
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