linux-kernel - Re: [PATCH 1/2] mm, oom: introduce oom reaper

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Date:	Thu, 17 Dec 2015 11:55:11 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	David Rientjes <rientjes@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Argangeli <andrea@...nel.org>,
	Rik van Riel <riel@...hat.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm, oom: introduce oom reaper

On Thu, Dec 17, 2015 at 5:02 AM, Michal Hocko <mhocko@...nel.org> wrote:
> Ups. You are right. I will go with msleep_interruptible(100).

I don't think that's right.

If a signal happens, that loop is now (again) just busy-looping. That
doesn't sound right, although with the maximum limit of 10 attempts,
maybe it's fine - the thing is technically "busylooping", but it will
definitely not busy-loop for very long.

So maybe that code is fine, but I think the signal case might at least
merit a comment?

Also, if you actually do want UNINTERRUPTIBLE (no reaction to signals
at all), but don't want to be seen as being "load" on the system, you
can use TASK_IDLE, which is a combination of TASK_UNINTERRUPTIBLE |
TASK_NOLOAD.

Because if you sleep interruptibly, you do generally need to handle
signals (although that limit count may make it ok in this case).

There's basically three levels:

 - TASK_UNINTERRUPTIBLE: no signal handling at all

 - TASK_KILLABLE: no normal signal handling, but ok to be killed
(needs to check fatal_signal_pending() and exit)

 - TASK_INTERRUPTIBLE: will react to signals

(and then that TASK_IDLE thing that is semantically the same as
uninterruptible, but doesn't count against the load average).

The main use for TASK_KILLABLE is in places where expected semantics
do not allow a EINTR return, but we know that because the process is
about to be killed, we can ignore that, for the simple reason that
nobody will ever *see* the EINTR.

Btw, I think you might want to re-run your test-case after this
change, since the whole "busy loop vs actually sleeping" might just
have changed the result..

              Linus
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