linux-kernel - Re: [RFC][PATCH] PM: Force GFP_NOIO during suspend/resume (was: Re: [linux-pm] Memory allocations in .suspend became very unreliable)

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Message-Id: <201001190033.16055.rjw@sisk.pl>
Date:	Tue, 19 Jan 2010 00:33:15 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Oliver Neukum <oliver@...kum.org>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-pm@...ts.linux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH] PM: Force GFP_NOIO during suspend/resume (was: Re: [linux-pm] Memory allocations in .suspend became very unreliable)

On Monday 18 January 2010, Benjamin Herrenschmidt wrote:
> On Mon, 2010-01-18 at 00:00 +0100, Rafael J. Wysocki wrote:
> > On Sunday 17 January 2010, Benjamin Herrenschmidt wrote:
> > > On Sun, 2010-01-17 at 14:27 +0100, Rafael J. Wysocki wrote:
> > ...
> > > However, it's hard to deal with the case of allocations that have
> > > already started waiting for IOs. It might be possible to have some VM
> > > hook to make them wakeup, re-evaluate the situation and get out of that
> > > code path but in any case it would be tricky.
> > 
> > In the second version of the patch I used an rwsem that made us wait for these
> > allocations to complete before we changed gfp_allowed_mask.
> > 
> > [This is kinda buggy in the version I sent, but I'm going to send an update
> > in a minute.]
> 
> And nobody screamed due to cache line ping pong caused by this in the
> fast path ? :-)

Apparently not. :-)

> We might want to look at something a bit smarter for that sort of
> read-mostly-really-really-mostly construct, though in this case I don't
> think RCU is the answer since we are happily scheduling.
> 
> I wonder if something per-cpu would do, it's thus the responsibility of
> the "writer" to take them all in order for all CPUs.

I think I'll get back to the first version of the patch which I think is not
going to have side effects (as long as no one will change gfp_allowed_mask
in parallel with suspend/resume), for now.

We can add more complicated things on top of it, then.

Rafael
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