Subject: Re: [RFC][PATCH] PM: Force GFP_NOIO during suspend/resume (was:
 Re: [linux-pm] Memory allocations in .suspend became very unreliable)
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Oliver Neukum <oliver@neukum.org>,
       Maxim Levitsky <maximlevitsky@gmail.com>,
       linux-pm@lists.linux-foundation.org,
       LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
       Andrew Morton <akpm@linux-foundation.org>
In-Reply-To: <201001180000.23376.rjw@sisk.pl>
References: <1263549544.3112.10.camel@maxim-laptop>
	 <201001171427.27954.rjw@sisk.pl> <1263754684.724.444.camel@pasglop>
	 <201001180000.23376.rjw@sisk.pl>
Content-Type: text/plain; charset="UTF-8"
Date: Tue, 19 Jan 2010 08:55:57 +1100
Message-ID: <1263851757.724.500.camel@pasglop>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1372
Lines: 34

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 ? :-)

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.

Cheers,
Ben.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/