linux-kernel - Re: [Xen-devel] [PATCHv1] x86: don't schedule when handling #NM exception

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Message-ID: <53266F56.9030909@zytor.com>
Date:	Sun, 16 Mar 2014 20:43:18 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Sarah Newman <srn@...mr.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xen.org, Suresh Siddha <sbsiddha@...il.com>
Subject: Re: [Xen-devel] [PATCHv1] x86: don't schedule when handling #NM exception

On 03/16/2014 08:35 PM, Sarah Newman wrote:
> Can you please review my patch first?  It's only enabled when absolutely required.

It doesn't help.  It means you're running on Xen, and you will have
processes subjected to random SIGKILL because they happen to touch the
FPU when the atomic pool is low.

However, there is probably a happy medium: you don't actually need eager
FPU restore, you just need eager FPU *allocation*.  We have been
intending to allocate the FPU state at task creation time for eagerfpu,
and Suresh Siddha has already produced such a patch; it just needs some
minor fixups due to an __init failure.

http://lkml.kernel.org/r/1391325599.6481.5.camel@europa

In the Xen case we could turn on eager allocation but not eager fpu.  In
fact, it might be justified to *always* do eager allocation...

	-hpa

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