linux-kernel - Re: [RFC v2 8/8] x86, xsave: remove lazy allocation of xstate area

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Message-ID: <20110329141712.GM13@escobedo.osrc.amd.com>
Date:	Tue, 29 Mar 2011 16:17:12 +0200
From:	Hans Rosenfeld <hans.rosenfeld@....com>
To:	Brian Gerst <brgerst@...il.com>
CC:	"hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>, "x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"suresh.b.siddha@...el.com" <suresh.b.siddha@...el.com>,
	"eranian@...gle.com" <eranian@...gle.com>,
	"Richter, Robert" <robert.richter@....com>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>
Subject: Re: [RFC v2 8/8] x86, xsave: remove lazy allocation of xstate area

On Thu, Mar 24, 2011 at 07:39:13AM -0400, Brian Gerst wrote:
> On Wed, Mar 23, 2011 at 11:27 AM, Hans Rosenfeld <hans.rosenfeld@....com> wrote:
> > This patch completely removes lazy allocation of the xstate area. All
> > tasks will always have an xstate area preallocated, just like they
> > already do when non-lazy features are present. The size of the xsave
> > area ranges from 112 to 960 bytes, depending on the xstates present and
> > enabled. Since it is common to use SSE etc. for optimization, the actual
> > overhead is expected to negligible.
> >
> > This removes some of the special-case handling of non-lazy xstates. It
> > also greatly simplifies init_fpu() by removing the allocation code, the
> > check for presence of the xstate area or init_fpu() return value.
> >
> > Signed-off-by: Hans Rosenfeld <hans.rosenfeld@....com>
> 
> I'm not sure I like this.  I did a quick test on 64-bit, and found
> that while most if not all user processes allocated the fpu save area
> (probably because of glibc blindly initializing the fpu), kernel
> threads did not.  This patch would force kernel threads to allocate
> memory they would never use.

Yes, up to a few kilobytes would be wasted by kernel threads. The
related code gets much simpler. I think that is a good thing.

Anyway, the patch is not essential for the rework and LWP support, so I
don't really care that much about it.


Did you take a look at the other patches? I haven't yet received a
single comment on them.


Hans


-- 
%SYSTEM-F-ANARCHISM, The operating system has been overthrown

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