linux-kernel - Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

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Message-ID: <20150221172914.GB32073@pd.tnic>
Date:	Sat, 21 Feb 2015 18:29:14 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

On Sat, Feb 21, 2015 at 05:38:40PM +0100, Borislav Petkov wrote:
> My assumption is that libc uses SSE for memcpy and thus the FPU will
> be used. (I'll trace FPU-specific PMCs later to confirm).

Ok, so I slapped a trace_printk() at the beginning of fpu_save_init()
and did a kernel build once with default 3.19 and once with Andy's
patch. So we end up with this count:

default: 712000
eager:   780000

This would explain the very small difference in the performance counters
data from the previous email.

Provided I've not made a mistake, this leads me to think that this
simple workload and pretty much everything else uses the FPU through
glibc which does the SSE memcpy and so on. Which basically kills the
whole idea behind lazy FPU as practically you don't really encounter
workloads nowadays which don't use the FPU thanks to glibc and the lazy
strategy doesn't really bring anything.

Which would then mean, we don't really need the lazy handling as
userspace is making it eager, so to speak, for us.

Thoughts?

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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