linux-kernel - Re: [PATCH 207/208] x86/fpu: Add FPU performance measurement subsystem

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Date:	Wed, 6 May 2015 06:11:04 +0200
From:	Ingo Molnar <mingo2.kernel.org@...il.com>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 207/208] x86/fpu: Add FPU performance measurement
 subsystem


* Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> On 05/05/2015 10:58 AM, Ingo Molnar wrote:
> >   x86/fpu: Cost of: XSAVE                       insn          :   104 cycles
> >   x86/fpu: Cost of: XRSTOR                      insn          :    80 cycles
> 
> Isn't there going to be pretty huge variability here depending on 
> how much state you are xsave/xrstor'ing and if the init/modified 
> optimizations are in play?

Hopefully there's such variability! :)

I thought to add measurements for that as well:

 - to see the costs of this instruction family when various xstate 
   components are in 'init state' or not

 - maybe even measure whether it can optimize based on whether things
   got changed since the last save (which the SDM kind of alludes to 
   but which I doubt the hw does)?

This initial version only measures trivial init state save/restore 
cost.

Thanks,

	Ingo
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