linux-kernel - Re: [lkp] [x86/hweight] 65ea11ec6a: will-it-scale.per_process

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Message-ID: <20160817064800.GN30192@twins.programming.kicks-ass.net>
Date:	Wed, 17 Aug 2016 08:48:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...e.de>,
	kernel test robot <xiaolong.ye@...el.com>,
	Ville Syrj?l? <ville.syrjala@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org
Subject: Re: [lkp] [x86/hweight]  65ea11ec6a:  will-it-scale.per_process_ops
 9.3% improvement

On Tue, Aug 16, 2016 at 04:09:19PM -0700, H. Peter Anvin wrote:
> On August 16, 2016 10:16:35 AM PDT, Borislav Petkov <bp@...e.de> wrote:
> >On Tue, Aug 16, 2016 at 09:59:00AM -0700, H. Peter Anvin wrote:
> >> Dang...
> >
> >Isn't 9.3% improvement a good thing(tm) ?
> 
> Yes, it's huge.  The only explanation I could imagine is that scrambling %rdi caused the scheduler to do completely the wrong thing.

Not entirely surprising. We have plenty bitmasks and if hweight is
corrupting the source data instead of computing the weight then we end
up having two bits of wrong information.

After that, all we can do is more wrong of course...