linux-kernel - Re: [PATCH 3/4] x86,asm: Re-work smp_store

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Message-ID: <56958467.6010808@zytor.com>
Date:	Tue, 12 Jan 2016 14:55:35 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Davidlohr Bueso <dbueso@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()

On 01/12/16 14:21, Michael S. Tsirkin wrote:
> 
> OK so I'll have to tweak the test to put something
> on stack to measure the difference: my test tweaks a
> global variable instead.
> I'll try that by tomorrow.
> 
> I couldn't measure any difference between mfence and lock+addl
> except in a micro-benchmark, but hey since we are tweaking this,
> let's do the optimal thing.
> 

Be careful with this: if it only shows up in a microbenchmark, we may
introduce a hard-to-debug regression for no real benefit.

	-hpa