linux-kernel - Re: [PATCH RFC] kvm: optimize out smp_mb using srcu_read

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Message-ID: <52723B87.1050609@redhat.com>
Date:	Thu, 31 Oct 2013 12:14:15 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	paulmck@...ux.vnet.ibm.com,
	linux-kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
	gleb@...hat.com
Subject: Re: [PATCH RFC] kvm: optimize out smp_mb using srcu_read_unlock

Il 30/10/2013 20:09, Michael S. Tsirkin ha scritto:
> I noticed that srcu_read_lock/unlock both have a memory barrier,
> so just by moving srcu_read_unlock earlier we can get rid of
> one call to smp_mb().
> 
> Unsurprisingly, the gain is small but measureable using the unit test
> microbenchmark:
> before
> 	vmcall 1407
> after
> 	vmcall 1357
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

Across how many runs?  Best or average or "all runs were in that
ballpark", :) and what's the minimum/maximum before and after the patch?

As you say the benefit is not surprising, but the experiments should be
documented properly.

Paolo
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