linux-kernel - Re: [PATCH] x86: Add clflush before monitor for Intel 7400 series

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Message-Id: <200902091653.26570.nickpiggin@yahoo.com.au>
Date:	Mon, 9 Feb 2009 16:53:25 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, H Peter Anvin <hpa@...or.com>,
	Ingo Molnar <mingo@...e.hu>,
	"thomas@...ux-os.sc.intel.com" <thomas@...ux-os.sc.intel.com>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Add clflush before monitor for Intel 7400 series - v2

On Sunday 08 February 2009 03:47:07 Pallipadi, Venkatesh wrote:
> On Sat, Feb 07, 2009 at 06:02:45AM -0800, Alan Cox wrote:
> > On Fri, 6 Feb 2009 16:47:29 -0800
> >
> > "Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com> wrote:
> > > For Intel 7400 series CPUs, the recommendation is to use a clflush on
> > > the monitored address just before monitor and mwait pair [1]. This
> > > clflush makes sure that there are no false wakeups from mwait when the
> > > monitored address was recently written to.
> >
> > Given our mwait usages will very quickly go back to sleep in such a case
> > and it would almost certainly be one sleep only is this really worth the
> > effort ?
>
> Yes. If we only consider the CPU idle behavior, we really do not need the
> patch as we will go back to idle. But, there are other factors:
> - drivers/idle/i7300_idle.c  which tries to save memory power based on CPU
>   idle time. It gets confused with these short idles.
> - cpuidle menu governor  These platforms may also support more than one
> C-state. C1 and CC3. So, we will go through the C-state policy in menu
> governor, which again looks at idle time and may end up taking wrong
> decisions due to these short idles.
>
> We can make the above code to be more clever, to ignore short idles. But,
> this patch seems to be the easier and clean way as the errata is only in a
> particular CPU model.

Have you benchmarked it? With something like tbench which IIRC should
generate a good number of idle/busy transitions?

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