linux-kernel - Re: [PATCH] x86: Voluntary leave

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Date:	Wed, 19 Dec 2007 18:08:47 -0800
From:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Venki Pallipadi <venkatesh.pallipadi@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Len Brown <lenb@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Voluntary leave_mm before entering ACPI C3

On Wed, Dec 19, 2007 at 08:32:55PM +0100, Ingo Molnar wrote:
> 
> * Venki Pallipadi <venkatesh.pallipadi@...el.com> wrote:
> 
> > Aviod TLB flush IPIs during C3 states by voluntary leave_mm() before 
> > entering C3.
> > 
> > The performance impact of TLB flush on C3 should not be significant 
> > with respect to C3 wakeup latency. Also, CPUs tend to flush TLB in 
> > hardware while in C3 anyways.
> > 
> > On a 8 logical CPU system, running make -j2, the number of tlbflush 
> > IPIs goes down from 40 per second to ~ 0. Total number of interrupts 
> > during the run of this workload was ~1200 per second, which makes it 
> > ~3% savings in wakeups.
> > 
> > There was no measurable performance or power impact however.
> 
> thanks, applied to x86.git. Nice and elegant patch!
> 
> Btw., since the TLB flush state machine is really subtle and fragile, 
> could you try to run the following mmap stresstest i wrote some time 
> ago:
> 
>    http://redhat.com/~mingo/threaded-mmap-stresstest/
> 
> for a couple of hours. It runs nr_cpus threads which then do a "random 
> crazy mix" of mappings/unmappings/remappings of a 800 MB memory window. 
> The more sockets/cores, the crazier the TLB races get ;-)
> 

Ingo,

I ran this stress test on two systems (8 cores and 2 cores) for over
4 hours without any issues. There was more than 20% C3 time during the
run. So, this C3 tlbflush path must have been stressed well during the run.

And sorry about the patch not working on UP config. That was a silly oversight
on my part.

Thanks,
Venki
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