linux-kernel - Re: [patch 0/2] x86,pat: Reduce contention on the memtype

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Date:	Wed, 24 Mar 2010 21:32:11 +0100
From:	"Rafael J. Wysocki" <rjw@...ell.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Robin Holt <holt@....com>,
	Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Takashi Iwai <tiwai@...e.de>
Subject: Re: [patch 0/2] x86,pat: Reduce contention on the memtype_lock -V4

On Wednesday 24 March 2010, Suresh Siddha wrote:
> On Wed, 2010-03-24 at 04:15 -0700, Thomas Gleixner wrote:
> > On Wed, 24 Mar 2010, Robin Holt wrote:
> > > On Wed, Mar 24, 2010 at 03:16:14AM +0100, Andi Kleen wrote:
> > > > holt@....com writes:
> > > > 
> > > > > Tracking memtype on x86 uses a single global spin_lock for either reading
> > > > > or changing the memory type.  This includes changes made to page flags
> > > > > which is perfectly parallel.
> > > > >
> > > > > Part one of the patchset makes the page-based tracking use cmpxchg
> > > > > without a need for a lock.
> > > > >
> > > > > Part two of the patchset converts the spin_lock into a read/write lock.
> > > > 
> > > > I'm curious: in what workloads did you see contention?
> > > > 
> > > > For any scalability patches it would be always good to have a description
> > > > of the workload.
> > > 
> > > It was a job using xpmem (an out of tree kernel module) which uses
> > > vm_insert_pfn to establish ptes.  The scalability issues were shown
> > > in the first patch.  I do not have any test which shows a performance
> > > difference  with the spin_lock to rw_lock conversion.
> > 
> > And what's exactly the point of converting it to a rw_lock then ?
> 
> Thomas, As I mentioned earlier I am ok in not doing this conversion. If
> we see any performance issues with this spinlock, we can use RCU based
> logic to address that.
> 
> For now, first patch in this series (which avoid the lock for RAM pages)
> is good to go. Thanks Rafael for spotting the page flags bit
> manipulation issue.

In fact Takashi did that, to put the record straight. :-)

Thanks,
Rafael
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