linux-kernel - Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay factor

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Message-ID: <1357229439.21409.25042.camel@edumazet-glaptop>
Date:	Thu, 03 Jan 2013 08:10:39 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jan Beulich <JBeulich@...e.com>, Rik van Riel <riel@...hat.com>,
	therbert@...gle.com, walken@...gle.com, jeremy@...p.org,
	tglx@...utronix.de, aquini@...hat.com, lwoodman@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay
 factor

On Thu, 2013-01-03 at 10:32 -0500, Steven Rostedt wrote:
> On Thu, 2013-01-03 at 05:35 -0800, Eric Dumazet wrote:
> > On Thu, 2013-01-03 at 08:24 -0500, Steven Rostedt wrote:
> > > On Thu, 2013-01-03 at 09:05 +0000, Jan Beulich wrote:
> > > 
> > > > > How much bus traffic do monitor/mwait cause behind the scenes?
> > > > 
> > > > I would suppose that this just snoops the bus for writes, but the
> > > > amount of bus traffic involved in this isn't explicitly documented.
> > > > 
> > > > One downside of course is that unless a spin lock is made occupy
> > > > exactly a cache line, false wakeups are possible.
> > > 
> > > And that would probably be very likely, as the whole purpose of Rik's
> > > patches was to lower cache stalls due to other CPUs pounding on spin
> > > locks that share the cache line of what is being protected (and
> > > modified).
> > 
> > A monitor/mwait would be an option only if using MCS (or K42 variant)
> > locks, where each cpu would wait on a private and dedicated cache line.
> 
> 
> But then would the problem even exist? If the lock is on its own cache
> line, it shouldn't cause a performance issue if other CPUs are spinning
> on it. Would it?

Not sure I understand the question.

The lock itself would not consume a whole cache line, only the items
chained on it would be percpu, and cache line aligned.

http://www.cs.rochester.edu/research/synchronization/pseudocode/ss.html#mcs

Instead of spinning in :

repeat while I->next = nil 

This part could use monitor/mwait


But :

1) We dont have such lock implementation

2) Trying to save power while waiting on a spinlock would be a clear
sign something is wrong in the implementation. A spinlock should not
protect a long critical section.


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