linux-kernel - Re: [PATCH RFC ticketlock] Auto-queued ticketlock

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Message-ID: <20130611164503.GI5146@linux.vnet.ibm.com>
Date:	Tue, 11 Jun 2013 09:45:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>, niv@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Darren Hart <darren@...art.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	sbw@....edu
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On Tue, Jun 11, 2013 at 11:22:45AM -0400, Steven Rostedt wrote:
> On Tue, 2013-06-11 at 03:14 -0700, Paul E. McKenney wrote:
> >  
> > > Off-topic, although I am in this community for several years,
> > > I am not exactly clear with this problem.
> > > 
> > > 1) In general case, which lock is the most competitive in the kernel? what it protects for?
> > > 2) In which special case, which lock is the most competitive in the kernel? what it protects for?
> > > 3) In general case, which list is the most hot list?
> > > 4) In which special case, which list is the most hot list?
> > 
> > Others would know better than I, but mmap_sem has been called out as a
> 
> If the contention is with mmap_sem, then I doubt this is going to help
> much, as that's a sleeping rw semaphore. Now, rw semaphores are
> implemented with raw spinlocks, but I doubt that would be the main point
> of contention, compared to the sleeping part.

If I remember correctly, someone actually hit this earlier this year,
which prompted use of a special-purpose queued lock to guard the
semaphore data.  I don't recall whether it was mmap_sem or not, so
cannot say whether it was a straight mutex or an rw semaphore.

							Thanx, Paul

> -- Steve
> 
> > prime offender for some workloads.  There is of course some debate as
> > to whether the fault lies mmap_sem or with the workloads.  There have
> > been some efforts to solve this one on LKML, plus some in academia have
> > worked on this as well:
> > 
> 
> 

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