linux-kernel - Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

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Message-ID: <20131001081011.GM3081@twins.programming.kicks-ass.net>
Date:	Tue, 1 Oct 2013 10:10:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Waiman Long <Waiman.Long@...com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Alex Shi <alex.shi@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

On Tue, Oct 01, 2013 at 09:48:02AM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Sat, Sep 28, 2013 at 11:55:26AM -0700, Linus Torvalds wrote:
> > > So if the primary reason for this is really just that f*cking anon_vma
> > > lock, then I would seriously suggest:
> > 
> > I would still like to see the rwsem patches merged; even if we end up 
> > going back to a spin style anon_vma lock.
> > 
> > There's been various reports in the past about how programs are 
> > significantly faster if they wrap their mmap() calls in a pthread_mutex. 
> > And this was purely down to the fact that rwsem writer-writer contention 
> > blows chunks.
> 
> That's about the mm->mmap_sem rwsem, right?

Yep.

> That impact would have to be measured carefully, and not just for 
> workloads where we know that better contention logic helps, but other MM 
> workloads that are hitting hard on mmap_sem.

Sure.
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