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

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Message-ID: <20131001080921.GL3081@twins.programming.kicks-ass.net>
Date:	Tue, 1 Oct 2013 10:09:21 +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 09:28:15AM +0200, Ingo Molnar wrote:
> That I mostly agree with, except that without a serious usecase do we have 
> a guarantee that bugs in fancies queueing in rwsems gets ironed out?

Methinks mmap_sem is still a big enough lock to work out a locking
primitive :-)

In fact, try something like this from userspace:

n-threads:

  pthread_mutex_lock(&mutex);
  foo = mmap();
  pthread_mutex_lock(&mutex);

  /* work */

  pthread_mutex_unlock(&mutex);
  munma(foo);
  pthread_mutex_unlock(&mutex);

vs

n-threads:

  foo = mmap();
  /* work */
  munmap(foo);


I've had reports that the former was significantly faster than the
latter.
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