Date: Thu, 3 Oct 2013 09:32:12 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>, Alex Shi <alex.shi@linaro.org>,
        Andi Kleen <andi@firstfloor.org>,
        Michel Lespinasse <walken@google.com>,
        Davidlohr Bueso <davidlohr.bueso@hp.com>,
        Matthew R Wilcox <matthew.r.wilcox@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Rik van Riel <riel@redhat.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        "Paul E.McKenney" <paulmck@linux.vnet.ibm.com>,
        Jason Low <jason.low2@hp.com>, Waiman Long <Waiman.Long@hp.com>,
        linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH v8 0/9] rwsem performance optimizations
Message-ID: <20131003073212.GC5775@gmail.com>
References: <cover.1380748401.git.tim.c.chen@linux.intel.com>
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* Tim Chen <tim.c.chen@linux.intel.com> wrote:

> For version 8 of the patchset, we included the patch from Waiman to 
> streamline wakeup operations and also optimize the MCS lock used in 
> rwsem and mutex.

I'd be feeling a lot easier about this patch series if you also had 
performance figures that show how mmap_sem is affected.

These:

> Tim got the following improvement for exim mail server 
> workload on 40 core system:
> 
> Alex+Tim's patchset:    	   +4.8%
> Alex+Tim+Waiman's patchset:        +5.3%

appear to be mostly related to the anon_vma->rwsem. But once that lock is 
changed to an rwlock_t, this measurement falls away.

Peter Zijlstra suggested the following testcase:

===============================>
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.
<===============================

this could be put into a standalone testcase, or you could add it as a new 
subcommand of 'perf bench', which already has some pthread code, see for 
example in tools/perf/bench/sched-messaging.c. Adding:

   perf bench mm threads

or so would be a natural thing to have.

Thanks,

	Ingo
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