linux-kernel - Re: [RFC PATCH RT] rwsem: The return of multi-reader PI rwsems

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Message-ID: <1397188356.5139.33.camel@marge.simpson.net>
Date:	Fri, 11 Apr 2014 05:52:36 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH RT] rwsem: The return of multi-reader PI rwsems

On Thu, 2014-04-10 at 23:25 -0400, Steven Rostedt wrote: 
> On Fri, 11 Apr 2014 04:50:26 +0200
> Mike Galbraith <umgwanakikbuti@...il.com> wrote:
> 
> > Oh, there was another useful bit. 
> > 
> > 3.12.17-rt25       403991     384216     377786
> >                    405466     394011     392850
> > 
> > 3.12.17-nopreempt  109049     186133     118766  !?!.. ew, TTWU_QUEUE
> >                    350385     318786     367336  !TTWU_QUEUE
> >                    326009     356939     378215  !TTWU_QUEUE
> > 
> > IPI == low-pass filter.
> 
> I'm sorry, but I don't have the foggiest clue to what the above means.

It means..

# Overhead                                                                                                                                                                                   Symbol
# ........  .......................................................................................................................................................................................
#
    17.33%  [k] native_write_msr_safe
            |
            |--88.45%-- __x2apic_send_IPI_mask
            |          |
            |          |--97.89%-- try_to_wake_up
            |          |          |
            |          |          |--99.91%-- wake_futex
            |          |          |          |
            |          |          |          |--99.77%-- futex_wake_op
            |          |          |          |          do_futex
            |          |          |          |          sys_futex
            |          |          |          |          system_call_fastpath

..wakeup frequency is restricted by the IPI.

	-Mike

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