linux-kernel - Re: [GIT PULL] Re: REGRESSION: Performance regressions from switching anon

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Date:	Thu, 16 Jun 2011 09:03:35 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Russell King <rmk@....linux.org.uk>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Tony Luck <tony.luck@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Namhyung Kim <namhyung@...il.com>, ak@...ux.intel.com,
	shaohua.li@...el.com, alex.shi@...el.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [GIT PULL] Re: REGRESSION: Performance regressions from
 switching anon_vma->lock to mutex

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> Ingo Molnar <mingo@...e.hu> wrote:
> >
> > I have this fix queued up currently:
> >
> >  09223371deac: rcu: Use softirq to address performance regression
> 
> I really don't think that is even close to enough.

Yeah.

> It still does all the callbacks in the threads, and according to 
> Peter, about half the rcu time in the threads remained..

You are right - things that are a few percent on a 24 core machine 
will definitely go exponentially worse on larger boxen. We'll get rid 
of the kthreads entirely.

The funny thing about this workload is that context-switches are 
really a fastpath here and we are using anonymous IRQ-triggered 
softirqs embedded in random task contexts as a workaround for that.

[ I think we'll have to revisit this issue and do it properly:
  quiescent state is mostly defined by context-switches here, so we
  could do the RCU callbacks from the task that turns a CPU
  quiescent, right in the scheduler context-switch path - perhaps
  with an option for SCHED_FIFO tasks to *not* do GC.

  That could possibly be more cache-efficient than softirq execution,
  as we'll process a still-hot pool of callbacks instead of doing
  them only once per timer tick. It will also make the RCU GC
  behavior HZ independent. ]

In any case the proxy kthread model clearly sucked, no argument about 
that.

Thanks,

	Ingo
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