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

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Message-ID: <20110617004536.GP2582@linux.vnet.ibm.com>
Date:	Thu, 16 Jun 2011 17:45:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	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>,
	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>, 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

On Fri, Jun 17, 2011 at 12:58:03AM +0200, Ingo Molnar wrote:
> 
> * Andi Kleen <ak@...ux.intel.com> wrote:
> 
> > > There's a crazy solution for that: the idle thread could process 
> > > RCU callbacks carefully, as if it was running user-space code.
> > 
> > In Ben's kernel NFS server case the system may not be idle.
> 
> An always-100%-busy NFS server is very unlikely, but even in the 
> hypothetical case a kernel NFS server is really performing system 
> calls from a kernel thread in essence. If it doesn't do it explicitly 
> then its main loop can easily include a "check RCU callbacks" call.

As long as they make sure to call it in a clean environment: no locks
held and so on.  But I am a bit worried about the possibility of someone
forgetting to put one of these where it is needed -- it would work just
fine for most workloads, but could fail only for rare workloads.

That said, invoking RCU core/callback processing from the scheduler
context certainly sounds like an interesting way to speed up grace
periods.

							Thanx, Paul
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