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

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Message-ID: <20110617094333.GB19235@elte.hu>
Date:	Fri, 17 Jun 2011 11:43:33 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> 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.

Yeah, some sort of worst-case-tick mechanism would guarantee that we 
wont remain without RCU GC.

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

It also moves whatever priority logic is needed closer to the 
scheduler that has to touch those data structures anyway.

RCU, at least partially, is a scheduler driven garbage collector even 
today: beyond context switch quiescent states the main practical role 
of the per CPU timer tick itself is scheduling. So having it close to 
when we do context-switches anyway looks pretty natural - worth 
trying.

It might not work out in practice, but at first sight it would 
simplify a few things i think.

Thanks,

	Ingo
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