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

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Message-ID: <20110616230247.GB28557@elte.hu>
Date:	Fri, 17 Jun 2011 01:02:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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>,
	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


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Thu, Jun 16, 2011 at 10:25:50PM +0200, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > > 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.
> > > 
> > > The other thing that the IRQ-triggered softirqs do is to get the 
> > > callbacks invoked in cases where a CPU-bound user thread is never 
> > > context switching.
> > 
> > Yeah - but this workload didnt have that.
> > 
> > > Of course, one alternative might be to set_need_resched() to force 
> > > entry into the scheduler as needed.
> > 
> > No need for that: we can just do the callback not in softirq but in 
> > regular syscall context in that case, in the return-to-userspace 
> > notifier. (see TIF_USER_RETURN_NOTIFY and the USER_RETURN_NOTIFIER 
> > facility)
> > 
> > Abusing a facility like setting need_resched artificially will 
> > generally cause trouble.
> 
> If the task enqueued callbacks in the kernel, thus started a new 
> grace period, it might return to userspace before every CPUs have 
> completed that grace period, and you need that full completion to 
> happen before invoking the callbacks.
> 
> I think you need to keep the tick in such case because you can't 
> count on the other CPUs to handle that completion as they may be 
> all idle.
> 
> So when you resume to userspace and you started a GP, either you 
> find another CPU to handle the GP completion and callbacks 
> executions, or you keep the tick until you are done.

We'll have a scheduler tick in any case, which will act as a 
worst-case RCU tick.

My main point is that we need to check whether this solution improves 
performance over the current softirq code. I think there's a real 
chance that it improves things like VFS workloads, because it 
provides (much!) lower grace period latencies hence provides 
fundamentally better cache locality.

If a workload pays the cost of frequent scheduling then it might as 
well use a beneficial side-effect of that scheduling: high-freq grace 
periods ...

If it improves performance we can figure out all the loose ends. If 
it doesnt then the loose ends are not worth worrying about.

Thanks,

	Ingo
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