Re: [PATCH] cpuacct: add a branch prediction

From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Bharata B Rao <bharata.rao@gmail.com>,
	Li Zefan <lizf@cn.fujitsu.com>, Ingo Molnar <mingo@elte.hu>,
	Paul Menage <menage@google.com>,
	Balbir Singh <balbir@linux.vnet.ibm.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] cpuacct: add a branch prediction
Date: Thu, 26 Feb 2009 12:28:02 +0100	[thread overview]
Message-ID: <1235647682.4948.15.camel@laptop> (raw)
In-Reply-To: <934198669efa83e838a52284e2c4f8b5.squirrel@webmail-b.css.fujitsu.com>

On Thu, 2009-02-26 at 20:17 +0900, KAMEZAWA Hiroyuki wrote:
> Peter Zijlstra さんは書きました：
> > On Thu, 2009-02-26 at 19:28 +0900, KAMEZAWA Hiroyuki wrote:
> >
> >> Taking hierarchy mutex while reading will make read-side stable.
> >
> > We're talking about scheduling here, taking a mutex to stop scheduling
> > won't work, nor will it be acceptible to use anything that will.
> >
> No mutex is necessary, anyway.
> hierarchy-walker function completely works well under rcu read lock,
> if small jitter is allowed.

Right, should be doable -- and looking at the code, we have this
horrible 32 bit exception in there that locks the rq in order to read
the 64bit value.

Would be grand to get rid of that,. how bad would it be for userspace to
get the occasionally fubarred value?

But aside from that, the cpu controller itself is also summing directly
up the hierarchy, so cpuacct doing the same doesn't seem odd.