linux-kernel - Re: [PATCH] cpuacct: add a branch prediction

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Message-ID: <145d0010d65060bb089d5a87e06cbd0d.squirrel@webmail-b.css.fujitsu.com>
Date:	Thu, 26 Feb 2009 21:06:24 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Bharata B Rao" <bharata.rao@...il.com>,
	"Li Zefan" <lizf@...fujitsu.com>, "Ingo Molnar" <mingo@...e.hu>,
	"Paul Menage" <menage@...gle.com>,
	"Balbir Singh" <balbir@...ux.vnet.ibm.com>,
	"LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpuacct: add a branch prediction

Peter Zijlstra wrote：
> On Thu, 2009-02-26 at 20:17 +0900, KAMEZAWA Hiroyuki wrote:
>> Peter Zijlstra wrote：
>> > 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?
>
>From view of user-support saler, if terrible broken value is reported,
it will be user-incident and annoy me(us) ;)

I'd like to get rid of rq->lock, too..Hmm.. some routine like
atomic64_read() can help this ? (But I don't want to use atomic_t here..)

> But aside from that, the cpu controller itself is also summing directly
> up the hierarchy, so cpuacct doing the same doesn't seem odd.
>
I'll post some idea if I can think of something reasonable.
But I tend to hesitate to modify sched.c ;)

Thanks,
-Kame

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