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Date: Thu, 26 Feb 2009 21:06:24 +0900 (JST)
Subject: Re: [PATCH] cpuacct: add a branch prediction
From: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>
To: "Peter Zijlstra" <a.p.zijlstra@chello.nl>
Cc: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>,
       "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>
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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