Re: [PATCH] cpuacct: add a branch prediction

["KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>]

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