linux-kernel - Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

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Date:	Thu, 11 Nov 2010 11:37:28 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mike Galbraith <efault@....de>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

On Thu, Nov 11, 2010 at 11:08 AM, Mike Galbraith <efault@....de> wrote:
>
>>  - the reason I suggested the signal struct was really that I thought
>> it would avoid extra (unnecessary) cost in thread creation/teardown.
>>
>>    Maybe I should have made that clear, but this seems to
>> unnecessarily do the whole atomic_inc/dec for each thread. That seems
>> a bit sad.
>>
>>    That said, if not having to dereference ->signal simplifies the
>> scheduler interaction, I guess the extra atomic ref at thread
>> creation/deletion is fine. So I don't think this is wrong, it's just
>> something I wanted to bring up.
>
> Ah, ok.  Anything that cuts overhead is worth doing.

Well, it cuts both ways. Maybe your approach is simpler and avoids
overhead at scheduling time. And "tsk->signal" may not be reliable due
to races with exit etc, so it may well be that going through the
signal struct could end up being a source of nasty races. I didn't
look whether the scheduler already derefenced ->signal for some other
reason, for example.

So your patch may well have done the exact right thing.

                              Linus
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