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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 11 Nov 2010 11:37:28 -0800
Message-ID: <AANLkTi=hKhsU0Nsenq-jezDVNYQ4fCXFBbUy5c7EFk=U@mail.gmail.com>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups
To: Mike Galbraith <efault@gmx.de>
Cc: Oleg Nesterov <oleg@redhat.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
        Markus Trippelsdorf <markus@trippelsdorf.de>
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On Thu, Nov 11, 2010 at 11:08 AM, Mike Galbraith <efault@gmx.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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