linux-kernel - Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid at once

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Date:	Fri, 7 Aug 2009 18:41:21 -0700
From:	Benjamin Blum <bblum@...gle.com>
To:	Paul Menage <menage@...gle.com>
Cc:	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid 
	at once

On Thu, Aug 6, 2009 at 4:40 AM, Paul Menage<menage@...gle.com> wrote:
> On Thu, Aug 6, 2009 at 4:24 AM, Louis Rilling<Louis.Rilling@...labs.com> wrote:
>>
>> You meant signal_struct, right? sighand_struct can be shared by several
>> thread groups, while signal_struct can't.
>>
>
> No, I meant sighand_struct. I realise that it *can* be shared between
> processes, but I didn't think that NPTL actually did so. (Are there
> common cases of this happening?) And in cases where it was shared, it
> wouldn't affect correctness, but simply create the potential for a
> little more contention.
>
> I agree that signal_struct might in principle be a better place for
> it, but the first cacheline of signal_struct appears to be occupied
> with performance-sensitive things (a couple of counters and a queue
> used in do_wait()) already, whereas the first cacheline of
> sighand_struct only appears to be used incremented/decremented during
> fork/exit, and when delivering a bunch of mostly-fatal signals.
>
> But having said that, if having it in signal_struct isn't considered a
> potential performance hit, it would be fine there too.
>
> Paul
>

I'm presently rewriting the locking scheme here to have the rwsem live
in sighand_struct, and writing two new functions lock_threadgroup_fork
and unlock_threadgroup_fork (for use in cgroup_attach_proc) which will
live in cgroup.c but are generic enough that they could be used by
anybody who wants to do threadgroup-wide fork-sensitive changes. I'm
also having the fork_lock under an #ifdef CONFIG_CGROUPS, which could
be expanded to, say, CONFIG_FORK_LOCK_THREADGROUP (which CGROUPS would
depend upon) if somebody else ever wanted to use this lock as well.
For genericness, I'll have the down_read and up_read on the lock in
do_fork() directly (instead of cgroup_fork and cgroup_post_fork as it
is in this version of the patch).

If there are no more comments/discussion on the locking scheme, I'll
resubmit the patch series with these changes approximately Monday.
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