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

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Message-ID: <20090806112450.GF26446@hawkmoon.kerlabs.com>
Date:	Thu, 6 Aug 2009 13:24:50 +0200
From:	Louis Rilling <Louis.Rilling@...labs.com>
To:	Paul Menage <menage@...gle.com>
Cc:	Benjamin Blum <bblum@...gle.com>,
	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 06/08/09  3:28 -0700, Paul Menage wrote:
> On Thu, Aug 6, 2009 at 2:58 AM, Louis Rilling<Louis.Rilling@...labs.com> wrote:
> >
> > mutex_lock_nested is not enough, since this would require putting each thread's
> > mutex in a different class. Again, something like mutex_lock_nest_lock() is
> > the solution, especially since Peter's recent improvement.
> >
> 
> OK, well if lockdep can't currently handle the "writer takes a lock on
> every thread" model, then maybe we should go with a simpler model
> until someone shows a performance issue with it? Ben's original
> patches had a per-task_struct lock, and a thread forking with CLONE_VM
> would down_read() its group leader's lock. Something that's even
> simpler (doesn't have to deal with thread group leader changing due to
> an execve()), and avoids the per-task_struct overhead would be to put
> the lock in sighand_struct instead (so only one per process). The
> procs file writer does a down_write(&tsk->sighand->fork_sem), and
> cgroup_fork() can do a down_read(&current->sighand->fork_sem) if
> flags&CLONE_SIGHAND.
> 
> If you put it as the second member of sighand_struct, there wouldn't
> even be any extra cacheline bouncing in the common case, since
> copy_sighand() would already have brought that line into cache in
> order to do atomic_inc(&current->sighand->count)

You meant signal_struct, right? sighand_struct can be shared by several
thread groups, while signal_struct can't.

Louis

-- 
Dr Louis Rilling			Kerlabs
Skype: louis.rilling			Batiment Germanium
Phone: (+33|0) 6 80 89 08 23		80 avenue des Buttes de Coesmes
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