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

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Date:	Thu, 06 Aug 2009 13:39:21 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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,
	Ingo Molnar <mingo@...e.hu>,
	paulmck <paulmck@...ux.vnet.ibm.com>, oleg <oleg@...hat.com>
Subject: Re: [PATCH 6/6] Makes procs file writable to move all threads by
 tgid  at once

On Thu, 2009-08-06 at 04:24 -0700, Paul Menage wrote:
> On Thu, Aug 6, 2009 at 4:02 AM, Peter Zijlstra<a.p.zijlstra@...llo.nl> wrote:
> >
> > Taking that many locks in general, some apps (JVM based usually) tend to
> > be thread heavy and can easily have hundreds of them, even on relatively
> 
> Oh, I'm well aware that apps can be heavily multi-threaded - we have
> much worse cases at Google.
> 
> >
> > Now that's not real nice is it ;-)
> 
> Not particularly - but who exactly is going to be moving processes
> with thousands of threads between cgroups on a lockdep-enabled debug
> kernel?

All it takes are: 8 or 48 (or soon 2048) depending on your particular
annotation. I might and then I'd have to come and kick you ;-)

Really, lockdep not being able to deal with something is a strong
indication that you're doing something wonky.

Stronger, you can even do wonky things which lockdep thinks are
absolutely fine.

And doing wonky things should be avoided :-)

Luckily we seem to have found a sensible solution.

> What benefits does the additional complexity of SRCU give, over the
> simple solution of putting an rwsem in the same cache line as
> sighand->count ?

I said:

> Then again, clone() might already serialize on the process as a whole
> (not sure though, Oleg/Ingo?), in which case you can indeed take a
> process wide lock.

Which looking up sighand->count seems to be the case:

static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
{
        struct sighand_struct *sig;

        if (clone_flags & CLONE_SIGHAND) {
                atomic_inc(&current->sighand->count);
                return 0;
        }


So yes, putting a rwsem in there sounds fine, you're already bouncing
it.

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