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

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Date:	Thu, 6 Aug 2009 08:19:22 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Paul Menage <menage@...gle.com>, Benjamin Blum <bblum@...gle.com>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Ingo Molnar <mingo@...e.hu>, oleg <oleg@...hat.com>
Subject: Re: [PATCH 6/6] Makes procs file writable to move all threads by
	tgid at once

On Thu, Aug 06, 2009 at 01:39:21PM +0200, Peter Zijlstra wrote:
> 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.

If the critical section is small, is an rwsem really better than a
straight mutex?

							Thanx, Paul
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