linux-kernel - Re: [PATCH] move put_task_struct() reaping into a thread [Re: 2.6.18-rt1]

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Message-ID: <20060927161847.GB1291@us.ibm.com>
Date:	Wed, 27 Sep 2006 09:18:47 -0700
From:	"Paul E. McKenney" <paulmck@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Bill Huey <billh@...ppy.monkey.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH] move put_task_struct() reaping into a thread [Re: 2.6.18-rt1]

On Wed, Sep 27, 2006 at 04:06:59PM +0200, Ingo Molnar wrote:
> 
> * Eric W. Biederman <ebiederm@...ssion.com> wrote:
> 
> > I'm still wondering if we can move put_task_struct a little lower in 
> > the logic in the places where it is called, so it isn't called under a 
> > lock, or with preemption disabled.  The only downside I see is that it 
> > might convolute the logic into unreadability.
> 
> well it's all a function of the task reaping logic: right now we in 
> essence complete the reaping from the scheduler, via prev_state == 
> TASK_DEAD. We cannot do it sooner because the task is still in use. I 
> had one other implementation upstream some time ago, which was a 
> single-slot cache for reaped tasks - but that uglified other codepaths 
> because _something_ has to notice that the task has been unscheduled.

I believe that we are way too far into the task-teardown process for
something like synchronize_rcu() to be feasible (not enough of the
task left to be able to sleep!), but thought I should bring up the
possibility on the off-chance that it caused someone to come up with a
better approach.

Another possible approach would be workqueues.  The disadvantages here are
(1) higher overhead (2) workqueues can be delayed for a -long- time in a
realtime environment, which increases vulnerability to memory exhaustion.

Again, hoping this provokes some better ideas...

							Thanx, Paul
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