linux-kernel - Re: [patch] sched: prevent bound kthreads from changing cpus

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Message-Id: <1213080240.31518.5.camel@twins>
Date:	Tue, 10 Jun 2008 08:44:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Max Krasnyanskiy <maxk@...lcomm.com>
Cc:	David Rientjes <rientjes@...gle.com>, Paul Jackson <pj@....com>,
	mingo@...e.hu, menage@...gle.com, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...sign.ru>
Subject: Re: [patch] sched: prevent bound kthreads from changing
	cpus_allowed

On Mon, 2008-06-09 at 13:59 -0700, Max Krasnyanskiy wrote:
> David Rientjes wrote:
> >>  2) Sometimes calls to kthread_bind are binding to any online cpu, such as in:
> >>
> >> 	drivers/infiniband/hw/ehca/ehca_irq.c:	kthread_bind(cct->task, any_online_cpu(cpu_online_map));
> >>
> >>     In such cases, the PF_THREAD_BOUND seems inappropriate.  The caller of
> >>     kthread_bind() really doesn't seem to care where that thread is bound;
> >>     they just want it on a CPU that is still online.
> >>
> > 
> > This particular case is simply moving the thread to any online cpu so that 
> > it survives long enough for the subsequent kthread_stop() in 
> > destroy_comp_task().  So I don't see a problem with this instance.
> > 
> > A caller to kthread_bind() can always remove PF_THREAD_BOUND itself upon 
> > return, but I haven't found any cases in the tree where that is currently 
> > necessary.  And doing that would defeat the semantics of kthread_bind() 
> > where these threads are supposed to be bound to a specific cpu and not 
> > allowed to run on others.
> 
> Actually I have another use case here. Above example in particular may be ok 
> but it does demonstrate the issue nicely. Which is that in some cases kthreads 
> are bound to a CPU but do not have a strict "must run here" requirement and 
> could be moved if needed.
> For example I need an ability to move workqueue threads. Workqueue threads do 
> kthread_bind().

Per cpu workqueues should stay on their cpu.

What you're really looking for is a more fine grained alternative to
flush_workqueue().

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