linux-kernel - Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

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Date:	Mon, 15 Jan 2007 21:48:10 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...l.org>, David Howells <dhowells@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...l.org>,
	linux-kernel@...r.kernel.org, Gautham shenoy <ego@...ibm.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Subject: Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

On Mon, Jan 15, 2007 at 03:54:01PM +0300, Oleg Nesterov wrote:
> > - singlethread_cpu needs to be hotplug safe (broken currently)
> 
> Why? Could you explain?

What if 'singlethread_cpu' dies?

> > - Any reason why cpu_populated_map is not modified on CPU_DEAD?
> 
> Because CPU_DEAD/CPU_UP_CANCELED doesn't wait for cwq->thread to exit.
> cpu_populated_map never shrinks, it only grows on CPU_UP_PREPARE.
> 
> We can change this, but it needs some more code, and I am not sure
> we need it. Note that a "false" bit in cpu_populated_map only means
> that flush_work/flush_workqueue/destroy_workqueu will do lock/unlock
> of cwq->lock, nothing more.

What abt __create_workqueue/schedule_on_each_cpu?

> > - I feel more comfortable if workqueue_cpu_callback were to take
> >   workqueue_mutex in LOCK_ACQ and release it in LOCK_RELEASE
> >   notifications.
> 
> The whole purpose of this change to avoid this!

I guess it depends on how __create_workqueue/schedule_on_each_cpu is
modified (whether we take/release lock upon LOCK_ACQ/LOCK_RELEASE)

> > Finally, I wonder if these changes will be unnecessary if we move to
> > process freezer based hotplug locking ...
> 
> This change ir not strictly necessary but imho make the code better and
> shrinks .text by 379 bytes.
> 
> But I believe that freezer will change nothing for workqueue. We still
> need take_over_work(), and hacks like migrate_sequence. And no, CPU_DEAD
> can't just thaw cwq->thread which was bound to the dead CPU to complete
> kthread_stop(), we should thaw all processes.

What abt stopping that thread in CPU_DOWN_PREPARE (before freezing
processes)? I understand that it may add to the latency, but compared to
the overall latency of process freezer, I suspect it may not be much.

-- 
Regards,
vatsa
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