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

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Date:	Tue, 9 Jan 2007 15:39:26 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	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>
Subject: Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

On Tue, Jan 09, 2007 at 01:51:52AM -0800, Andrew Morton wrote:
> > This thread makes absolutely -no- calls to try_to_freeze() in its lifetime.
> 
> Looks like a bug to me.  powerpc does appear to try to support the freezer.
> 
> > 1. Does this mean that the thread can't be frozen? (lets say that the
> >    thread's PF_NOFREEZE is not set)
> 
> yup.  I'd expect the freeze_processes() call would fail if this thread is
> running.

ok.

> 
> >    AFAICS it can still be frozen by sending it a signal and have the signal
> >    delivery code call try_to_freeze() ..
> 
> kernel threads don't take signals in the same manner as userspace.  A
> kernel thread needs to explicitly poll, via
> 
> 	if (signal_pending(current))
> 		do_something()

Thanks for the education! I feel much better about the use of process
freezer now ..

> > 2. If the thread can be frozen at any arbitrary point of its execution, then I
> >    dont see what prevents cpu_online_map from changing under the feet of rtasd
> >    thread,
> 
> It cannot.

Excellent ..

I just hope the latency of freeze_processes() is tolerable ..

-- 
Regards,
vatsa
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