linux-kernel - Re: [PATCH v3 1/1] ftrace, workqueuetrace: Make workqueuetracepoints use TRACE

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Date:	Tue, 21 Apr 2009 21:37:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zhaolei <zhaolei@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tom Zanussi <tzanussi@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] ftrace, workqueuetrace: Make
	workqueuetracepoints use TRACE_EVENT macro

On 04/21, Frederic Weisbecker wrote:
>
> On Tue, Apr 21, 2009 at 05:28:43PM +0200, Oleg Nesterov wrote:
> > More precisely, at this stage tsk->cpu_allowed is not bound, it has
> > "all CPUS", == cpu_possible_mask. This means that cpumask_first()
> > returns the "random" value. The fix is simple, I think you should
> > just add "int cpu" argument to cleanup_workqueue_thread().
>
> Ok.
> But where is it done? I mean, by looking at workqueue_cpu_callback()
> in workqueue.c, I only see this part related to _cpu_down():
>
> case CPU_POST_DEAD:
> 	cleanup_workqueue_thread(cwq);
> 	break;

There are two case. Please note that create_workqueue_thread() does not
bind the thread, we delay kthread_bind() untill start_workqueue_thread().
So, if we have CPU_UP_CANCELED (after CPU_UP_PREPARE) we call
cleanup_workqueue_thread() but the thread still has ->cpu_allowed ==
cpu_all_mask.

Another case is a normal cpu_down(). When CPU_POST_DEAD happens, this
CPU is really dead. All tasks were alredy moved from the dead cpu.
And since wq_thread was affine to that CPU, it gets the "full" cpu mask.

OK, I don't think it is possible to parse the text above ;) Please
look at  /* No more Mr. Nice Guy. */ in move_task_off_dead_cpu().

Oleg.

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