linux-kernel - Re: [PATCH v12 07/13] task_isolation: add debug boot flag

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Date:	Thu, 19 May 2016 14:05:27 -0400
From:	Chris Metcalf <cmetcalf@...lanox.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Andy Lutomirski <luto@...capital.net>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v12 07/13] task_isolation: add debug boot flag

On 5/19/2016 1:54 PM, Peter Zijlstra wrote:
> So the 'simple' thing is:
>
> 	struct rq *rq = cpu_rq(cpu);
> 	struct task_struct *task;
>
> 	raw_spin_lock_irq(&rq->lock);
> 	task = rq->curr;
> 	get_task_struct(task);
> 	raw_spin_unlock_irq(&rq->lock);
>
> Because by holding rq->lock, the remote CPU cannot schedule and the
> current task_must_  still be valid.

I will plan to use that idiom in the next patch series.  Thanks!

> And note; the above can result in a task which already has PF_EXITING
> set.

I think that should be benign though; we may generate an unnecessary
warning, but somebody was doing something that could have resulted in
interrupting an isolated task anyway, so warning about it is reasonable.  And
presumably PF_EXITING just means we don't wake any threads and leave
the signal queued, but that gets flushed when the task finally exits.

> The complex thing is described in the linked thread and will likely make
> your head hurt.

I read the linked thread and was entertained. :-)  I suspect locking the
runqueue may be the more robust solution anyway, and since this is
presumably not a hot path, it seems easier to reason about this way.

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com