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

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Message-ID: <2da7620e-dc23-6a36-e5ad-757441560127@mellanox.com>
Date:	Wed, 18 May 2016 12:36:46 -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

(Oops, missed one that I should have forced to text/plain. Resending.)

On 5/18/2016 9:56 AM, Peter Zijlstra wrote:
> On Tue, Apr 05, 2016 at 01:38:36PM -0400, Chris Metcalf wrote:
>> +#ifdef CONFIG_TASK_ISOLATION
>> +void task_isolation_debug(int cpu)
>> +{
>> +	struct task_struct *p;
>> +
>> +	if (!task_isolation_possible(cpu))
>> +		return;
>> +
>> +	rcu_read_lock();
>> +	p = cpu_curr(cpu);
>> +	get_task_struct(p);
>> +	rcu_read_unlock();
>> +	task_isolation_debug_task(cpu, p);
>> +	put_task_struct(p);
> This is still broken...

I don't know how or why, though. :-)  Can you give me a better idiom?
This looks to my eye just like how it's done for something like
sched_setaffinity() by one task on another task, and I would have
assumed the risks there of the other task evaporating part way
through would be the same as the risks here.

> Also, I really don't like how you sprinkle a call all over the core
> kernel. At the very least make an inline fast path for this function to
> avoid the call whenever possible.

I can boost the "task_isolation_possible()" test up into a static inline,
and only call in the case where we have a target cpu that is actually
in the "task_isolation=" boot argument set.

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