linux-kernel - Re: [PATCH v7 05/11] task_isolation: add debug boot flag

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Date:	Mon, 28 Sep 2015 17:55:39 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.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>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 05/11] task_isolation: add debug boot flag

On 09/28/2015 04:59 PM, Andy Lutomirski wrote:
> On Mon, Sep 28, 2015 at 11:17 AM, Chris Metcalf <cmetcalf@...hip.com> wrote:
>> The new "task_isolation_debug" flag simplifies debugging
>> of TASK_ISOLATION kernels when processes are running in
>> PR_TASK_ISOLATION_ENABLE mode.  Such processes should get no
>> interrupts from the kernel, and if they do, when this boot flag is
>> specified a kernel stack dump on the console is generated.
>>
>> It's possible to use ftrace to simply detect whether a task_isolation
>> core has unexpectedly entered the kernel.  But what this boot flag
>> does is allow the kernel to provide better diagnostics, e.g. by
>> reporting in the IPI-generating code what remote core and context
>> is preparing to deliver an interrupt to a task_isolation core.
>>
>> It may be worth considering other ways to generate useful debugging
>> output rather than console spew, but for now that is simple and direct.
> This may be addressed elsewhere, but is there anything that alerts the
> task or the admin if it's PR_TASK_ISOLATION_ENABLE and *not* on a
> nohz_full core?

No, and I've thought about it without coming up with a great
solution.  We could certainly fail the initial prctl() if the caller
was not on a nohz_full core.  But this seems a little asymmetric
since the task could be on such a core at prctl() time, and then
do a sched_setaffinity() later to a non-nohz-full core.  Would
we want to fail that call?  Seems heavy-handed.  Or we could
then clear the task-isolation state and emit a console message.

I suppose we could notice that we were on a nohz-full
enabled system and the task isolation flags were set on return
to userspace, but we were not on a nohz-full core, and emit
a console message and clear the task-isolation state at that point.
But that also seems a little questionable; maybe the user for
some reason was doing some odd migratory thing with their
tasks or threads and was going to end up migrating them to
a final destination where the prctl() would apply.

Any suggestions for a better approach?  Is it worth doing the
minimal printk-warning approach in the previous paragraph?
My instinct is to say that we just leave it as-is, I think.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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