linux-kernel - Re: sched: horrible way to detect whether a task has been preempted

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Message-ID: <20160411083846.GL5218@pathway.suse.cz>
Date:	Mon, 11 Apr 2016 10:38:46 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Jiri Kosina <jikos@...nel.org>, Jessica Yu <jeyu@...hat.com>,
	Miroslav Benes <mbenes@...e.cz>, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org, Vojtech Pavlik <vojtech@...e.com>
Subject: Re: sched: horrible way to detect whether a task has been preempted

On Fri 2016-04-08 09:31:31, Josh Poimboeuf wrote:
> On Fri, Apr 08, 2016 at 10:03:04AM +0200, Petr Mladek wrote:
> > The big advantage about checking the stack is that it does not add
> > any overhead to the scheduler code, does not eat any TIF flag or
> > memory. The overhead is only when we are migrating a task and it is
> > charged to a separate process.
> 
> My biggest concern about checking the stack for preempt_schedule_irq()
> is that it's kind of brittle:
> 
> - What if the preemption code changes such that it's no longer a
>   reliable indicator?  For example, what if preempt_schedule_irq() is
>   only called in some places, and a new __preempt_schedule_irq() is
>   called elsewhere?
> 
> - Or due to some obscure gcc optimization like partial inlining or
>   sibling tail calls, preempt_schedule_irq() doesn't show up on the
>   stack?  
> 
> - Or the code could silently break if there were another static
>   preempt_schedule_irq symbol somewhere (though we could prevent this by
>   searching all symbols to ensure there are no duplicates).
> 
> These scenarios are unlikely, but they could conceivably happen...

You are right.

> Anyway, we really wouldn't have to eat a TIF flag.  We could instead add
> something to task_struct.  I can at least propose it.  If anybody
> doesn't like it, maybe they'll suggest something else, or maybe then we
> can go with checking the stack.

Yup, we should give the extra task struct stuff a try. Another
advantage is that it would be easier to port for another
architectures.

Best Regards,
Petr