linux-kernel - Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop

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Date:	Thu, 18 Dec 2008 23:00:58 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Thomas Gleixner" <tglx@...utronix.de>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop

2008/12/18 Ingo Molnar <mingo@...e.hu>:
>
> * Frédéric Weisbecker <fweisbec@...il.com> wrote:
>
>> > Which means that it's not some hrtimer problem, but simply the traced
>> > timer tick takes more than 1 millisecond to execute under this
>> > virtualization.
>> >
>> >        Ingo
>> >
>>
>> Oh ok I see. Sorry I'm a bit slow today... So the solution would be to
>> adapt dynamically the timeout between hrtimer irq. But I don't know that
>> much hrtimer to implement such a feature...
>
> just if you want this lockup to go away. I think you did the hardest bit
> already: to detect the situation reliably, without false positives. Now
> the 'action' needs to change: instead of 'turning off ftrace' (which is
> brutal - ftrace was just the last drop of water that pushed the system
> over the edge), we can instead do 'double the minimum clockevent delta
> threshold'.
>
> there's already such code in kernel/time/tick-oneshot.c:
>
>                /*
>                 * We tried 2 times to program the device with the given
>                 * min_delta_ns. If that's not working then we double it
>                 * and emit a warning.
>                 */
>                if (++i > 2) {
>                        /* Increase the min. delta and try again */
>                        if (!dev->min_delta_ns)
>                                dev->min_delta_ns = 5000;
>                        else
>                                dev->min_delta_ns += dev->min_delta_ns >> 1;
>
> what would be needed is to simply double ->min_delta_ns on every such
> situation you detect? Once you do that, it takes effect on the next tick
> automatically.
>
> Or something like that. In theory. :-)
>
>        Ingo
>

Ok. I understand. Thanks for these pointers, that seems more easy than
I thought.
I will send a patch in a few days....
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