linux-kernel - Re: [PATCH 2/3] clockevents: Restart clockevent device before using it again

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Date:	Thu, 2 Apr 2015 19:20:50 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kevin Hilman <khilman@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 2/3] clockevents: Restart clockevent device before using
 it again

On 2 April 2015 at 19:04, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Mar 27, 2015 at 10:44:28PM +0530, Viresh Kumar wrote:

>> +++ b/kernel/time/hrtimer.c
>> @@ -566,6 +566,7 @@ static int hrtimer_reprogram(struct hrtimer *timer,
>>  {

>> +     /* Switchback to ONESHOT state */
>> +     if (unlikely(dev->state == CLOCK_EVT_STATE_ONESHOT_STOPPED))
>> +             clockevents_set_state(dev, CLOCK_EVT_STATE_ONESHOT);
>> +
>>       /*
>>        * Clockevents returns -ETIME, when the event was in the past.
>>        */
>
> Should we not do this in tick_program_event() instead? Note that there
> are a few more places that call that, the two in the hrtimer_interrupt()
> should be safe because if we're handling the interrupt its cannot be
> stopped anyhow.
>
> hrtimer_force_reprogram() seems to need the annotation regardless.

Do you mean that we need the same modification here as well? In order
to save myself against any bugs, I have added following to
clockevents_program_event():

+       /* We must be in ONESHOT state here */
+       WARN_ONCE(dev->state != CLOCK_EVT_STATE_ONESHOT, "Current state: %d\n",
+                 dev->state);

And I never faced this WARN, or any such reports from Fengguang. So
probably after all the hrtimers are gone, we will always call
hrtimer_reprogram().

> Furthermore, by putting it in tick_program_event() you also don't need
> to fixup tick_nohz_restart().
>
> Or am I completely missing something?

So yes, if we would have done that in tick_program_event(), it would have
been a single place for doing this change..

But, when Thomas ranted [1] at me on this earlier, he said:

"
No, we are not doing a state change behind the scene and a magic
restore.

2B) Implement the ONESHOT_STOPPED logic and make sure all of the core
    code is aware of it.
"

And so I did it explicitly, wherever it is required.

--
viresh

[1] https://lkml.org/lkml/2014/5/9/508
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