Re: [PATCH v2] nohz: Fix collision between tick and other hrtimers

[Peter Zijlstra]

On Wed, Jan 04, 2017 at 03:12:04PM +0100, Frederic Weisbecker wrote:
> When the tick is stopped and an interrupt occurs afterward, we check on
> that interrupt exit if the next tick needs to be rescheduled. If it
> doesn't need any update, we don't want to do anything.
> 
> In order to check if the tick needs an update, we compare it against the
> clockevent device deadline. Now that's a problem because the clockevent
> device is at a lower level than the tick itself if it is implemented
> on top of hrtimer.
> 
> Every hrtimer share this clockevent device. So comparing the next tick
> deadline against the clockevent device deadline is wrong because the
> device may be programmed for another hrtimer whose deadline collides
> with the tick. As a result we may end up not reprogramming the tick
> accidentally.
> 
> In a worst case scenario under full dynticks mode, the tick stops firing
> as it is supposed to every 1hz, leaving /proc/stat stalled:
> 
>       Task in a full dynticks CPU
>       ----------------------------
> 
>       * hrtimer A is queued 2 seconds ahead
>       * the tick is stopped, scheduled 1 second ahead
>       * tick fires 1 second later
>       * on tick exit, nohz schedules the tick 1 second ahead but sees
>         the clockevent device is already programmed to that deadline,
>         fooled by hrtimer A, the tick isn't rescheduled.
>       * hrtimer A is cancelled before its deadline
>       * tick never fires again until an interrupt happens...
> 
> In order to fix this, store the next tick deadline to the tick_sched
> local structure and reuse that value later to check whether we need to
> reprogram the clock after an interrupt.
> 
> On the other hand, ts->sleep_length still wants to know about the next
> clock event and not just the tick, so we want to improve the related
> comment to avoid confusion.
> 
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Reviewed-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> Reported-by: James Hartsock <hartsjc@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>