Re: [PATCH -rt] timer: upper bound on loops of __run_timers processing

Rik van Riel
2014-12-30T21:05:52Z

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On 12/30/2014 02:52 PM, Marcelo Tosatti wrote:
>
> Commit "timers: do not raise softirq unconditionally", allows for
> timer wheel processing (__run_timers) to be delayed for long
> periods of time.
>
> The effect is that
>
> loops = jiffies - base->timer_jiffies
>
> Can grow to very large values resulting in __run_timers taking
> hundreds of milliseconds to execute.
>
> Fix by creating an upper bound on the number of loops to be
> processed. This allows a nohz=off kernel to achieve desired
> latencies.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> diff --git a/kernel/timer.c b/kernel/timer.c index f59e18c..c128416
> 100644 --- a/kernel/timer.c +++ b/kernel/timer.c @@ -1488,6
> +1488,12 @@ void run_local_timers(void) } #endif
>
> + if (time_after_eq(jiffies, base->timer_jiffies)) { + unsigned
> long jiffies_delta = jiffies - base->timer_jiffies; + if
> (jiffies_delta > TVR_SIZE) + raise_softirq(TIMER_SOFTIRQ); + }

It certainly looks like cascading the timers over and over and
over again could be a lot of work.

Acked-by: Rik van Riel <riel@redhat.com>

- --
All rights reversed
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