linux-kernel - Re: [PATCH -rt] timer: upper bound on loops of __run

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Date:	Tue, 30 Dec 2014 16:05:36 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Clark Williams <clark.williams@...il.com>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -rt] timer: upper bound on loops of __run_timers processing

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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@...hat.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@...hat.com>

- -- 
All rights reversed
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