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

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Message-ID: <54E3C9DF.1040607@redhat.com>
Date:	Tue, 17 Feb 2015 18:08:15 -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.

Turns out that when we make nohz=off actually work correctly
with realtime tasks, and the scheduler tick is really disabled,
run_local_timers does not get called while the scheduler tick
is disabled, and this patch does nothing...

> +++ 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); +	} + 
> if (!base->active_timers) goto out;

I suspect we may need an alternate approach, where we
change the way __run_timers works, so it does not go
around its main loop thousands of times.

Is there any sane way we could going around this list
thousands of times?

This is especially annoying when there are no timers
pending in the first place :)

static inline void __run_timers(struct tvec_base *base)
{
        struct timer_list *timer;

        spin_lock_irq(&base->lock);
        while (time_after_eq(jiffies, base->timer_jiffies)) {
                struct list_head work_list;
                struct list_head *head = &work_list;
                int index = base->timer_jiffies & TVR_MASK;

                /*
                 * Cascade timers:
                 */
                if (!index &&
                        (!cascade(base, &base->tv2, INDEX(0))) &&
                                (!cascade(base, &base->tv3, INDEX(1))) &&
                                        !cascade(base, &base->tv4,
INDEX(2)))
                        cascade(base, &base->tv5, INDEX(3));
                ++base->timer_jiffies;
                list_replace_init(base->tv1.vec + index, &work_list);
                while (!list_empty(head)) {


- -- 
All rights reversed
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