linux-kernel - [RFC PATCH 00/10] timer: Reduce timers softirq (and other optimizations)

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Date:   Wed,  1 Jul 2020 03:10:20 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: [RFC PATCH 00/10] timer: Reduce timers softirq (and other optimizations)

Juri Lelli has reported that threaded IRQs don't go along well with
full dynticks.

The tick never manages to stop due to the following endless loop:

* Timer tick raises timer softirq
* Ksoftirqd wakes up, restarts the tick (2 tasks in the runqueue)
* Timer tick raises timer softirq
* Ksoftirqd wakes up, etc...

The main issue here is that the tick fires the timer tick unconditionally,
whether timers have expired or not. This set is a proposal to lower that.

The 1st patch is actually a bugfix for a theoretical issue that I haven't
observed in practice. But who knows?

Patch 2 is a consolidation.

Patch 3 and 4 are optimizations.

The rest is about timer softirqs.

git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
	timers/softirq

HEAD: d0567dd5546d1f32eca3772a431488f8b0ac26a1

Thanks,
	Frederic
---

Frederic Weisbecker (10):
      timer: Prevent base->clk from moving backward
      timer: Move trigger_dyntick_cpu() to enqueue_timer()
      timer: Simplify LVL_START() and calc_index()
      timer: Optimize _next_timer_interrupt() level iteration
      timers: Always keep track of next expiry
      timer: Reuse next expiry cache after nohz exit
      timer: Expand clk forward logic beyond nohz
      timer: Spare timer softirq until next expiry
      timer: Remove must_forward_clk
      timer: Lower base clock forwarding threshold

 kernel/time/timer.c | 179 +++++++++++++++++++---------------------------------
 1 file changed, 64 insertions(+), 115 deletions(-)