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

Frederic Weisbecker
2020-07-01T01:10:38Z

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(-)