Re: [PATCH 1/3] tick/nohz: Re-evaluate the tick on polling idle and on user entry

[Frederic Weisbecker <frederic@kernel.org>]

Le Mon, Nov 09, 2026 at 06:24:22PM +0100, Marco Crivellari a écrit :
> Mmh I'm not entirely sure. Look at this:
> 
> Without that hunk:
> 
> <idle>-0       [002] dN.1.    21.838685: csd_function_entry:
> func=idle_cycle [nohz_poll_timer_test], csd=0000000006a53f0a
> <idle>-0       [002] dN.2.    21.889256: hrtimer_start:
> hrtimer=000000007acc0071 function=tick_nohz_handler
> expires=21879000000 softexpires=21879000000 mode=ABS|PINNED|HARD
> was_armed=1
> <idle>-0       [002] d.h1.    21.900238: local_timer_entry: vector=236
> <idle>-0       [002] d.h1.    21.900238: hrtimer_expire_entry:
> hrtimer=000000007acc0071 function=tick_nohz_handler now=21879002056
> <idle>-0       [002] ..s1.    21.900246: timer_expire_entry:
> timer=0000000076975009 function=test_timer_fn [nohz_poll_timer_test]
> now=4294689147 baseclk=4294689147
> <idle>-0       [002] dN.2.    23.737243: hrtimer_start:
> hrtimer=000000007acc0071 function=tick_nohz_handler
> expires=23716000000 softexpires=23716000000 mode=ABS|PINNED|HARD
> was_armed=0
> <idle>-0       [002] dNh1.    23.737250: local_timer_entry: vector=236
> <idle>-0       [002] dNh1.    23.737251: hrtimer_expire_entry:
> hrtimer=000000007acc0071 function=tick_nohz_handler now=23716014595
> <idle>-0       [002] .Ns1.    23.737255: timer_expire_entry:
> timer=00000000134bb09d function=test_timer_fn [nohz_poll_timer_test]
> now=4294690984 baseclk=4294689177
> <idle>-0       [002] dN.1.    23.737261: csd_function_entry:
> func=watchdog_check_skew_remote, csd=00000000ecfea5e9
> 
> With the hunk applied:
> 
> <idle>-0       [002] dN.1.    10.363861: csd_function_entry:
> func=idle_cycle [nohz_poll_timer_test], csd=0000000046d09235
> <idle>-0       [002] dN.2.    10.414660: hrtimer_start:
> hrtimer=00000000ccc99756 function=tick_nohz_handler
> expires=10405000000 softexpires=10405000000 mode=ABS|PINNED|HARD
> was_armed=1
> <idle>-0       [002] d.h1.    10.425656: local_timer_entry: vector=236
> <idle>-0       [002] d.h1.    10.425656: hrtimer_expire_entry:
> hrtimer=00000000ccc99756 function=tick_nohz_handler now=10405001482
> <idle>-0       [002] .ns1.    10.425660: timer_expire_entry:
> timer=00000000654cb5a4 function=test_timer_fn [nohz_poll_timer_test]
> now=4294677672 baseclk=4294677672
> <idle>-0       [002] dN.2.    10.425661: hrtimer_start:
> hrtimer=00000000ccc99756 function=tick_nohz_handler
> expires=10435000000 softexpires=10435000000 mode=ABS|PINNED|HARD
> was_armed=0
> <idle>-0       [002] d.h1.    10.455656: local_timer_entry: vector=236
> <idle>-0       [002] d.h1.    10.455657: hrtimer_expire_entry:
> hrtimer=00000000ccc99756 function=tick_nohz_handler now=10435001994
> <idle>-0       [002] .ns1.    10.455659: timer_expire_entry:
> timer=00000000a060ca9a function=test_timer_fn [nohz_poll_timer_test]
> now=4294677702 baseclk=4294677702
> 
> With the hunk, the stopped tick firing for the first timer sets
> TIF_NEED_RESCHED, the polling loop exits and the tick is reprogrammed
> for the second timer within a few microseconds; the 10 iterations
> captured take 0.92s instead of ~18s. Without that hunk, the CPU stops
> spinning on an unrelated event.
> 
> So in my understanding tick_nohz_idle_exit() program the tick only for
> the earliest timer; when it expire, the tick is stopped and not
> rearmed in tick_nohz_handler() (HRTIMER_NORESTART). Without setting
> NEED_RESCHED if another timer has been queued it has to wait for
> another event that set NEED_RESCHED.

Hehe, you're right :)

Let's try another strategy then. TS_FLAG_INIDLE is what tells nohz_fool whether
to manage the tick through the idle loop or through the IRQ exit.

So why not calling tick_nohz_idle_enter() on each idle loop iterations? That is,
from cpuidle_idle_call(). And call tick_nohz_idle_exit() right before
cpu_idle_poll()?

Then you can remove tick_nohz_idle_restart_tick().

Just be careful: tick_nohz_idle_enter() and tick_nohz_idle_exit() may now be
unbalanced. If TS_FLAG_INIDLE is already set, tick_nohz_idle_enter() should
return early and tick_nohz_idle_exit() as well if !TS_FLAG_INIDLE.

Also IRQs may now be disabled while calling tick_nohz_idle_exit(), so it should
use local_irq_save() and not local_irq_disable(). OTOH tick_nohz_idle_enter()
should always expect IRQs disabled.

So the purpose is to move cpu_idle_poll() outside the dynticks idle logic, then
it's safe against nohz_full timer management, the tick keeps being stopped if
possible and it's only restarted for dynticks idle.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs