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hrtimers: Unconditionally update target CPU base after offline timer migration 

[ Upstream commit e895f8e291 ]

When testing softirq based hrtimers on an ARM32 board, with high resolution
mode and NOHZ inactive, softirq based hrtimers fail to expire after being
moved away from an offline CPU:

CPU0				CPU1
				hrtimer_start(..., HRTIMER_MODE_SOFT);
cpu_down(CPU1)			...
				hrtimers_cpu_dying()
				  // Migrate timers to CPU0
				  smp_call_function_single(CPU0, returgger_next_event);
  retrigger_next_event()
    if (!highres && !nohz)
        return;

As retrigger_next_event() is a NOOP when both high resolution timers and
NOHZ are inactive CPU0's hrtimer_cpu_base::softirq_expires_next is not
updated and the migrated softirq timers never expire unless there is a
softirq based hrtimer queued on CPU0 later.

Fix this by removing the hrtimer_hres_active() and tick_nohz_active() check
in retrigger_next_event(), which enforces a full update of the CPU base.
As this is not a fast path the extra cost does not matter.

[ tglx: Massaged change log ]

Fixes: 5c0930ccaa ("hrtimers: Push pending hrtimers away from outgoing CPU earlier")
Co-developed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250805081025.54235-1-wangxiongfeng2@huawei.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Xiongfeng Wang 2025-08-05 16:10:25 +08:00 committed by Greg Kroah-Hartman
parent b1fa39fb30
commit 156677ea10
1 changed files with 3 additions and 8 deletions
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11
kernel/time/hrtimer.c
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@ -808,10 +808,10 @@ static void retrigger_next_event(void *arg)
* of the next expiring timer is enough. The return from the SMP
* function call will take care of the reprogramming in case the
* CPU was in a NOHZ idle sleep.
*
* In periodic low resolution mode, the next softirq expiration
* must also be updated.
*/
if (!hrtimer_hres_active(base) && !tick_nohz_active)
return;
raw_spin_lock(&base->lock);
hrtimer_update_base(base);
if (hrtimer_hres_active(base))
@ -2289,11 +2289,6 @@ int hrtimers_cpu_dying(unsigned int dying_cpu)
&new_base->clock_base[i]);
}
/*
* The migration might have changed the first expiring softirq
* timer on this CPU. Update it.
*/
__hrtimer_get_next_event(new_base, HRTIMER_ACTIVE_SOFT);
/* Tell the other CPU to retrigger the next event */
smp_call_function_single(ncpu, retrigger_next_event, NULL, 0);