5.15-stable review patch. If anyone has any objections, please let me know.
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From: Xiongfeng Wang wangxiongfeng2@huawei.com
[ Upstream commit e895f8e29119c8c966ea794af9e9100b10becb88 ]
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: 5c0930ccaad5 ("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 --- kernel/time/hrtimer.c | 11 +++-------- 1 file changed, 3 insertions(+), 8 deletions(-)
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c index 74a71b3a064dc..7e2ed34e9803b 100644 --- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c @@ -770,10 +770,10 @@ 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)) @@ -2229,11 +2229,6 @@ 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);