6.12-stable review patch. If anyone has any objections, please let me know.
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From: Xiongfeng Wang wangxiongfeng2@huawei.com
commit e895f8e29119c8c966ea794af9e9100b10becb88 upstream.
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: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/time/hrtimer.c | 11 +++-------- 1 file changed, 3 insertions(+), 8 deletions(-)
--- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c @@ -798,10 +798,10 @@ static void retrigger_next_event(void *a * 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)) @@ -2286,11 +2286,6 @@ int hrtimers_cpu_dying(unsigned int dyin &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);