6.12-stable review patch. If anyone has any objections, please let me know.
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From: K Prateek Nayak kprateek.nayak@amd.com
[ Upstream commit e932c4ab38f072ce5894b2851fea8bc5754bb8e5 ]
Scheduler raises a SCHED_SOFTIRQ to trigger a load balancing event on from the IPI handler on the idle CPU. If the SMP function is invoked from an idle CPU via flush_smp_call_function_queue() then the HARD-IRQ flag is not set and raise_softirq_irqoff() needlessly wakes ksoftirqd because soft interrupts are handled before ksoftirqd get on the CPU.
Adding a trace_printk() in nohz_csd_func() at the spot of raising SCHED_SOFTIRQ and enabling trace events for sched_switch, sched_wakeup, and softirq_entry (for SCHED_SOFTIRQ vector alone) helps observing the current behavior:
<idle>-0 [000] dN.1.: nohz_csd_func: Raising SCHED_SOFTIRQ from nohz_csd_func <idle>-0 [000] dN.4.: sched_wakeup: comm=ksoftirqd/0 pid=16 prio=120 target_cpu=000 <idle>-0 [000] .Ns1.: softirq_entry: vec=7 [action=SCHED] <idle>-0 [000] .Ns1.: softirq_exit: vec=7 [action=SCHED] <idle>-0 [000] d..2.: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=ksoftirqd/0 next_pid=16 next_prio=120 ksoftirqd/0-16 [000] d..2.: sched_switch: prev_comm=ksoftirqd/0 prev_pid=16 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120 ...
Use __raise_softirq_irqoff() to raise the softirq. The SMP function call is always invoked on the requested CPU in an interrupt handler. It is guaranteed that soft interrupts are handled at the end.
Following are the observations with the changes when enabling the same set of events:
<idle>-0 [000] dN.1.: nohz_csd_func: Raising SCHED_SOFTIRQ for nohz_idle_balance <idle>-0 [000] dN.1.: softirq_raise: vec=7 [action=SCHED] <idle>-0 [000] .Ns1.: softirq_entry: vec=7 [action=SCHED]
No unnecessary ksoftirqd wakeups are seen from idle task's context to service the softirq.
Fixes: b2a02fc43a1f ("smp: Optimize send_call_function_single_ipi()") Closes: https://lore.kernel.org/lkml/fcf823f-195e-6c9a-eac3-25f870cb35ac@inria.fr/ [1] Reported-by: Julia Lawall julia.lawall@inria.fr Suggested-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Signed-off-by: K Prateek Nayak kprateek.nayak@amd.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Link: https://lore.kernel.org/r/20241119054432.6405-5-kprateek.nayak@amd.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 33bc43b223cba..6cc12777bb11a 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1244,7 +1244,7 @@ static void nohz_csd_func(void *info) rq->idle_balance = idle_cpu(cpu); if (rq->idle_balance) { rq->nohz_idle_balance = flags; - raise_softirq_irqoff(SCHED_SOFTIRQ); + __raise_softirq_irqoff(SCHED_SOFTIRQ); } }