On Wed, Oct 07, 2020 at 03:01:52PM -0300, Marcelo Tosatti wrote: > When adding a tick dependency to a task, its necessary to > wakeup the CPU where the task resides to reevaluate tick > dependencies on that CPU. > > However the current code wakes up all nohz_full CPUs, which > is unnecessary. > > Switch to waking up a single CPU, by using ordering of writes > to task->cpu and task->tick_dep_mask. > > From: Frederic Weisbecker <[email protected]> > Suggested-by: Peter Zijlstra <[email protected]> > Signed-off-by: Frederic Weisbecker <[email protected]> > Signed-off-by: Marcelo Tosatti <[email protected]> > > Index: linux-2.6/kernel/time/tick-sched.c > =================================================================== > --- linux-2.6.orig/kernel/time/tick-sched.c > +++ linux-2.6/kernel/time/tick-sched.c > @@ -274,6 +274,31 @@ void tick_nohz_full_kick_cpu(int cpu) > irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu); > } > > +static void tick_nohz_kick_task(struct task_struct *tsk) > +{ > + int cpu = task_cpu(tsk); > + > + /* > + * If the task concurrently migrates to another cpu, > + * we guarantee it sees the new tick dependency upon > + * schedule. > + * > + * > + * set_task_cpu(p, cpu); > + * STORE p->cpu = @cpu > + * __schedule() (switch to task 'p') > + * LOCK rq->lock > + * smp_mb__after_spin_lock() STORE p->tick_dep_mask > + * tick_nohz_task_switch() smp_mb() (atomic_fetch_or()) > + * LOAD p->tick_dep_mask LOAD p->cpu > + */ > + > + preempt_disable(); > + if (cpu_online(cpu)) > + tick_nohz_full_kick_cpu(cpu); > + preempt_enable(); > +}
So we need to kick the CPU unconditionally, or only when the task is actually running? AFAICT we only care about current->tick_dep_mask.
