Messages in this thread | | | Date | Mon, 18 May 2020 10:57:58 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] tick/nohz: Narrow down noise while setting current task's tick dependency |
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On Fri, May 15, 2020 at 02:34:29AM +0200, Frederic Weisbecker wrote: > So far setting a tick dependency on any task, including current, used to > trigger an IPI to all CPUs. That's of course suboptimal but it wasn't > an issue as long as it was only used by posix-cpu-timers on nohz_full, > a combo that nobody seemed to use in real life. > > But RCU started to use task tick dependency on current task to fix > stall issues on callbacks processing. These trigger regular and > undesired system wide IPIs on nohz_full. > > The fix is very easy while setting a tick dependency on the current > task, only its CPU needs an IPI. > > Fixes: 6a949b7af82d (rcu: Force on tick when invoking lots of callbacks) > Reported-by: Matt Fleming <matt@codeblueprint.co.uk> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > Cc: stable@kernel.org > Cc: Paul E. McKenney <paulmck@kernel.org> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Ingo Molnar <mingo@kernel.org> > --- > kernel/time/tick-sched.c | 22 +++++++++++++++------- > 1 file changed, 15 insertions(+), 7 deletions(-) > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index 3e2dc9b8858c..f0199a4ba1ad 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -351,16 +351,24 @@ void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit) > EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_cpu); > > /* > - * Set a per-task tick dependency. Posix CPU timers need this in order to elapse > - * per task timers. > + * Set a per-task tick dependency. RCU need this. Also posix CPU timers > + * in order to elapse per task timers. > */ > void tick_nohz_dep_set_task(struct task_struct *tsk, enum tick_dep_bits bit) > { > - /* > - * We could optimize this with just kicking the target running the task > - * if that noise matters for nohz full users. > - */ > - tick_nohz_dep_set_all(&tsk->tick_dep_mask, bit); > + if (!atomic_fetch_or(BIT(bit), &tsk->tick_dep_mask)) {
So why not simply:
tick_nohz_full_kick_cpu(task_cpu(tsk)); ?
If it got preempted, the scheduling involved would already have observed the bit we just set and kept the tick on anyway, same for migration.
Or am I missing something?
> + if (tsk == current) { > + preempt_disable(); > + tick_nohz_full_kick(); > + preempt_enable(); > + } else { > + /* > + * Some future tick_nohz_full_kick_task() > + * should optimize this. > + */ > + tick_nohz_full_kick_all(); > + } > + }
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