Re: [PATCH] tick/nohz: Narrow down noise while setting current task's tick dependency

[Peter Zijlstra]

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@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxx
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  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();
> +		}
> +	}