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.
This passes moderate rcutorture testing. If you want me to take it, please
let me know, and otherwise:
Tested-by: Paul E. McKenney <paul...@kernel.org>
> Fixes: 6a949b7af82d (rcu: Force on tick when invoking lots of callbacks)
> Reported-by: Matt Fleming <m...@codeblueprint.co.uk>
> Signed-off-by: Frederic Weisbecker <frede...@kernel.org>
> Cc: sta...@kernel.org
> Cc: Paul E. McKenney <paul...@kernel.org>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Ingo Molnar <mi...@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)) {
> + 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();
> + }
> + }
> }
> EXPORT_SYMBOL_GPL(tick_nohz_dep_set_task);
>
> --
> 2.26.2
>
