| From 3c8920e2dbd1a55f72dc14d656df9d0097cf5c72 Mon Sep 17 00:00:00 2001 |
| From: Frederic Weisbecker <frederic@kernel.org> |
| Date: Fri, 15 May 2020 02:34:29 +0200 |
| Subject: tick/nohz: Narrow down noise while setting current task's tick dependency |
| |
| From: Frederic Weisbecker <frederic@kernel.org> |
| |
| commit 3c8920e2dbd1a55f72dc14d656df9d0097cf5c72 upstream. |
| |
| Setting a tick dependency on any task, including the case where a task |
| sets that dependency on itself, triggers an IPI to all CPUs. That is |
| of course suboptimal but it had previously not been an issue because it |
| was only used by POSIX CPU timers on nohz_full, which apparently never |
| occurs in latency-sensitive workloads in production. (Or users of such |
| systems are suffering in silence on the one hand or venting their ire |
| on the wrong people on the other.) |
| |
| But RCU now sets a task tick dependency on the current task in order |
| to fix stall issues that can occur during RCU callback processing. |
| Thus, RCU callback processing triggers frequent system-wide IPIs from |
| nohz_full CPUs. This is quite counter-productive, after all, avoiding |
| IPIs is what nohz_full is supposed to be all about. |
| |
| This commit therefore optimizes tasks' self-setting of a task tick |
| dependency by using tick_nohz_full_kick() to avoid the system-wide IPI. |
| Instead, only the execution of the one task is disturbed, which is |
| acceptable given that this disturbance is well down into the noise |
| compared to the degree to which the RCU callback processing itself |
| disturbs execution. |
| |
| 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> |
| Signed-off-by: Paul E. McKenney <paulmck@kernel.org> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| |
| --- |
| kernel/time/tick-sched.c | 22 +++++++++++++++------- |
| 1 file changed, 15 insertions(+), 7 deletions(-) |
| |
| --- a/kernel/time/tick-sched.c |
| +++ b/kernel/time/tick-sched.c |
| @@ -351,16 +351,24 @@ void tick_nohz_dep_clear_cpu(int cpu, en |
| 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); |
| |