diff options
| author | Joel Fernandes (Google) <joel@joelfernandes.org> | 2023-10-20 01:40:27 +0000 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2023-10-20 09:56:21 +0200 |
| commit | fb064e5ae1657595c090ebbc5b15787a3ef603e9 (patch) | |
| tree | 9b21d14e872766468d628c09e69ab2ed02e3e959 /kernel/sched | |
| parent | 1b7ef2d94ff4cb0b1186a224a97349864820c606 (diff) | |
sched/nohz: Update comments about NEWILB_KICK
How ILB is triggered without IPIs is cryptic. Out of mercy for future
code readers, document it in code comments.
The comments are derived from a discussion with Vincent in a past
review.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231020014031.919742-2-joel@joelfernandes.org
Diffstat (limited to 'kernel/sched')
| -rw-r--r-- | kernel/sched/fair.c | 15 |
1 files changed, 13 insertions, 2 deletions
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 9ae2208089e4..8c486ffcb779 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -12005,8 +12005,19 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) } /* - * Check if we need to run the ILB for updating blocked load before entering - * idle state. + * Check if we need to directly run the ILB for updating blocked load before + * entering idle state. Here we run ILB directly without issuing IPIs. + * + * Note that when this function is called, the tick may not yet be stopped on + * this CPU yet. nohz.idle_cpus_mask is updated only when tick is stopped and + * cleared on the next busy tick. In other words, nohz.idle_cpus_mask updates + * don't align with CPUs enter/exit idle to avoid bottlenecks due to high idle + * entry/exit rate (usec). So it is possible that _nohz_idle_balance() is + * called from this function on (this) CPU that's not yet in the mask. That's + * OK because the goal of nohz_run_idle_balance() is to run ILB only for + * updating the blocked load of already idle CPUs without waking up one of + * those idle CPUs and outside the preempt disable / irq off phase of the local + * cpu about to enter idle, because it can take a long time. */ void nohz_run_idle_balance(int cpu) { |