Commit fb064e5a authored by Joel Fernandes (Google)'s avatar Joel Fernandes (Google) Committed by Ingo Molnar
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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: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231020014031.919742-2-joel@joelfernandes.org
parent 1b7ef2d9

kernel/sched/fair.c

+13 −2
Original line number Diff line number Diff line
@@ -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)

{