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sched/nohz: Update comments about NEWILB_KICK

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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
This commit is contained in:
Joel Fernandes (Google) 2023-10-20 01:40:27 +00:00 committed by Ingo Molnar
parent 1b7ef2d94f
commit fb064e5ae1
1 changed files with 13 additions and 2 deletions
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15
kernel/sched/fair.c
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@ -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)
{