| Message ID | 20240913214205.12359-3-frederic@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | rcu/nocb updates | expand |
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 08608fe1792c..0c9eca1cc76e 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -216,10 +216,7 @@ static bool __wake_nocb_gp(struct rcu_data *rdp_gp, raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags); if (needwake) { trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DoWake")); - if (cpu_is_offline(raw_smp_processor_id())) - swake_up_one_online(&rdp_gp->nocb_gp_wq); - else - wake_up_process(rdp_gp->nocb_gp_kthread); + swake_up_one_online(&rdp_gp->nocb_gp_wq); } return needwake;
A callback enqueuer currently wakes up the rcuo kthread if it is adding the first non-done callback of a CPU, whether the kthread is waiting on a grace period or not (unless the CPU is offline). This looks like a desired behaviour because then the rcuo kthread doesn't wait for the end of the current grace period to handle the callback. It is accelerated right away and assigned to the next grace period. The GP kthread is notified about that fact and iterates with the upcoming GP without sleeping in-between. However this best-case scenario is contradicted by a few details, depending on the situation: 1) If the callback is a non-bypass one queued with IRQs enabled, the wake up only occurs if no other pending callbacks are on the list. Therefore the theoretical "optimization" actually applies on rare occasions. 2) If the callback is a non-bypass one queued with IRQs disabled, the situation is similar with even more uncertainty due to the deferred wake up. 3) If the callback is lazy, a few jiffies don't make any difference. 4) If the callback is bypass, the wake up timer is programmed 2 jiffies ahead by rcuo in case the regular pending queue has been handled in the meantime. The rare storm of callbacks can otherwise wait for the currently elapsing grace period to be flushed and handled. For all those reasons, the optimization is only theoretical and occasional. Therefore it is reasonable that callbacks enqueuers only wake up the rcuo kthread when it is not already waiting on a grace period to complete. Signed-off-by: Frederic Weisbecker <frederic@kernel.org> --- kernel/rcu/tree_nocb.h | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-)