* Paul E. McKenney <[email protected]> wrote:
> rcu: Fix comparison sense in rcu_needs_cpu()
>
> Commit c0f4dfd4f (rcu: Make RCU_FAST_NO_HZ take advantage of numbered
> callbacks) introduced a bug that can result in excessively long grace
> periods. This bug reverse the senes of the "if" statement checking
> for lazy callbacks, so that RCU takes a lazy approach when there are
> in fact non-lazy callbacks. This can result in excessive boot, suspend,
> and resume times.
>
> This commit therefore fixes the sense of this "if" statement.
>
> Reported-by: Borislav Petkov <[email protected]>
> Reported-by: Bj?rn Mork <[email protected]>
> Reported-by: Joerg Roedel <[email protected]>
> Signed-off-by: Paul E. McKenney <[email protected]>
>
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 170814d..6d939a6 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -1667,7 +1667,7 @@ int rcu_needs_cpu(int cpu, unsigned long *dj)
> rdtp->last_accelerate = jiffies;
>
> /* Request timer delay depending on laziness, and round. */
> - if (rdtp->all_lazy) {
> + if (!rdtp->all_lazy) {
> *dj = round_up(rcu_idle_gp_delay + jiffies,
> rcu_idle_gp_delay) - jiffies;
Neat - could this explain sporadic long (but not infinite) boot times with
NOHZ_FULL?
We changed HZ to be at least 1 Hz pretty recently, which might have worked
around this bug.
Thanks,
Ingo
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