Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ

[Ingo Molnar <mingo@kernel.org>]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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 <bp@alien8.de>
> Reported-by: Bj?rn Mork <bjorn@mork.no>
> Reported-by: Joerg Roedel <joro@8bytes.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> 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