Re: [PATCH rcu 6/9] rcu: Add rcutree.nocb_patience_delay to reduce nohz_full OS jitter

[Frederic Weisbecker <frederic@kernel.org>]

Le Tue, Jun 04, 2024 at 03:23:52PM -0700, Paul E. McKenney a écrit :
> If a CPU is running either a userspace application or a guest OS in
> nohz_full mode, it is possible for a system call to occur just as an
> RCU grace period is starting.  If that CPU also has the scheduling-clock
> tick enabled for any reason (such as a second runnable task), and if the
> system was booted with rcutree.use_softirq=0, then RCU can add insult to
> injury by awakening that CPU's rcuc kthread, resulting in yet another
> task and yet more OS jitter due to switching to that task, running it,
> and switching back.
> 
> In addition, in the common case where that system call is not of
> excessively long duration, awakening the rcuc task is pointless.
> This pointlessness is due to the fact that the CPU will enter an extended
> quiescent state upon returning to the userspace application or guest OS.
> In this case, the rcuc kthread cannot do anything that the main RCU
> grace-period kthread cannot do on its behalf, at least if it is given
> a few additional milliseconds (for example, given the time duration
> specified by rcutree.jiffies_till_first_fqs, give or take scheduling
> delays).
> 
> This commit therefore adds a rcutree.nocb_patience_delay kernel boot
> parameter that specifies the grace period age (in milliseconds)
> before which RCU will refrain from awakening the rcuc kthread.
> Preliminary experiementation suggests a value of 1000, that is,
> one second.  Increasing rcutree.nocb_patience_delay will increase
> grace-period latency and in turn increase memory footprint, so systems
> with constrained memory might choose a smaller value.  Systems with
> less-aggressive OS-jitter requirements might choose the default value
> of zero, which keeps the traditional immediate-wakeup behavior, thus
> avoiding increases in grace-period latency.
> 
> [ paulmck: Apply Leonardo Bras feedback.  ]
> 
> Link: https://lore.kernel.org/all/20240328171949.743211-1-leobras@redhat.com/
> 
> Reported-by: Leonardo Bras <leobras@redhat.com>
> Suggested-by: Leonardo Bras <leobras@redhat.com>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Reviewed-by: Leonardo Bras <leobras@redhat.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  8 ++++++++
>  kernel/rcu/tree.c                               | 10 ++++++++--
>  kernel/rcu/tree_plugin.h                        | 10 ++++++++++
>  3 files changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 500cfa7762257..2d4a512cf1fc6 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5018,6 +5018,14 @@
>  			the ->nocb_bypass queue.  The definition of "too
>  			many" is supplied by this kernel boot parameter.
>  
> +	rcutree.nocb_patience_delay= [KNL]
> +			On callback-offloaded (rcu_nocbs) CPUs, avoid
> +			disturbing RCU unless the grace period has
> +			reached the specified age in milliseconds.
> +			Defaults to zero.  Large values will be capped
> +			at five seconds.  All values will be rounded down
> +			to the nearest value representable by jiffies.
> +
>  	rcutree.qhimark= [KNL]
>  			Set threshold of queued RCU callbacks beyond which
>  			batch limiting is disabled.
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 35bf4a3736765..408b020c9501f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -176,6 +176,9 @@ static int gp_init_delay;
>  module_param(gp_init_delay, int, 0444);
>  static int gp_cleanup_delay;
>  module_param(gp_cleanup_delay, int, 0444);
> +static int nocb_patience_delay;
> +module_param(nocb_patience_delay, int, 0444);
> +static int nocb_patience_delay_jiffies;
>  
>  // Add delay to rcu_read_unlock() for strict grace periods.
>  static int rcu_unlock_delay;
> @@ -4344,11 +4347,14 @@ static int rcu_pending(int user)
>  		return 1;
>  
>  	/* Is this a nohz_full CPU in userspace or idle?  (Ignore RCU if so.) */
> -	if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu())
> +	gp_in_progress = rcu_gp_in_progress();
> +	if ((user || rcu_is_cpu_rrupt_from_idle() ||
> +	     (gp_in_progress &&
> +	      time_before(jiffies, READ_ONCE(rcu_state.gp_start) + nocb_patience_delay_jiffies))) &&
> +	    rcu_nohz_full_cpu())

The rcu_nohz_full_cpu() test should go before anything in order to benefit from
the static key in tick_nohz_full_cpu().

And since it only applies to nohz_full, should it be called
nohz_full_patience_delay ?

Or do we want to generalize it to all nocb uses
(which means only rely on rcu_is_cpu_rrupt_from_idle() if not nohz_full). Not
sure if that would make sense...

Thanks.

>  		return 0;
>  
>  	/* Is the RCU core waiting for a quiescent state from this CPU? */
> -	gp_in_progress = rcu_gp_in_progress();
>  	if (rdp->core_needs_qs && !rdp->cpu_no_qs.b.norm && gp_in_progress)
>  		return 1;
>  
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 340bbefe5f652..31c539f09c150 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -93,6 +93,16 @@ static void __init rcu_bootup_announce_oddness(void)
>  		pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay);
>  	if (gp_cleanup_delay)
>  		pr_info("\tRCU debug GP cleanup slowdown %d jiffies.\n", gp_cleanup_delay);
> +	if (nocb_patience_delay < 0) {
> +		pr_info("\tRCU NOCB CPU patience negative (%d), resetting to zero.\n", nocb_patience_delay);
> +		nocb_patience_delay = 0;
> +	} else if (nocb_patience_delay > 5 * MSEC_PER_SEC) {
> +		pr_info("\tRCU NOCB CPU patience too large (%d), resetting to %ld.\n", nocb_patience_delay, 5 * MSEC_PER_SEC);
> +		nocb_patience_delay = 5 * MSEC_PER_SEC;
> +	} else if (nocb_patience_delay) {
> +		pr_info("\tRCU NOCB CPU patience set to %d milliseconds.\n", nocb_patience_delay);
> +	}
> +	nocb_patience_delay_jiffies = msecs_to_jiffies(nocb_patience_delay);
>  	if (!use_softirq)
>  		pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
>  	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
> -- 
> 2.40.1
> 
>