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

Leonardo Bras
2024-06-10T05:05:42Z

On Tue, Jun 04, 2024 at 03:23:52PM -0700, Paul E. McKenney wrote:
> 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 rcutexperiementationree.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,

Just a nit I found when cherry-picking here
s/experiementation/experimentation/

Thanks!
Leo

> 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())
> 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
>