Re: [RFC PATCH v1 0/2] Avoid rcu_core() if CPU just left guest vcpu

Paul E. McKenney
2024-05-09T22:41:37Z

On Thu, May 09, 2024 at 05:16:57AM -0300, Leonardo Bras wrote:
> On Wed, May 08, 2024 at 08:32:40PM -0700, Paul E. McKenney wrote:
> > On Wed, May 08, 2024 at 07:01:29AM -0700, Sean Christopherson wrote:
> > > On Wed, May 08, 2024, Leonardo Bras wrote:
> > > > Something just hit me, and maybe I need to propose something more generic.
> > >
> > > Yes. This is what I was trying to get across with my complaints about keying off
> > > of the last VM-Exit time. It's effectively a broad stroke "this task will likely
> > > be quiescent soon" and so the core concept/functionality belongs in common code,
> > > not KVM.
> >
> > OK, we could do something like the following wholly within RCU, namely
> > to make rcu_pending() refrain from invoking rcu_core() until the grace
> > period is at least the specified age, defaulting to zero (and to the
> > current behavior).
> >
> > Perhaps something like the patch shown below.
>
> That's exactly what I was thinking :)
>
> >
> > Thoughts?
>
> Some suggestions below:
>
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit abc7cd2facdebf85aa075c567321589862f88542
> > Author: Paul E. McKenney <paulmck@kernel.org>
> > Date: Wed May 8 20:11:58 2024 -0700
> >
> > rcu: Add rcutree.nocb_patience_delay to reduce nohz_full OS jitter
> >
> > 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.
> >
> > 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>
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 0a3b0fd1910e6..42383986e692b 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4981,6 +4981,13 @@
> > 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.
> > +
> > 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 7560e204198bb..6e4b8b43855a0 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -176,6 +176,8 @@ 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);
> >
> > // Add delay to rcu_read_unlock() for strict grace periods.
> > static int rcu_unlock_delay;
> > @@ -4334,6 +4336,8 @@ EXPORT_SYMBOL_GPL(cond_synchronize_rcu_full);
> > static int rcu_pending(int user)
> > {
> > bool gp_in_progress;
> > + unsigned long j = jiffies;
>
> I think this is probably taken care by the compiler, but just in case I would move the
> j = jiffies;
> closer to it's use, in order to avoid reading 'jiffies' if rcu_pending
> exits before the nohz_full testing.

Good point! I just removed j and used jiffies directly.

> > + unsigned int patience = msecs_to_jiffies(nocb_patience_delay);
>
> What do you think on processsing the new parameter in boot, and saving it
> in terms of jiffies already?
>
> It would make it unnecessary to convert ms -> jiffies every time we run
> rcu_pending.
>
> (OOO will probably remove the extra division, but may cause less impact in
> some arch)

This isn't exactly a fastpath, but it is easy enough to do the conversion
in rcu_bootup_announce_oddness() and place it into another variable
(for the benefit of those using drgn or going through crash dumps).

> > struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> > struct rcu_node *rnp = rdp->mynode;
> >
> > @@ -4347,11 +4351,13 @@ 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(j + patience, rcu_state.gp_start))) &&
>
> I think you meant:
> time_before(j, rcu_state.gp_start + patience)
>
> or else this always fails, as we can never have now to happen before a
> previously started gp, right?
>
> Also, as per rcu_nohz_full_cpu() we probably need it to be read with
> READ_ONCE():
>
> time_before(j, READ_ONCE(rcu_state.gp_start) + patience)

Good catch on both counts, fixed!

> > + 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..174333d0e9507 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -93,6 +93,15 @@ 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) {
>
> Here you suggest that we don't print if 'nocb_patience_delay == 0',
> as it's the default behavior, right?

Exactly, in keeping with the function name rcu_bootup_announce_oddness().

This approach allows easy spotting of deviations from default settings,
which can be very helpful when debugging.

> I think printing on 0 could be useful to check if the feature exists, even
> though we are zeroing it, but this will probably add unnecessary verbosity.

It could be quite useful to people learning the RCU implementation,
and I encourage those people to remove all those "if" statements from
rcu_bootup_announce_oddness() in order to get the full story.

> > + pr_info("\tRCU NOCB CPU patience set to %d milliseconds.\n", nocb_patience_delay);
> > + }
>
> Here I suppose something like this can take care of not needing to convert
> ms -> jiffies every rcu_pending():
>
> + nocb_patience_delay = msecs_to_jiffies(nocb_patience_delay);

Agreed, but I used a separate variable to help people looking at crash
dumps or using drgn.

And thank you for your review and comments! Applying these changes
with attribution.

Thanx, Paul

> > if (!use_softirq)
> > pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
> > if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
> >
>
>
> Thanks!
> Leo
>