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

Leonardo Bras
2024-05-10T16:07:17Z

On Thu, May 09, 2024 at 04:45:53PM -0700, Paul E. McKenney wrote:
> On Thu, May 09, 2024 at 07:14:18AM -0300, Leonardo Bras wrote:
> > On Thu, May 09, 2024 at 05:16:57AM -0300, Leonardo Bras wrote:
>
> [ . . . ]
>
> > > 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);
> > >
> >
> > Uh, there is more to it, actually. We need to make sure the user
> > understands that we are rounding-down the value to multiple of a jiffy
> > period, so it's not a surprise if the delay value is not exactly the same
> > as the passed on kernel cmdline.
> >
> > So something like bellow diff should be ok, as this behavior is explained
> > in the docs, and pr_info() will print the effective value.
> >
> > What do you think?
>
> Good point, and I have taken your advice on making the documentation
> say what it does.

Thanks :)

>
> > Thanks!
> > Leo
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 0a3b0fd1910e..9a50be9fd9eb 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4974,20 +4974,28 @@
> > otherwise be caused by callback floods through
> > use of the ->nocb_bypass list. However, in the
> > common non-flooded case, RCU queues directly to
> > the main ->cblist in order to avoid the extra
> > overhead of the ->nocb_bypass list and its lock.
> > But if there are too many callbacks queued during
> > a single jiffy, RCU pre-queues the callbacks into
> > 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. Values rounded-down to a multiple
> > + of a jiffy period.
> > +
> > rcutree.qhimark= [KNL]
> > Set threshold of queued RCU callbacks beyond which
> > batch limiting is disabled.
> >
> > rcutree.qlowmark= [KNL]
> > Set threshold of queued RCU callbacks below which
> > batch limiting is re-enabled.
> >
> > rcutree.qovld= [KNL]
> > Set threshold of queued RCU callbacks beyond which
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index fcf2b4aa3441..62ede401420f 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -512,20 +512,21 @@ do { \
> > local_irq_save(flags); \
> > if (rcu_segcblist_is_offloaded(&(rdp)->cblist)) \
> > raw_spin_lock(&(rdp)->nocb_lock); \
> > } while (0)
> > #else /* #ifdef CONFIG_RCU_NOCB_CPU */
> > #define rcu_nocb_lock_irqsave(rdp, flags) local_irq_save(flags)
> > #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
> >
> > static void rcu_bind_gp_kthread(void);
> > static bool rcu_nohz_full_cpu(void);
> > +static bool rcu_on_patience_delay(void);
>
> I don't think we need an access function, but will check below.
>
> > /* Forward declarations for tree_stall.h */
> > static void record_gp_stall_check_time(void);
> > static void rcu_iw_handler(struct irq_work *iwp);
> > static void check_cpu_stall(struct rcu_data *rdp);
> > static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
> > const unsigned long gpssdelay);
> >
> > /* Forward declarations for tree_exp.h. */
> > static void sync_rcu_do_polled_gp(struct work_struct *wp);
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 340bbefe5f65..639243b0410f 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -5,20 +5,21 @@
> > * or preemptible semantics.
> > *
> > * Copyright Red Hat, 2009
> > * Copyright IBM Corporation, 2009
> > *
> > * Author: Ingo Molnar <mingo@elte.hu>
> > * Paul E. McKenney <paulmck@linux.ibm.com>
> > */
> >
> > #include "../locking/rtmutex_common.h"
> > +#include <linux/jiffies.h>
>
> This is already pulled in by the enclosing tree.c file, so it should not
> be necessary to include it again.

Even better :)

> (Or did you get a build failure when
> leaving this out?)

I didn't, it's just that my editor complained the symbols were not getting
properly resolved, so I included it and it was fixed. But since clangd is
know to make some mistakes, I should have compile-test'd before adding it.

>
> > static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
> > {
> > /*
> > * In order to read the offloaded state of an rdp in a safe
> > * and stable way and prevent from its value to be changed
> > * under us, we must either hold the barrier mutex, the cpu
> > * hotplug lock (read or write) or the nocb lock. Local
> > * non-preemptible reads are also safe. NOCB kthreads and
> > * timers have their own means of synchronization against the
> > @@ -86,20 +87,33 @@ static void __init rcu_bootup_announce_oddness(void)
> > if (rcu_kick_kthreads)
> > pr_info("\tKick kthreads if too-long grace period.\n");
> > if (IS_ENABLED(CONFIG_DEBUG_OBJECTS_RCU_HEAD))
> > pr_info("\tRCU callback double-/use-after-free debug is enabled.\n");
> > if (gp_preinit_delay)
> > pr_info("\tRCU debug GP pre-init slowdown %d jiffies.\n", gp_preinit_delay);
> > if (gp_init_delay)
> > 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 = msecs_to_jiffies(5 * MSEC_PER_SEC);
> > + } else if (nocb_patience_delay) {
> > + nocb_patience_delay = msecs_to_jiffies(nocb_patience_delay);
> > + pr_info("\tRCU NOCB CPU patience set to %d milliseconds.\n",
> > + jiffies_to_msecs(nocb_patience_delay);
> > + }
>
> I just did this here at the end:
>
> nocb_patience_delay_jiffies = msecs_to_jiffies(nocb_patience_delay);
>
> Ah, you are wanting to print out the milliseconds after the rounding
> to jiffies.

That's right, just to make sure the user gets the effective patience time,
instead of the before-rounding one, which was on input.

>
> I am going to hold off on that for the moment, but I hear your request
> and I have not yet said "no". ;-)

Sure :)
It's just something I think it's nice to have (as a user).

>
> > if (!use_softirq)
> > pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
> > if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
> > pr_info("\tRCU debug extended QS entry/exit.\n");
> > rcupdate_announce_bootup_oddness();
> > }
> >
> > #ifdef CONFIG_PREEMPT_RCU
> >
> > static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
> > @@ -1260,10 +1274,29 @@ static bool rcu_nohz_full_cpu(void)
> >
> > /*
> > * Bind the RCU grace-period kthreads to the housekeeping CPU.
> > */
> > static void rcu_bind_gp_kthread(void)
> > {
> > if (!tick_nohz_full_enabled())
> > return;
> > housekeeping_affine(current, HK_TYPE_RCU);
> > }
> > +
> > +/*
> > + * Is this CPU a NO_HZ_FULL CPU that should ignore RCU if the time since the
> > + * start of current grace period is smaller than nocb_patience_delay ?
> > + *
> > + * This code relies on the fact that all NO_HZ_FULL CPUs are also
> > + * RCU_NOCB_CPU CPUs.
> > + */
> > +static bool rcu_on_patience_delay(void)
> > +{
> > +#ifdef CONFIG_NO_HZ_FULL
>
> You lost me on this one. Why do we need the #ifdef instead of
> IS_ENABLED()? Also, please note that rcu_nohz_full_cpu() is already a
> compile-time @false in CONFIG_NO_HZ_FULL=n kernels.

You are right. rcu_nohz_full_cpu() has a high chance of being inlined on
if ((...) && rcu_nohz_full_cpu())
And since it returns false, this whole statement will be compiled out, and
the new function will not exist in CONFIG_NO_HZ_FULL=n, so there is no
need to test it.

>
> > + if (!nocb_patience_delay)
> > + return false;
>
> We get this automatically with the comparison below, right?

Right

> If so, we
> are not gaining much by creating the helper function. Or am I missing
> some trick here?

Well, it's a fastpath. Up to here, we just need to read
nocb_patience_delay{,_jiffies} from memory.

If we don't include the fastpath we have to read jiffies and
rcu_state.gp_start, which can take extra time: up to 2 cache misses.

I thought it could be relevant, as we reduce the overhead of the new
parameter when it's disabled (patience=0).

Do you think that could be relevant?

Thanks!
Leo

>
> Thanx, Paul
>
> > + if (time_before(jiffies, READ_ONCE(rcu_state.gp_start) + nocb_patience_delay))
> > + return true;
> > +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> > + return false;
> > +}
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 7560e204198b..7a2d94370ab4 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -169,20 +169,22 @@ static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
> > module_param(kthread_prio, int, 0444);
> >
> > /* Delay in jiffies for grace-period initialization delays, debug only. */
> >
> > static int gp_preinit_delay;
> > module_param(gp_preinit_delay, int, 0444);
> > 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;
> > #ifdef CONFIG_RCU_STRICT_GRACE_PERIOD
> > module_param(rcu_unlock_delay, int, 0444);
> > #endif
> >
> > /*
> > * This rcu parameter is runtime-read-only. It reflects
> > * a minimum allowed number of objects which can be cached
> > @@ -4340,25 +4342,27 @@ static int rcu_pending(int user)
> > lockdep_assert_irqs_disabled();
> >
> > /* Check for CPU stalls, if enabled. */
> > check_cpu_stall(rdp);
> >
> > /* Does this CPU need a deferred NOCB wakeup? */
> > if (rcu_nocb_need_deferred_wakeup(rdp, RCU_NOCB_WAKE))
> > 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 && rcu_on_patience_delay())) &&
> > + 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;
> >
> > /* Does this CPU have callbacks ready to invoke? */
> > if (!rcu_rdp_is_offloaded(rdp) &&
> > rcu_segcblist_ready_cbs(&rdp->cblist))
> > return 1;
> >
> > /* Has RCU gone idle with this CPU needing another grace period? */
> > if (!gp_in_progress && rcu_segcblist_is_enabled(&rdp->cblist) &&
> >
> >
> >
>