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

Paul E. McKenney
2024-05-10T16:21:59Z

On Fri, May 10, 2024 at 01:06:40PM -0300, Leonardo Bras wrote:
> 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.

Ah, got 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 you would like to do a separate patch adding this, here are the
requirements:

o If the current code prints nothing, nothing additional should
be printed.

o If the rounding ended up with the same value (as it should in
systems with HZ=1000), nothing additional should be printed.

o Your choice as to whether or not you want to print out the
jiffies value.

o If the additional message is on a new line, it needs to be
indented so that it is clear that it is subordinate to the
previous message.

Otherwise, you can use pr_cont() to continue the previous
line, of course being careful about "\n".

Probably also something that I am forgetting, but that is most of it.

> > > 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.

Very good! You had me going there for a bit. ;-)

> > > + 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.

Just nocb_patience_delay_jiffies, correct? Unless I am missing something,
nocb_patience_delay is unused after boot.

> 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?

Well, the hardware's opinion is what matters. ;-)

But the caller's code path reads jiffies a few times, so it should
be hot in the cache, correct?

But that does lead to another topic, namely the possibility of tagging
nocb_patience_delay_jiffies with __read_mostly. And there might be
a number of other of RCU's variables that could be similarly tagged
in order to avoid false sharing. (But is there any false sharing?
This might be worth testing.)

Thanx, Paul

> 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) &&
> > >
> > >
> > >
> >
>
>