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

[Leonardo Bras <leobras@redhat.com>]

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) &&
> > 
> > 
> > 
>