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Date: Fri, 10 May 2024 10:41:53 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Leonardo Bras <leobras@...hat.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Frederic Weisbecker <frederic@...nel.org>,
Neeraj Upadhyay <quic_neeraju@...cinc.com>,
Joel Fernandes <joel@...lfernandes.org>,
Josh Triplett <josh@...htriplett.org>,
Boqun Feng <boqun.feng@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Zqiang <qiang.zhang1211@...il.com>,
Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [RFC PATCH v1 0/2] Avoid rcu_core() if CPU just left guest vcpu
On Fri, May 10, 2024 at 02:12:32PM -0300, Leonardo Bras wrote:
> On Fri, May 10, 2024 at 09:21:59AM -0700, Paul E. McKenney wrote:
> > 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@...e.hu>
> > > > > * Paul E. McKenney <paulmck@...ux.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.
>
> Thanks!
> I will work on a patch doing that :)
Very good, looking forward to seeing what you come up with!
My current state is on the "dev" branch of the -rcu tree, so please base
on that.
> > > > > 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.
>
> Right, I used both because I was referring to the older version and the
> current version with _jiffies.
Fair enough!
> > > 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?
>
> Right, but I wonder how are the chances of it getting updated between
> caller's use and this function's. Same for gp_start.
Well, jiffies is updated at most once per millisecond, and gp_start is
updated at most once per few milliseconds. So the chances of it being
updated within that code sequence are quite small.
> > But that does lead to another topic, namely the possibility of tagging
> > nocb_patience_delay_jiffies with __read_mostly.
>
> Oh, right. This was supposed to be in the diff I sent earlier, but I
> completelly forgot to change before sending. So, yeah, I agree on
> nocb_patience_delay being __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.)
>
> Maybe there isn't, but I wonder if it would hurt performance if they were
> tagged as __read_only anyway.
Let's be at least a little careful here. It is just as easy to hurt
performance by marking things __read_mostly or __read_only as it is
to help performance. ;-)
Thanx, Paul
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