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Message-ID: <20140726005104.GK11241@linux.vnet.ibm.com>
Date: Fri, 25 Jul 2014 17:51:04 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Pranith Kumar <bobby.prani@...il.com>
Cc: linux-kernel@...r.kernel.org, mingo@...nel.org,
laijs@...fujitsu.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
sasha.levin@...cle.com
Subject: Re: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading
callbacks unless requested
On Fri, Jul 25, 2014 at 08:10:57PM -0400, Pranith Kumar wrote:
> On 07/25/2014 07:36 PM, Paul E. McKenney wrote:
> > [ Note: This applies on top of commit 187497fa5e9e (rcu: Allow for NULL
> > tick_nohz_full_mask when nohz_full= missing) in -tip
> > or -rcu. To make this work on top of rcu/next, move the
> > call to rcu_organize_nocb_kthreads(rsp) to the end of the
> > for_each_rcu_flavor(rsp) loop in rcu_init_nohz(). ]
> >
> > Commit b58cc46c5f6b (rcu: Don't offload callbacks unless specifically
> > requested) failed to adjust the callback lists of the CPUs that are
> > known to be no-CBs CPUs only because they are also nohz_full= CPUs.
> > This failure can result in callbacks that are posted during early boot
> > getting stranded on nxtlist for CPUs whose no-CBs property becomes
> > apparent late, and there can also be spurious warnings about offline
> > CPUs posting callbacks.
> >
> > This commit fixes these problems by adding an early-boot rcu_init_nohz()
> > that properly initializes the no-CBs CPUs.
> >
> > Note that kernels built with CONFIG_RCU_NOCB_CPU_ALL=y or with
> > CONFIG_RCU_NOCB_CPU=n do not exhibit this bug. Neither do kernels
> > booted without the nohz_full= boot parameter.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>
> Please find two points below.
>
> <snip>
> > #ifdef CONFIG_TREE_PREEMPT_RCU
> > @@ -2451,6 +2424,66 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
> > trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
> > }
> >
> > +void rcu_init_nohz(void)
> > +{
> > + int cpu;
> > + bool need_rcu_nocb_mask = true;
> > + struct rcu_state *rsp;
> > +
> > +#ifdef CONFIG_RCU_NOCB_CPU_NONE
> > + need_rcu_nocb_mask = false;
> > +#endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
> > +
> > +#if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
> > + if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
> > + need_rcu_nocb_mask = true;
> > +#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */
> > +
> > + if (!have_rcu_nocb_mask && need_rcu_nocb_mask) {
> > + zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
>
> Please check the return value unless you want to increase my commit count ;)
I have already queued an adapted version of your patch on top of this
one, see below. ;-)
> > + have_rcu_nocb_mask = true;
> > + }
> > + if (!have_rcu_nocb_mask)
> > + return;
> > +
> > +#ifdef CONFIG_RCU_NOCB_CPU_ZERO
> > + pr_info("\tOffload RCU callbacks from CPU 0\n");
> > + cpumask_set_cpu(0, rcu_nocb_mask);
> > +#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
> > +#ifdef CONFIG_RCU_NOCB_CPU_ALL
> > + pr_info("\tOffload RCU callbacks from all CPUs\n");
> > + cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
> > +#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
> > +#if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
> > + cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> > +#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */
>
> I understand that if CONFIG_NO_HZ_FULL_ALL is set then CONFIG_NOCB_CPU_ALL
> will also be set and there is no need for this cpumask_or().
>
> Is there any reason for the coupling between CONFIG_NO_HZ_FULL_ALL
> and CONFIG_NOCB_CPU_ALL?
>
> I ask because a user can override CONFIG_NO_HZ_FULL_ALL=y at boot time
> using the nohz_full= boot time parameter. In this case even if a user marks
> CPU 0 as the only nohz_full cpu, we will offload call backs from all CPUs.
> Is this behavior what you have in mind?
Yep. The normal setup will be CONFIG_NO_HZ_FULL=y and
CONFIG_NO_HZ_FULL_ALL=n, and that works as you advocate.
If someone builds with CONFIG_NO_HZ_FULL_ALL=y, then they get
all CPUs offloaded.
Longer term, the hope is that offloading is unconditional, so that
CONFIG_NOCB_CPU and friends disappear, but the current code is most
definitely not up to that task yet.
Thanx, Paul
------------------------------------------------------------------------
rcu: Check the return value of zalloc_cpumask_var()
This commit checks the return value of the zalloc_cpumask_var() used for
allocating cpumask for rcu_nocb_mask.
Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 095d6e4d2fd7..5a6398e21bfc 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2440,7 +2440,10 @@ void rcu_init_nohz(void)
#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */
if (!have_rcu_nocb_mask && need_rcu_nocb_mask) {
- zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
+ if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
+ pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
+ return;
+ }
have_rcu_nocb_mask = true;
}
if (!have_rcu_nocb_mask)
--
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