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Date: Fri, 16 Mar 2012 12:56:44 -0500
From: Dimitri Sivanich <sivanich@....com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] rcu: Limit GP initialization to CPUs that have
been online
On Fri, Mar 16, 2012 at 10:51:42AM -0700, Paul E. McKenney wrote:
> On Fri, Mar 16, 2012 at 12:28:50PM -0500, Dimitri Sivanich wrote:
> > On Fri, Mar 16, 2012 at 09:45:35AM +0100, Mike Galbraith wrote:
> > > On Fri, 2012-03-16 at 09:09 +0100, Mike Galbraith wrote:
> > > > On Fri, 2012-03-16 at 08:27 +0100, Mike Galbraith wrote:
> > > > > On Thu, 2012-03-15 at 12:59 -0500, Dimitri Sivanich wrote:
> > >
> > > > > > Could I try your 3.0 enterprise patch?
> > > > >
> > > > > Sure, v3 below.
> > > >
> > > > Erm, a bit of that went missing. I'll put it back.
> > >
> > > OK, box finally finished rebuild/boot.
> > >
> >
> > This patch also shows great improvement in the two
> > rcu_for_each_node_breadth_first() (nothing over 20 usec and most less than
> > 10 in initial testing).
> >
> > However, there are spinlock holdoffs at the following tracebacks (my nmi
> > handler does work on the 3.0 kernel):
> >
> > [ 584.157019] [<ffffffff8144c5a0>] nmi+0x20/0x30
> > [ 584.157023] [<ffffffff8144bc8a>] _raw_spin_lock_irqsave+0x1a/0x30
> > [ 584.157026] [<ffffffff810c5f18>] force_qs_rnp+0x58/0x170
>
> This is a holdoff, not a deadlock hang, correct?
Correct. Likely < 200 usec.
>
> If so...
>
> Presumably this is the last raw_spin_lock_irqsave() in force_qs_rnp(),
> the one right before the call to rcu_initiate_boost(). Could you please
> verify for me?
Stay tuned. I need to reproduce this without the extra tracecode I had
added around the loops this morning (it -shouldn't- have had an effect..).
>
> If so, someone is holding the root rcu_node structure's lock for longer
> than is good. Or that there is significant contention on that lock,
> which there might well be on large configurations. Any info on who
> is holding or contending for this lock would be very helpful!
>
> Are you running TREE_RCU or TREE_PREEMPT_RCU?
TREE_RCU
>
> Thanx, Paul
>
> > [ 584.157030] [<ffffffff810c6192>] force_quiescent_state+0x162/0x1d0
> > [ 584.157033] [<ffffffff810c6c95>] __rcu_process_callbacks+0x165/0x200
> > [ 584.157037] [<ffffffff810c6d4d>] rcu_process_callbacks+0x1d/0x80
> > [ 584.157041] [<ffffffff81061eaf>] __do_softirq+0xef/0x220
> > [ 584.157044] [<ffffffff81454cbc>] call_softirq+0x1c/0x30
> > [ 584.157048] [<ffffffff810043a5>] do_softirq+0x65/0xa0
> > [ 584.157051] [<ffffffff81061c85>] irq_exit+0xb5/0xe0
> > [ 584.157054] [<ffffffff810212c8>] smp_apic_timer_interrupt+0x68/0xa0
> > [ 584.157057] [<ffffffff81454473>] apic_timer_interrupt+0x13/0x20
> > [ 584.157061] [<ffffffff8102b352>] native_safe_halt+0x2/0x10
> > [ 584.157064] [<ffffffff8100adf5>] default_idle+0x145/0x150
> > [ 584.157067] [<ffffffff810020c6>] cpu_idle+0x66/0xc0
> >
> > >
> > > rcu: Limit GP initialization to CPUs that have been online
> > >
> > > The current grace-period initialization initializes all leaf rcu_node
> > > structures, even those corresponding to CPUs that have never been online.
> > > This is harmless in many configurations, but results in 200-microsecond
> > > latency spikes for kernels built with NR_CPUS=4096.
> > >
> > > This commit therefore keeps track of the largest-numbered CPU that has
> > > ever been online, and limits grace-period initialization to rcu_node
> > > structures corresponding to that CPU and to smaller-numbered CPUs.
> > >
> > > Reported-by: Dimitri Sivanich <sivanich@....com>
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > Acked-by: Mike Galbraith <mgalbraith@...e.de>
> > >
> > > ---
> > > kernel/rcutree.c | 36 ++++++++++++++++++++++++++++++++----
> > > kernel/rcutree.h | 16 ++++++++++++++--
> > > 2 files changed, 46 insertions(+), 6 deletions(-)
> > >
> > > --- a/kernel/rcutree.c
> > > +++ b/kernel/rcutree.c
> > > @@ -84,6 +84,8 @@ DEFINE_PER_CPU(struct rcu_data, rcu_bh_d
> > >
> > > static struct rcu_state *rcu_state;
> > >
> > > +int rcu_max_cpu __read_mostly; /* Largest # CPU that has ever been online. */
> > > +
> > > /*
> > > * The rcu_scheduler_active variable transitions from zero to one just
> > > * before the first task is spawned. So when this variable is zero, RCU
> > > @@ -827,25 +829,31 @@ rcu_start_gp(struct rcu_state *rsp, unsi
> > > struct rcu_node *rnp = rcu_get_root(rsp);
> > >
> > > if (!cpu_needs_another_gp(rsp, rdp) || rsp->fqs_active) {
> > > + struct rcu_node *rnp_root = rnp;
> > > +
> > > if (cpu_needs_another_gp(rsp, rdp))
> > > rsp->fqs_need_gp = 1;
> > > if (rnp->completed == rsp->completed) {
> > > - raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > > + raw_spin_unlock_irqrestore(&rnp_root->lock, flags);
> > > return;
> > > }
> > > - raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> > >
> > > /*
> > > * Propagate new ->completed value to rcu_node structures
> > > * so that other CPUs don't have to wait until the start
> > > * of the next grace period to process their callbacks.
> > > + * We must hold the root rcu_node structure's ->lock
> > > + * across rcu_for_each_node_breadth_first() in order to
> > > + * synchronize with CPUs coming online for the first time.
> > > */
> > > rcu_for_each_node_breadth_first(rsp, rnp) {
> > > + raw_spin_unlock(&rnp_root->lock); /* remain disabled. */
> > > raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> > > rnp->completed = rsp->completed;
> > > raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> > > + raw_spin_lock(&rnp_root->lock); /* already disabled. */
> > > }
> > > - local_irq_restore(flags);
> > > + raw_spin_unlock_irqrestore(&rnp_root->lock, flags);
> > > return;
> > > }
> > >
> > > @@ -935,7 +943,7 @@ static void rcu_report_qs_rsp(struct rcu
> > > rsp->gp_max = gp_duration;
> > > rsp->completed = rsp->gpnum;
> > > rsp->signaled = RCU_GP_IDLE;
> > > - rcu_start_gp(rsp, flags); /* releases root node's rnp->lock. */
> > > + rcu_start_gp(rsp, flags); /* releases root node's ->lock. */
> > > }
> > >
> > > /*
> > > @@ -1862,6 +1870,7 @@ rcu_init_percpu_data(int cpu, struct rcu
> > > unsigned long mask;
> > > struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
> > > struct rcu_node *rnp = rcu_get_root(rsp);
> > > + struct rcu_node *rnp_init;
> > >
> > > /* Set up local state, ensuring consistent view of global state. */
> > > raw_spin_lock_irqsave(&rnp->lock, flags);
> > > @@ -1882,6 +1891,20 @@ rcu_init_percpu_data(int cpu, struct rcu
> > > /* Exclude any attempts to start a new GP on large systems. */
> > > raw_spin_lock(&rsp->onofflock); /* irqs already disabled. */
> > >
> > > + /*
> > > + * Initialize any rcu_node structures that will see their first use.
> > > + * Note that rcu_max_cpu cannot change out from under us because the
> > > + * hotplug locks are held.
> > > + */
> > > + raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> > > + for (rnp_init = per_cpu_ptr(rsp->rda, rcu_max_cpu)->mynode + 1;
> > > + rnp_init <= rdp->mynode;
> > > + rnp_init++) {
> > > + rnp_init->gpnum = rsp->gpnum;
> > > + rnp_init->completed = rsp->completed;
> > > + }
> > > + raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> > > +
> > > /* Add CPU to rcu_node bitmasks. */
> > > rnp = rdp->mynode;
> > > mask = rdp->grpmask;
> > > @@ -1907,6 +1930,11 @@ static void __cpuinit rcu_prepare_cpu(in
> > > rcu_init_percpu_data(cpu, &rcu_sched_state, 0);
> > > rcu_init_percpu_data(cpu, &rcu_bh_state, 0);
> > > rcu_preempt_init_percpu_data(cpu);
> > > + if (cpu > rcu_max_cpu) {
> > > + smp_mb(); /* Initialization before rcu_max_cpu assignment. */
> > > + rcu_max_cpu = cpu;
> > > + smp_mb(); /* rcu_max_cpu assignment before later uses. */
> > > + }
> > > }
> > >
> > > /*
> > > --- a/kernel/rcutree.h
> > > +++ b/kernel/rcutree.h
> > > @@ -191,11 +191,23 @@ struct rcu_node {
> > >
> > > /*
> > > * Do a full breadth-first scan of the rcu_node structures for the
> > > - * specified rcu_state structure.
> > > + * specified rcu_state structure. The caller must hold either the
> > > + * ->onofflock or the root rcu_node structure's ->lock.
> > > */
> > > +extern int rcu_max_cpu;
> > > +static inline int rcu_get_max_cpu(void)
> > > +{
> > > + int ret;
> > > +
> > > + smp_mb(); /* Pairs with barriers in rcu_prepare_cpu(). */
> > > + ret = rcu_max_cpu;
> > > + smp_mb(); /* Pairs with barriers in rcu_prepare_cpu(). */
> > > + return ret;
> > > +}
> > > #define rcu_for_each_node_breadth_first(rsp, rnp) \
> > > for ((rnp) = &(rsp)->node[0]; \
> > > - (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
> > > + (rnp) <= per_cpu_ptr((rsp)->rda, rcu_get_max_cpu())->mynode; \
> > > + (rnp)++)
> > >
> > > /*
> > > * Do a breadth-first scan of the non-leaf rcu_node structures for the
> > >
> >
--
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