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Message-ID: <20080903153446.44f2cb64@bull.net>
Date: Wed, 3 Sep 2008 15:34:46 +0200
From: Sebastien Dugue <sebastien.dugue@...l.net>
To: benh@...nel.crashing.org
Cc: linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
linux-rt-users@...r.kernel.org, paulus@...ba.org,
michael@...erman.id.au, jean-pierre.dion@...l.net,
gilles.carry@....bull.net, tinytim@...ibm.com, tglx@...utronix.de,
rostedt@...dmis.org, dwalker@...sta.com
Subject: Re: [PATCH 2/2] powerpc - Make the irq reverse mapping radix tree
lockless
On Wed, 20 Aug 2008 15:22:06 +1000 Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:
> On Wed, 2008-08-06 at 15:30 +0200, Sebastien Dugue wrote:
> > The radix trees used by interrupt controllers for their irq reverse mapping
> > (currently only the XICS found on pSeries) have a complex locking scheme
> > dating back to before the advent of the lockless radix tree.
> >
> > Take advantage of this and of the fact that the items of the tree are
> > pointers to a static array (irq_map) elements which can never go under us
> > to simplify the locking.
> >
> > Concurrency between readers and writers is handled by the intrinsic
> > properties of the lockless radix tree. Concurrency between writers is handled
> > with a spinlock added to the irq_host structure.
>
> No need for a spinlock in the irq_host. Make it one global lock, it's
> not like scalability of irq_create_mapping() was a big deal and there's
> usually only one of those type of hosts anyway.
Right, done.
Thanks,
Sebastien.
>
> >
> > Signed-off-by: Sebastien Dugue <sebastien.dugue@...l.net>
> > Cc: Paul Mackerras <paulus@...ba.org>
> > Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > Cc: Michael Ellerman <michael@...erman.id.au>
> > ---
> > arch/powerpc/include/asm/irq.h | 1 +
> > arch/powerpc/kernel/irq.c | 74 ++++++----------------------------------
> > 2 files changed, 12 insertions(+), 63 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/irq.h b/arch/powerpc/include/asm/irq.h
> > index 0a51376..72fd036 100644
> > --- a/arch/powerpc/include/asm/irq.h
> > +++ b/arch/powerpc/include/asm/irq.h
> > @@ -119,6 +119,7 @@ struct irq_host {
> > } linear;
> > struct radix_tree_root tree;
> > } revmap_data;
> > + spinlock_t tree_lock;
> > struct irq_host_ops *ops;
> > void *host_data;
> > irq_hw_number_t inval_irq;
> > diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
> > index dc8663a..7a19103 100644
> > --- a/arch/powerpc/kernel/irq.c
> > +++ b/arch/powerpc/kernel/irq.c
> > @@ -439,8 +439,6 @@ void do_softirq(void)
> >
> > static LIST_HEAD(irq_hosts);
> > static DEFINE_SPINLOCK(irq_big_lock);
> > -static DEFINE_PER_CPU(unsigned int, irq_radix_reader);
> > -static unsigned int irq_radix_writer;
> > static atomic_t revmap_trees_allocated = ATOMIC_INIT(0);
> > struct irq_map_entry irq_map[NR_IRQS];
> > static unsigned int irq_virq_count = NR_IRQS;
> > @@ -584,57 +582,6 @@ void irq_set_virq_count(unsigned int count)
> > irq_virq_count = count;
> > }
> >
> > -/* radix tree not lockless safe ! we use a brlock-type mecanism
> > - * for now, until we can use a lockless radix tree
> > - */
> > -static void irq_radix_wrlock(unsigned long *flags)
> > -{
> > - unsigned int cpu, ok;
> > -
> > - spin_lock_irqsave(&irq_big_lock, *flags);
> > - irq_radix_writer = 1;
> > - smp_mb();
> > - do {
> > - barrier();
> > - ok = 1;
> > - for_each_possible_cpu(cpu) {
> > - if (per_cpu(irq_radix_reader, cpu)) {
> > - ok = 0;
> > - break;
> > - }
> > - }
> > - if (!ok)
> > - cpu_relax();
> > - } while(!ok);
> > -}
> > -
> > -static void irq_radix_wrunlock(unsigned long flags)
> > -{
> > - smp_wmb();
> > - irq_radix_writer = 0;
> > - spin_unlock_irqrestore(&irq_big_lock, flags);
> > -}
> > -
> > -static void irq_radix_rdlock(unsigned long *flags)
> > -{
> > - local_irq_save(*flags);
> > - __get_cpu_var(irq_radix_reader) = 1;
> > - smp_mb();
> > - if (likely(irq_radix_writer == 0))
> > - return;
> > - __get_cpu_var(irq_radix_reader) = 0;
> > - smp_wmb();
> > - spin_lock(&irq_big_lock);
> > - __get_cpu_var(irq_radix_reader) = 1;
> > - spin_unlock(&irq_big_lock);
> > -}
> > -
> > -static void irq_radix_rdunlock(unsigned long flags)
> > -{
> > - __get_cpu_var(irq_radix_reader) = 0;
> > - local_irq_restore(flags);
> > -}
> > -
> > static int irq_setup_virq(struct irq_host *host, unsigned int virq,
> > irq_hw_number_t hwirq)
> > {
> > @@ -789,7 +736,6 @@ void irq_dispose_mapping(unsigned int virq)
> > {
> > struct irq_host *host;
> > irq_hw_number_t hwirq;
> > - unsigned long flags;
> >
> > if (virq == NO_IRQ)
> > return;
> > @@ -825,9 +771,9 @@ void irq_dispose_mapping(unsigned int virq)
> > /* Check if radix tree allocated yet */
> > if (atomic_read(&revmap_trees_allocated) == 0)
> > break;
> > - irq_radix_wrlock(&flags);
> > + spin_lock(&host->tree_lock);
> > radix_tree_delete(&host->revmap_data.tree, hwirq);
> > - irq_radix_wrunlock(flags);
> > + spin_unlock(&host->tree_lock);
> > break;
> > }
> >
> > @@ -881,7 +827,6 @@ unsigned int irq_radix_revmap_lookup(struct irq_host *host,
> > {
> > struct irq_map_entry *ptr;
> > unsigned int virq = NO_IRQ;
> > - unsigned long flags;
> >
> > WARN_ON(host->revmap_type != IRQ_HOST_MAP_TREE);
> >
> > @@ -893,9 +838,11 @@ unsigned int irq_radix_revmap_lookup(struct irq_host *host,
> > return irq_find_mapping(host, hwirq);
> >
> > /* Now try to resolve */
> > - irq_radix_rdlock(&flags);
> > + /*
> > + * No rcu_read_lock(ing) needed, the ptr returned can't go under us
> > + * as it's referencing an entry in the static irq_map table.
> > + */
> > ptr = radix_tree_lookup(&host->revmap_data.tree, hwirq);
> > - irq_radix_rdunlock(flags);
> >
> > /* Found it, return */
> > if (ptr)
> > @@ -907,7 +854,6 @@ unsigned int irq_radix_revmap_lookup(struct irq_host *host,
> > void irq_radix_revmap_insert(struct irq_host *host, unsigned int virq,
> > irq_hw_number_t hwirq)
> > {
> > - unsigned long flags;
> >
> > WARN_ON(host->revmap_type != IRQ_HOST_MAP_TREE);
> >
> > @@ -920,10 +866,10 @@ void irq_radix_revmap_insert(struct irq_host *host, unsigned int virq,
> > return;
> >
> > if (virq != NO_IRQ) {
> > - irq_radix_wrlock(&flags);
> > + spin_lock(&host->tree_lock);
> > radix_tree_insert(&host->revmap_data.tree, hwirq,
> > &irq_map[virq]);
> > - irq_radix_wrunlock(flags);
> > + spin_unlock(&host->tree_lock);
> > }
> > }
> >
> > @@ -1041,8 +987,10 @@ static int irq_late_init(void)
> > * revmap_trees_allocated.
> > */
> > list_for_each_entry(h, &irq_hosts, link) {
> > - if (h->revmap_type == IRQ_HOST_MAP_TREE)
> > + if (h->revmap_type == IRQ_HOST_MAP_TREE) {
> > INIT_RADIX_TREE(&h->revmap_data.tree, GFP_ATOMIC);
> > + spin_lock_init(&h->tree_lock);
> > + }
> > }
> >
> > /*
>
>
--
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