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SubjectRe: [PATCH 1/2] scalable rw_mutex
On Sat, 12 May 2007 11:06:24 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On 12 May 2007 20:55:28 +0200 Andi Kleen <andi@firstfloor.org> wrote:
>
> > Andrew Morton <akpm@linux-foundation.org> writes:
> >
> > > On Fri, 11 May 2007 10:07:17 -0700 (PDT)
> > > Christoph Lameter <clameter@sgi.com> wrote:
> > >
> > > > On Fri, 11 May 2007, Andrew Morton wrote:
> > > >
> > > > > yipes. percpu_counter_sum() is expensive.
> > > >
> > > > Capable of triggering NMI watchdog on 4096+ processors?
> > >
> > > Well. That would be a millisecond per cpu which sounds improbable. And
> > > we'd need to be calling it under local_irq_save() which we presently don't.
> > > And nobody has reported any problems against the existing callsites.
> > >
> > > But it's no speed demon, that's for sure.
> >
> > There is one possible optimization for this I did some time ago. You don't really
> > need to sum all over the possible map, but only all CPUs that were ever
> > online. But this only helps on systems where the possible map is bigger
> > than online map in the common case. But that shouldn't be the case anymore on x86
> > -- it just used to be. If it's true on some other architectures it might
> > be still worth it.
> >
>
> hm, yeah.
>
> We could put a cpumask in percpu_counter, initialise it to
> cpu_possible_map. Then, those callsites which have hotplug notifiers can
> call into new percpu_counter functions which clear and set bits in that
> cpumask and which drain percpu_counter.counts[cpu] into
> percpu_counter.count.
>
> And percpu_counter_sum() gets taught to do for_each_cpu_mask(fbc->cpumask).

Like this:


From: Andrew Morton <akpm@linux-foundation.org>

per-cpu counters presently must iterate over all possible CPUs in the
exhaustive percpu_counter_sum().

But it can be much better to only iterate over the presently-online CPUs. To
do this, we must arrange for an offlined CPU's count to be spilled into the
counter's central count.

We can do this for all percpu_counters in the machine by linking them into a
single global list and walking that list at CPU_DEAD time.

(I hope. Might have race windows in which the percpu_counter_sum() count is
inaccurate?)


Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
include/linux/percpu_counter.h | 18 ++------
lib/percpu_counter.c | 66 +++++++++++++++++++++++++++++++
2 files changed, 72 insertions(+), 12 deletions(-)
diff -puN lib/percpu_counter.c~percpu_counters-use-cpu-notifiers lib/percpu_counter.c
--- a/lib/percpu_counter.c~percpu_counters-use-cpu-notifiers
+++ a/lib/percpu_counter.c
@@ -3,8 +3,17 @@
*/

#include <linux/percpu_counter.h>
+#include <linux/notifier.h>
+#include <linux/mutex.h>
+#include <linux/init.h>
+#include <linux/cpu.h>
#include <linux/module.h>

+#ifdef CONFIG_HOTPLUG_CPU
+static LIST_HEAD(percpu_counters);
+static DEFINE_MUTEX(percpu_counters_lock);
+#endif
+
void percpu_counter_mod(struct percpu_counter *fbc, s32 amount)
{
long count;
@@ -44,3 +53,60 @@ s64 percpu_counter_sum(struct percpu_cou
return ret < 0 ? 0 : ret;
}
EXPORT_SYMBOL(percpu_counter_sum);
+
+void percpu_counter_init(struct percpu_counter *fbc, s64 amount)
+{
+ spin_lock_init(&fbc->lock);
+ fbc->count = amount;
+ fbc->counters = alloc_percpu(s32);
+#ifdef CONFIG_HOTPLUG_CPU
+ mutex_lock(&percpu_counters_lock);
+ list_add(&fbc->list, &percpu_counters);
+ mutex_unlock(&percpu_counters_lock);
+#endif
+}
+EXPORT_SYMBOL(percpu_counter_init);
+
+void percpu_counter_destroy(struct percpu_counter *fbc)
+{
+ free_percpu(fbc->counters);
+#ifdef CONFIG_HOTPLUG_CPU
+ mutex_lock(&percpu_counters_lock);
+ list_del(&fbc->list);
+ mutex_unlock(&percpu_counters_lock);
+#endif
+}
+EXPORT_SYMBOL(percpu_counter_destroy);
+
+#ifdef CONFIG_HOTPLUG_CPU
+static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
+ unsigned long action, void *hcpu)
+{
+ unsigned int cpu;
+ struct percpu_counter *fbc;
+
+ if (action != CPU_DEAD)
+ return NOTIFY_OK;
+
+ cpu = (unsigned long)hcpu;
+ mutex_lock(&percpu_counters_lock);
+ list_for_each_entry(fbc, &percpu_counters, list) {
+ s32 *pcount;
+
+ spin_lock(&fbc->lock);
+ pcount = per_cpu_ptr(fbc->counters, cpu);
+ fbc->count += *pcount;
+ *pcount = 0;
+ spin_unlock(&fbc->lock);
+ }
+ mutex_unlock(&percpu_counters_lock);
+ return NOTIFY_OK;
+}
+
+static int __init percpu_counter_startup(void)
+{
+ hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
+ return 0;
+}
+module_init(percpu_counter_startup);
+#endif
diff -puN include/linux/percpu.h~percpu_counters-use-cpu-notifiers include/linux/percpu.h
diff -puN include/linux/percpu_counter.h~percpu_counters-use-cpu-notifiers include/linux/percpu_counter.h
--- a/include/linux/percpu_counter.h~percpu_counters-use-cpu-notifiers
+++ a/include/linux/percpu_counter.h
@@ -8,6 +8,7 @@

#include <linux/spinlock.h>
#include <linux/smp.h>
+#include <linux/list.h>
#include <linux/threads.h>
#include <linux/percpu.h>
#include <linux/types.h>
@@ -17,6 +18,9 @@
struct percpu_counter {
spinlock_t lock;
s64 count;
+#ifdef CONFIG_HOTPLUG_CPU
+ struct list_head list; /* All percpu_counters are on a list */
+#endif
s32 *counters;
};

@@ -26,18 +30,8 @@ struct percpu_counter {
#define FBC_BATCH (NR_CPUS*4)
#endif

-static inline void percpu_counter_init(struct percpu_counter *fbc, s64 amount)
-{
- spin_lock_init(&fbc->lock);
- fbc->count = amount;
- fbc->counters = alloc_percpu(s32);
-}
-
-static inline void percpu_counter_destroy(struct percpu_counter *fbc)
-{
- free_percpu(fbc->counters);
-}
-
+void percpu_counter_init(struct percpu_counter *fbc, s64 amount);
+void percpu_counter_destroy(struct percpu_counter *fbc);
void percpu_counter_mod(struct percpu_counter *fbc, s32 amount);
s64 percpu_counter_sum(struct percpu_counter *fbc);

_
and then this:


From: Andrew Morton <akpm@linux-foundation.org>

Now that we have implemented hotunplug-time counter spilling,
percpu_counter_sum() only needs to look at online CPus.


Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
lib/percpu_counter.c | 2 +-
1 files changed, 1 insertion(+), 1 deletion(-)
diff -puN lib/percpu_counter.c~percpu_counters-use-for_each_online_cpu lib/percpu_counter.c
--- a/lib/percpu_counter.c~percpu_counters-use-for_each_online_cpu
+++ a/lib/percpu_counter.c
@@ -45,7 +45,7 @@ s64 percpu_counter_sum(struct percpu_cou

spin_lock(&fbc->lock);
ret = fbc->count;
- for_each_possible_cpu(cpu) {
+ for_each_online_cpu(cpu) {
s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
ret += *pcount;
}
_
-
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