On Sat, 2007-04-21 at 14:15 +0200, Peter Zijlstra wrote:
> > > > +/*
> > > > + * maximal error of a stat counter.
> > > > + */
> > > > +static inline unsigned long bdi_stat_delta(void)
> > > > +{
> > > > +#ifdef CONFIG_SMP
> > > > + return NR_CPUS * FBC_BATCH;
> > >
> > > This is enormously wrong for CONFIG_NR_CPUS=1024 on a 2-way.
>
> Right, I knew about that but, uhm.
>
> I wanted to make that num_online_cpus(), and install a hotplug notifier
> to fold the percpu delta back into the total on cpu offline.
>
> But I have to look into doing that hotplug notifier stuff.
Something like this should do I think, I just looked at other hotplug
code and imitated the pattern.
I assumed CONFIG_HOTPLUG_CPU requires CONFIG_SMP, I didn't actually try
that one :-)
---
In order to estimate the per stat counter error more accurately, using
num_online_cpus() instead of NR_CPUS, install a cpu hotplug notifier
(when cpu hotplug is enabled) that flushes whatever percpu delta was
present into the total on cpu unplug.
Signed-off-by: Peter Zijlstra <[email protected]>
---
include/linux/backing-dev.h | 6 ++++-
include/linux/percpu_counter.h | 1
lib/percpu_counter.c | 11 +++++++++
mm/backing-dev.c | 47 +++++++++++++++++++++++++++++++++++++++++
4 files changed, 64 insertions(+), 1 deletion(-)
Index: linux-2.6/include/linux/backing-dev.h
===================================================================
--- linux-2.6.orig/include/linux/backing-dev.h 2007-04-21 21:32:49.000000000 +0200
+++ linux-2.6/include/linux/backing-dev.h 2007-04-21 21:33:28.000000000 +0200
@@ -51,6 +51,10 @@ struct backing_dev_info {
spinlock_t lock; /* protect the cycle count */
unsigned long cycles; /* writeout cycles */
int dirty_exceeded;
+
+#ifdef CONFIG_HOTPLUG_CPU
+ struct notifier_block hotplug_nb;
+#endif
};
void bdi_init(struct backing_dev_info *bdi);
@@ -137,7 +141,7 @@ static inline s64 bdi_stat_sum(struct ba
static inline unsigned long bdi_stat_delta(void)
{
#ifdef CONFIG_SMP
- return NR_CPUS * FBC_BATCH;
+ return num_online_cpus() * FBC_BATCH;
#else
return 1UL;
#endif
Index: linux-2.6/include/linux/percpu_counter.h
===================================================================
--- linux-2.6.orig/include/linux/percpu_counter.h 2007-04-21 21:32:49.000000000 +0200
+++ linux-2.6/include/linux/percpu_counter.h 2007-04-21 21:33:17.000000000 +0200
@@ -38,6 +38,7 @@ static inline void percpu_counter_destro
void percpu_counter_mod(struct percpu_counter *fbc, s32 amount);
void percpu_counter_mod64(struct percpu_counter *fbc, s64 amount);
s64 percpu_counter_sum(struct percpu_counter *fbc);
+void percpu_counter_fold(struct percpu_counter *fbx, int cpu);
static inline s64 percpu_counter_read(struct percpu_counter *fbc)
{
Index: linux-2.6/lib/percpu_counter.c
===================================================================
--- linux-2.6.orig/lib/percpu_counter.c 2007-04-21 21:32:49.000000000 +0200
+++ linux-2.6/lib/percpu_counter.c 2007-04-21 21:33:17.000000000 +0200
@@ -72,3 +72,14 @@ s64 percpu_counter_sum(struct percpu_cou
return ret < 0 ? 0 : ret;
}
EXPORT_SYMBOL(percpu_counter_sum);
+
+void percpu_counter_fold(struct percpu_counter *fbc, int cpu)
+{
+ s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
+ if (*pcount) {
+ spin_lock(&fbc->lock);
+ fbc->count += *pcount;
+ *pcount = 0;
+ spin_unlock(&fbc->lock);
+ }
+}
Index: linux-2.6/mm/backing-dev.c
===================================================================
--- linux-2.6.orig/mm/backing-dev.c 2007-04-21 21:32:49.000000000 +0200
+++ linux-2.6/mm/backing-dev.c 2007-04-21 21:34:47.000000000 +0200
@@ -4,6 +4,49 @@
#include <linux/fs.h>
#include <linux/sched.h>
#include <linux/module.h>
+#include <linux/cpu.h>
+
+#ifdef CONFIG_HOTPLUG_CPU
+static int bdi_stat_fold(struct notifier_block *nb,
+ unsigned long action, void *hcpu)
+{
+ struct backing_dev_info *bdi =
+ container_of(nb, struct backing_dev_info, hotplug_nb);
+ unsigned long flags;
+ int cpu = (unsigned long)hcpu;
+ int i;
+
+ if (action == CPU_DEAD) {
+ local_irq_save(flags);
+ for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
+ percpu_counter_fold(&bdi->bdi_stat[i], cpu);
+ local_irq_restore(flags);
+ }
+ return NOTIFY_OK;
+}
+
+static void bdi_init_hotplug(struct backing_dev_info *bdi)
+{
+ bdi->hotplug_nb = (struct notifier_block){
+ .notifier_call = bdi_stat_fold,
+ .priority = 0,
+ };
+ register_hotcpu_notifier(&bdi->hotplug_nb);
+}
+
+static void bdi_destroy_hotplug(struct backing_dev_info *bdi)
+{
+ unregister_hotcpu_notifier(&bdi->hotplug_nb);
+}
+#else
+static void bdi_init_hotplug(struct backing_dev_info *bdi)
+{
+}
+
+static void bdi_destroy_hotplug(struct backing_dev_info *bdi)
+{
+}
+#endif
void bdi_init(struct backing_dev_info *bdi)
{
@@ -17,6 +60,8 @@ void bdi_init(struct backing_dev_info *b
bdi->dirty_exceeded = 0;
for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
percpu_counter_init(&bdi->bdi_stat[i], 0);
+
+ bdi_init_hotplug(bdi);
}
EXPORT_SYMBOL(bdi_init);
@@ -27,6 +72,8 @@ void bdi_destroy(struct backing_dev_info
if (!(bdi_cap_writeback_dirty(bdi) || bdi_cap_account_dirty(bdi)))
return;
+ bdi_destroy_hotplug(bdi);
+
for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
percpu_counter_destroy(&bdi->bdi_stat[i]);
}
-
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