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Date: Sun, 07 Dec 2008 19:30:10 +0100
From: Eric Dumazet <dada1@...mosbay.com>
To: Andrew Morton <akpm@...ux-foundation.org>
CC: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Mike Travis <travis@....com>,
"David S. Miller" <davem@...emloft.net>,
linux kernel <linux-kernel@...r.kernel.org>,
Christoph Lameter <cl@...ux-foundation.org>,
linux-ext4@...r.kernel.org
Subject: Re: [PATCH, take2] percpu_counter: FBC_BATCH might be too big
Andrew Morton a écrit :
> On Sun, 07 Dec 2008 10:25:45 +0100 Eric Dumazet <dada1@...mosbay.com> wrote:
>
>> In this second version I guarded hotcpu_notifier() call by
>> a #ifdef CONFIG_HOTPLUG_CPU
>>
>> I wonder why hotcpu_notifier() is not a null op if !CONFIG_HOTPLUG_CPU
>>
>> Thank you
>>
>> [PATCH] percpu_counter: FBC_BATCH might be too big
>>
>> For NR_CPUS >= 16 values, FBC_BATCH is 2*NR_CPUS
>>
>> Considering more and more distros are using high NR_CPUS values,
>> it makes sense to use a more sensible value for FBC_BATCH.
>>
>> A sensible value is 2*num_online_cpus(), with a minimum value of 32
>> (This minimum value helps branch prediction in __percpu_counter_add())
>>
>> We already have a hotcpu notifier, so we can adjust FBC_BATCH dynamically.
>
> Yup, anything using NR_CPUS is probably wrong.
>
>> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
>> index 9007ccd..c42a184 100644
>> --- a/include/linux/percpu_counter.h
>> +++ b/include/linux/percpu_counter.h
>> @@ -24,11 +24,7 @@ struct percpu_counter {
>> s32 *counters;
>> };
>>
>> -#if NR_CPUS >= 16
>> -#define FBC_BATCH (NR_CPUS*2)
>> -#else
>> -#define FBC_BATCH (NR_CPUS*4)
>> -#endif
>> +extern int FBC_BATCH;
>
> y:/usr/src/linux-2.6.28-rc7> grep -r FBC_BATCH . | wc -l
> 7
>
> Can we fix this properly please? It should now become lower case, and
> it was a pretty dopey name anyway - now would be a good time to improve
> it. `percpu_counter_batch'?
Yes
>
>> int percpu_counter_init(struct percpu_counter *fbc, s64 amount);
>> int percpu_counter_init_irq(struct percpu_counter *fbc, s64 amount);
>> diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
>> index a866389..e21ce7c 100644
>> --- a/lib/percpu_counter.c
>> +++ b/lib/percpu_counter.c
>> @@ -114,6 +114,16 @@ void percpu_counter_destroy(struct percpu_counter *fbc)
>> }
>> EXPORT_SYMBOL(percpu_counter_destroy);
>>
>> +int FBC_BATCH __read_mostly = 32;
>> +EXPORT_SYMBOL(FBC_BATCH);
>> +
>> +static void compute_batch_value(void)
>> +{
>> + int nr = num_online_cpus();
>> +
>> + FBC_BATCH = max(32, nr*2);
>> +}
>> +
>> #ifdef CONFIG_HOTPLUG_CPU
>> static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
>> unsigned long action, void *hcpu)
>> @@ -121,6 +131,7 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
>> unsigned int cpu;
>> struct percpu_counter *fbc;
>>
>> + compute_batch_value();
>> if (action != CPU_DEAD)
>> return NOTIFY_OK;
>>
>> @@ -139,11 +150,14 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
>> mutex_unlock(&percpu_counters_lock);
>> return NOTIFY_OK;
>> }
>> +#endif
>>
>> static int __init percpu_counter_startup(void)
>> {
>> + compute_batch_value();
>> +#ifdef CONFIG_HOTPLUG_CPU
>> hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
>> +#endif
>> return 0;
>> }
>> module_init(percpu_counter_startup);
>> -#endif
>
> hm, now what's going on in there? We should be able to drop the #ifdef
> CONFIG_HOTPLUG_CPU from lib/percpu_counter.c altogether.
> hotcpu_notifier() will do the right thing, and the compiler should
> generate no code for percpu_counter_hotcpu_callback() if
> CONFIG_HOTPLUG_CPU=n.
>
> Do
>
> $EDITOR $(grep -l hotcpu_notifier */*.c)
>
> and you'll see lots of code gets it right, and lots of code gets it wrong.
I see nothing interesting, I must be blind.
lib/percpu_counter.c: In function 'percpu_counter_startup':
lib/percpu_counter.c:158: error: 'percpu_counter_hotcpu_callback' undeclared (first use in this function)
lib/percpu_counter.c:158: error: (Each undeclared identifier is reported only once
lib/percpu_counter.c:158: error: for each function it appears in.)
make[1]: *** [lib/percpu_counter.o] Error 1
static int __init percpu_counter_startup(void)
{
compute_batch_value();
hotcpu_notifier(percpu_counter_hotcpu_callback, 0); << ERROR >>
return 0;
}
module_init(percpu_counter_startup);
# grep hotcpu_notifier include/linux/cpu.h
#define hotcpu_notifier(fn, pri) { \
#define hotcpu_notifier(fn, pri) do { (void)(fn); } while (0) << ERROR >>
If changed to :
static struct notifier_block __cpuinitdata percpu_counter_cpu_notifier = {
.notifier_call = percpu_counter_hotcpu_callback,
};
static int __init percpu_counter_startup(void)
{
compute_batch_value();
register_hotcpu_notifier(&percpu_counter_cpu_notifier);
return 0;
}
module_init(percpu_counter_startup);
Then error is :
lib/percpu_counter.c:156: error: 'percpu_counter_hotcpu_callback' undeclared here (not in a function)
make[1]: *** [lib/percpu_counter.o] Error 1
So the only way seems to add an ugly
#define percpu_counter_hotcpu_callback NULL
Is is what you name "the right thing" ?
Thanks
[PATCH] percpu_counter: FBC_BATCH should be a variable
For NR_CPUS >= 16 values, FBC_BATCH is 2*NR_CPUS
Considering more and more distros are using high NR_CPUS values,
it makes sense to use a more sensible value for FBC_BATCH.
A sensible value is 2*num_online_cpus(), with a minimum value of 32
(This minimum value helps branch prediction in __percpu_counter_add())
We already have a hotcpu notifier, so we can adjust FBC_BATCH dynamically.
We rename FBC_BATCH to percpu_counter_batch since its not a constant
anymore.
Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
Acked-by: David S. Miller <davem@...emloft.net>
Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
fs/ext4/ext4.h | 6 +++---
fs/ext4/inode.c | 2 +-
include/linux/percpu_counter.h | 8 ++------
lib/percpu_counter.c | 16 +++++++++++++++-
4 files changed, 21 insertions(+), 11 deletions(-)
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index b0537c8..6c46c64 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1225,11 +1225,11 @@ do { \
} while (0)
#ifdef CONFIG_SMP
-/* Each CPU can accumulate FBC_BATCH blocks in their local
+/* Each CPU can accumulate percpu_counter_batch blocks in their local
* counters. So we need to make sure we have free blocks more
- * than FBC_BATCH * nr_cpu_ids. Also add a window of 4 times.
+ * than percpu_counter_batch * nr_cpu_ids. Also add a window of 4 times.
*/
-#define EXT4_FREEBLOCKS_WATERMARK (4 * (FBC_BATCH * nr_cpu_ids))
+#define EXT4_FREEBLOCKS_WATERMARK (4 * (percpu_counter_batch * nr_cpu_ids))
#else
#define EXT4_FREEBLOCKS_WATERMARK 0
#endif
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index be21a5a..db661e7 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2497,7 +2497,7 @@ static int ext4_nonda_switch(struct super_block *sb)
/*
* switch to non delalloc mode if we are running low
* on free block. The free block accounting via percpu
- * counters can get slightly wrong with FBC_BATCH getting
+ * counters can get slightly wrong with percpu_counter_batch getting
* accumulated on each CPU without updating global counters
* Delalloc need an accurate free block accounting. So switch
* to non delalloc when we are near to error range.
diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 9007ccd..99de7a3 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -24,11 +24,7 @@ struct percpu_counter {
s32 *counters;
};
-#if NR_CPUS >= 16
-#define FBC_BATCH (NR_CPUS*2)
-#else
-#define FBC_BATCH (NR_CPUS*4)
-#endif
+extern int percpu_counter_batch;
int percpu_counter_init(struct percpu_counter *fbc, s64 amount);
int percpu_counter_init_irq(struct percpu_counter *fbc, s64 amount);
@@ -39,7 +35,7 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc);
static inline void percpu_counter_add(struct percpu_counter *fbc, s64 amount)
{
- __percpu_counter_add(fbc, amount, FBC_BATCH);
+ __percpu_counter_add(fbc, amount, percpu_counter_batch);
}
static inline s64 percpu_counter_sum_positive(struct percpu_counter *fbc)
diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index a866389..519b877 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -114,6 +114,16 @@ void percpu_counter_destroy(struct percpu_counter *fbc)
}
EXPORT_SYMBOL(percpu_counter_destroy);
+int percpu_counter_batch __read_mostly = 32;
+EXPORT_SYMBOL(percpu_counter_batch);
+
+static void compute_batch_value(void)
+{
+ int nr = num_online_cpus();
+
+ percpu_counter_batch = max(32, nr*2);
+}
+
#ifdef CONFIG_HOTPLUG_CPU
static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
unsigned long action, void *hcpu)
@@ -121,6 +131,7 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
unsigned int cpu;
struct percpu_counter *fbc;
+ compute_batch_value();
if (action != CPU_DEAD)
return NOTIFY_OK;
@@ -139,11 +150,14 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
mutex_unlock(&percpu_counters_lock);
return NOTIFY_OK;
}
+#else
+#define percpu_counter_hotcpu_callback NULL
+#endif /* CONFIG_HOTPLUG_CPU */
static int __init percpu_counter_startup(void)
{
+ compute_batch_value();
hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
return 0;
}
module_init(percpu_counter_startup);
-#endif
--
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