Commit 179f7ebf authored by Eric Dumazet's avatar Eric Dumazet Committed by Linus Torvalds

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, and get rid of NR_CPUS.

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
Signed-off-by: default avatarEric Dumazet <>
Acked-by: default avatarDavid S. Miller <>
Acked-by: default avatarPeter Zijlstra <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent e3d5a27d
......@@ -1225,11 +1225,11 @@ do { \
} while (0)
/* 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))
......@@ -2498,7 +2498,7 @@ 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.
......@@ -24,11 +24,7 @@ struct percpu_counter {
s32 *counters;
#if NR_CPUS >= 16
#define FBC_BATCH (NR_CPUS*2)
#define FBC_BATCH (NR_CPUS*4)
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)
......@@ -9,10 +9,8 @@
#include <linux/cpu.h>
#include <linux/module.h>
static LIST_HEAD(percpu_counters);
static DEFINE_MUTEX(percpu_counters_lock);
void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
......@@ -111,13 +109,24 @@ void percpu_counter_destroy(struct percpu_counter *fbc)
int percpu_counter_batch __read_mostly = 32;
static void compute_batch_value(void)
int nr = num_online_cpus();
percpu_counter_batch = max(32, nr*2);
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;
......@@ -134,13 +143,14 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
spin_unlock_irqrestore(&fbc->lock, flags);
return NOTIFY_OK;
static int __init percpu_counter_startup(void)
hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
return 0;
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