On Thu, Feb 02, 2006 at 07:16:00PM -0800, Andrew Morton wrote:
> Ravikiran G Thirumalai <[EMAIL PROTECTED]> wrote:
> >
> > On Fri, Jan 27, 2006 at 03:01:06PM -0800, Andrew Morton wrote:
> > Here's an implementation which delegates tuning of batching to the user. We
> > don't really need local_t at all as percpu_counter_mod is not safe against
> > interrupts and softirqs as it is. If we have a counter which could be
> > modified in process context and irq/bh context, we just have to use a
> > wrapper like percpu_counter_mod_bh which will just disable and enable bottom
> > halves. Reads on the counters are safe as they are atomic_reads, and the
> > cpu local variables are always accessed by that cpu only.
> >
> > (PS: the maxerr for ext2/ext3 is just guesstimate)
>
> Well that's the problem. We need to choose production-quality values for
> use in there.
The guesstimate was loosely based on keeping the per-cpu batch at atleast 8
on reasonably sized systems, while not letting maxerr grow too big. I guess
machines with cpu counts more than 128 don't use ext3 :). And if they do,
they can tune the counters with a higher maxerr. I guess it might be a bit
ugly on the user side with all the if num_possibl_cpus(), but is there a
better alternative?
(I plan to test the counter values for ext2 and ext3 on a 16 way box, and
change these if they turn out to be not so good)
>
> > Comments?
>
> Using num_possible_cpus() in that header file is just asking for build
> errors. Probably best to uninline the function rather than adding the
> needed include of cpumask.h.
Yup,
Here it is.
Change the percpu_counter interface so that user can specify the maximum
tolerable deviation.
Signed-off-by: Ravikiran Thirumalai <[EMAIL PROTECTED]>
Index: linux-2.6.16-rc1mm4/include/linux/percpu_counter.h
===================================================================
--- linux-2.6.16-rc1mm4.orig/include/linux/percpu_counter.h 2006-02-02
11:18:54.000000000 -0800
+++ linux-2.6.16-rc1mm4/include/linux/percpu_counter.h 2006-02-03
11:04:05.000000000 -0800
@@ -16,24 +16,20 @@
struct percpu_counter {
atomic_long_t count;
+ int percpu_batch;
long *counters;
};
-#if NR_CPUS >= 16
-#define FBC_BATCH (NR_CPUS*2)
-#else
-#define FBC_BATCH (NR_CPUS*4)
-#endif
-
-static inline void percpu_counter_init(struct percpu_counter *fbc)
-{
- atomic_long_set(&fbc->count, 0);
- fbc->counters = alloc_percpu(long);
-}
+/*
+ * Choose maxerr carefully. maxerr/num_possible_cpus indicates per-cpu
+ * batching. Set maximum tolerance for better performance on large systems.
+ */
+void percpu_counter_init(struct percpu_counter *fbc, unsigned int maxerr);
static inline void percpu_counter_destroy(struct percpu_counter *fbc)
{
- free_percpu(fbc->counters);
+ if (fbc->percpu_batch)
+ free_percpu(fbc->counters);
}
void percpu_counter_mod(struct percpu_counter *fbc, long amount);
@@ -63,7 +59,8 @@ struct percpu_counter {
long count;
};
-static inline void percpu_counter_init(struct percpu_counter *fbc)
+static inline void percpu_counter_init(struct percpu_counter *fbc,
+ unsigned int maxerr)
{
fbc->count = 0;
}
Index: linux-2.6.16-rc1mm4/mm/swap.c
===================================================================
--- linux-2.6.16-rc1mm4.orig/mm/swap.c 2006-01-29 20:20:20.000000000 -0800
+++ linux-2.6.16-rc1mm4/mm/swap.c 2006-02-03 11:02:05.000000000 -0800
@@ -468,15 +468,39 @@ static int cpu_swap_callback(struct noti
#endif /* CONFIG_SMP */
#ifdef CONFIG_SMP
+
+/*
+ * Choose maxerr carefully. maxerr/num_possible_cpus indicates per-cpu
+ * batching. Set maximum tolerance for better performance on large systems.
+ */
+void percpu_counter_init(struct percpu_counter *fbc, unsigned int maxerr)
+{
+ atomic_long_set(&fbc->count, 0);
+ fbc->percpu_batch = maxerr/num_possible_cpus();
+ if (fbc->percpu_batch) {
+ fbc->counters = alloc_percpu(long);
+ if (!fbc->counters)
+ fbc->percpu_batch = 0;
+ }
+
+}
+
void percpu_counter_mod(struct percpu_counter *fbc, long amount)
{
- long count;
long *pcount;
- int cpu = get_cpu();
+ long count;
+ int cpu;
+ /* Slow mode */
+ if (unlikely(!fbc->percpu_batch)) {
+ atomic_long_add(amount, &fbc->count);
+ return;
+ }
+
+ cpu = get_cpu();
pcount = per_cpu_ptr(fbc->counters, cpu);
count = *pcount + amount;
- if (count >= FBC_BATCH || count <= -FBC_BATCH) {
+ if (count >= fbc->percpu_batch || count <= -fbc->percpu_batch) {
atomic_long_add(count, &fbc->count);
count = 0;
}
@@ -484,6 +508,7 @@ void percpu_counter_mod(struct percpu_co
put_cpu();
}
EXPORT_SYMBOL(percpu_counter_mod);
+EXPORT_SYMBOL(percpu_counter_init);
#endif
/*
Index: linux-2.6.16-rc1mm4/fs/ext2/super.c
===================================================================
--- linux-2.6.16-rc1mm4.orig/fs/ext2/super.c 2006-02-03 11:03:54.000000000
-0800
+++ linux-2.6.16-rc1mm4/fs/ext2/super.c 2006-02-03 11:04:10.000000000 -0800
@@ -610,6 +610,7 @@ static int ext2_fill_super(struct super_
int db_count;
int i, j;
__le32 features;
+ int maxerr;
sbi = kmalloc(sizeof(*sbi), GFP_KERNEL);
if (!sbi)
@@ -835,9 +836,14 @@ static int ext2_fill_super(struct super_
printk ("EXT2-fs: not enough memory\n");
goto failed_mount;
}
- percpu_counter_init(&sbi->s_freeblocks_counter);
- percpu_counter_init(&sbi->s_freeinodes_counter);
- percpu_counter_init(&sbi->s_dirs_counter);
+
+ if (num_possible_cpus() <= 16 )
+ maxerr = 256;
+ else
+ maxerr = 1024;
+ percpu_counter_init(&sbi->s_freeblocks_counter, maxerr);
+ percpu_counter_init(&sbi->s_freeinodes_counter, maxerr);
+ percpu_counter_init(&sbi->s_dirs_counter, maxerr);
bgl_lock_init(&sbi->s_blockgroup_lock);
sbi->s_debts = kmalloc(sbi->s_groups_count * sizeof(*sbi->s_debts),
GFP_KERNEL);
Index: linux-2.6.16-rc1mm4/fs/ext3/super.c
===================================================================
--- linux-2.6.16-rc1mm4.orig/fs/ext3/super.c 2006-02-03 11:03:54.000000000
-0800
+++ linux-2.6.16-rc1mm4/fs/ext3/super.c 2006-02-03 11:04:10.000000000 -0800
@@ -1353,6 +1353,7 @@ static int ext3_fill_super (struct super
int i;
int needs_recovery;
__le32 features;
+ int maxerr;
sbi = kmalloc(sizeof(*sbi), GFP_KERNEL);
if (!sbi)
@@ -1578,9 +1579,14 @@ static int ext3_fill_super (struct super
goto failed_mount;
}
- percpu_counter_init(&sbi->s_freeblocks_counter);
- percpu_counter_init(&sbi->s_freeinodes_counter);
- percpu_counter_init(&sbi->s_dirs_counter);
+ if (num_possible_cpus() <= 16)
+ maxerr = 256;
+ else
+ maxerr = 1024;
+
+ percpu_counter_init(&sbi->s_freeblocks_counter, maxerr);
+ percpu_counter_init(&sbi->s_freeinodes_counter, maxerr);
+ percpu_counter_init(&sbi->s_dirs_counter, maxerr);
bgl_lock_init(&sbi->s_blockgroup_lock);
for (i = 0; i < db_count; i++) {
-
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