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SubjectRe: [patch 3/4] net: Percpufy frequently used variables -- proto.sockets_allocated
On Fri, Jan 27, 2006 at 03:01:06PM -0800, Andrew Morton wrote:
> Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
>
>
> > >
> > > If the benchmarks say that we need to. If we cannot observe any problems
> > > in testing of existing code and if we can't demonstrate any benefit from
> > > the patched code then one option is to go off and do something else ;)
> >
> > We first tried plain per-CPU counters for memory_allocated, found that reads
> > on memory_allocated was causing cacheline transfers, and then
> > switched over to batching. So batching reads is useful. To avoid
> > inaccuracy, we can maybe change percpu_counter_init to:
> >
> > void percpu_counter_init(struct percpu_counter *fbc, int maxdev)
> >
> > the percpu batching limit would then be maxdev/num_possible_cpus. One would
> > use batching counters only when both reads and writes are frequent. With
> > the above scheme, we would go fetch cachelines from other cpus for read
> > often only on large cpu counts, which is not any worse than the global
> > counter alternative, but it would still be beneficial on smaller machines,
> > without sacrificing a pre-set deviation.
> >
> > Comments?
>
> Sounds sane.
>

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)

Comments?

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 2006-02-02 18:29:46.000000000 -0800
@@ -16,24 +16,32 @@

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)
+/*
+ * Choose maxerr carefully. maxerr/num_possible_cpus indicates per-cpu batching
+ * Set maximum tolerance for better performance on large systems.
+ */
+static inline void percpu_counter_init(struct percpu_counter *fbc,
+ unsigned int maxerr)
{
atomic_long_set(&fbc->count, 0);
- fbc->counters = alloc_percpu(long);
+ fbc->percpu_batch = maxerr/num_possible_cpus();
+ if (fbc->percpu_batch) {
+ fbc->counters = alloc_percpu(long);
+ if (!fbc->counters)
+ fbc->percpu_batch = 0;
+ }
+
}

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 +71,8 @@ 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 2006-02-02 18:36:21.000000000 -0800
@@ -470,13 +470,20 @@ static int cpu_swap_callback(struct noti
#ifdef CONFIG_SMP
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;
}
Index: linux-2.6.16-rc1mm4/fs/ext2/super.c
===================================================================
--- linux-2.6.16-rc1mm4.orig/fs/ext2/super.c 2006-02-02 18:30:28.000000000 -0800
+++ linux-2.6.16-rc1mm4/fs/ext2/super.c 2006-02-02 18:36:39.000000000 -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 2006-02-02 18:30:28.000000000 -0800
+++ linux-2.6.16-rc1mm4/fs/ext3/super.c 2006-02-02 18:38:10.000000000 -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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