Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

Rusty Russell
2008-12-15T12:54:17Z

On Wednesday 10 December 2008 16:19:21 Andrew Morton wrote:
> Rusty, Christoph: talk to me. If we add a new user of local_t in core
> kernel, will we regret it?

Interesting. There's new local_t infrastructure, but it's not used.

Here's a benchmark patch showing some results. x86 is pretty close to
optimal already, though my results on Power show atomic_long is a bad choice
there.

I'll do an audit of the users, then send out some local_t cleanup patches etc.

Benchmarks for local_t variants

(This patch also fixes the x86 cpu_local_* macros, which are obviously
unused).

I chose a large array (1M longs) for the inc/add/add_return tests so
the trivalue case would show some cache pressure.

The cpu_local_inc case is always cache-hot, so it's not comparable to
the others.

Time in ns per iteration (brackets is with CONFIG_PREEMPT=y):

inc add add_return cpu_local_inc read
x86-32: 2.13 Ghz Core Duo 2
atomic_long 118 118 115 17 17
irqsave/rest 77 78 77 23 16
trivalue 45 45 127 3(6) 21
local_t 36 36 36 1(5) 17

x86-64: 2.6 GHz Dual-Core AMD Opteron 2218
atomic_long 55 60 - 6 19
irqsave/rest 54 54 - 11 19
trivalue 47 47 - 5 28
local_t 47 46 - 1 19

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
arch/x86/include/asm/local.h | 20 ++--
init/main.c | 198 +++++++++++++++++++++++++++++++++++++++++++
2 files changed, 208 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h
--- a/arch/x86/include/asm/local.h
+++ b/arch/x86/include/asm/local.h
@@ -220,16 +220,16 @@ static inline long local_sub_return(long
preempt_enable(); \
}) \

-#define cpu_local_read(l) cpu_local_wrap_v(local_read(&__get_cpu_var((l))))
-#define cpu_local_set(l, i) cpu_local_wrap(local_set(&__get_cpu_var((l)), (i)))
-#define cpu_local_inc(l) cpu_local_wrap(local_inc(&__get_cpu_var((l))))
-#define cpu_local_dec(l) cpu_local_wrap(local_dec(&__get_cpu_var((l))))
-#define cpu_local_add(i, l) cpu_local_wrap(local_add((i), &__get_cpu_var((l))))
-#define cpu_local_sub(i, l) cpu_local_wrap(local_sub((i), &__get_cpu_var((l))))
+#define cpu_local_read(l) cpu_local_wrap_v(local_read(&__get_cpu_var(l)))
+#define cpu_local_set(l, i) cpu_local_wrap(local_set(&__get_cpu_var(l), (i)))
+#define cpu_local_inc(l) cpu_local_wrap(local_inc(&__get_cpu_var(l)))
+#define cpu_local_dec(l) cpu_local_wrap(local_dec(&__get_cpu_var(l)))
+#define cpu_local_add(i, l) cpu_local_wrap(local_add((i), &__get_cpu_var(l)))
+#define cpu_local_sub(i, l) cpu_local_wrap(local_sub((i), &__get_cpu_var(l)))

-#define __cpu_local_inc(l) cpu_local_inc((l))
-#define __cpu_local_dec(l) cpu_local_dec((l))
-#define __cpu_local_add(i, l) cpu_local_add((i), (l))
-#define __cpu_local_sub(i, l) cpu_local_sub((i), (l))
+#define __cpu_local_inc(l) cpu_local_inc(l)
+#define __cpu_local_dec(l) cpu_local_dec(l)
+#define __cpu_local_add(i, l) cpu_local_add((i), l)
+#define __cpu_local_sub(i, l) cpu_local_sub((i), l)

#endif /* _ASM_X86_LOCAL_H */
diff --git a/init/main.c b/init/main.c
--- a/init/main.c
+++ b/init/main.c
@@ -539,6 +539,200 @@ void __init __weak thread_info_cache_ini
{
}

+/* There are three obvious ways to implement local_t on an arch which
+ * can't do single-instruction inc/dec etc.
+ * 1) atomic_long
+ * 2) irq_save/irq_restore
+ * 3) multiple counters.
+ *
+ * This does a very rough benchmark on each one.
+ */
+struct local1 {
+ atomic_long_t v;
+};
+struct local2 {
+ unsigned long v;
+};
+struct local3 {
+ unsigned long v[3];
+};
+
+/* Enough to put some pressure on the caches. */
+#define NUM_LOCAL_TEST (1024*1024)
+#define NUM_LOCAL_RUNS (NUM_LOCAL_TEST*32)
+/* This will make it jump around looking random */
+#define STRIDE 514001
+
+static void *test_local_variants_mem;
+
+static void init_test_local_variants(void)
+{
+ unsigned long size;
+ size = max(sizeof(struct local1),
+ max(sizeof(struct local2),
+ max(sizeof(struct local3), sizeof(local_t))))
+ * NUM_LOCAL_TEST;
+ /* Assume this works in early boot. */
+ test_local_variants_mem = alloc_bootmem_nopanic(size);
+
+ if (!test_local_variants_mem) {
+ printk("test_local_variants: failed to allocate %lu bytes\n",
+ size);
+ return;
+ }
+}
+
+static void print_result(const char *str,
+ struct timespec start, struct timespec end)
+{
+ s64 diff;
+
+ diff = ktime_to_ns(ktime_sub(timespec_to_ktime(end), timespec_to_ktime(start)));
+ printk("%s=%lli/%lli ",
+ str, diff, diff/NUM_LOCAL_RUNS);
+}
+
+static unsigned int warm_local_test_cache(const void *mem, size_t len)
+{
+ unsigned int i, total = 0;
+ for (i = 0; i < len; i++)
+ total += ((char *)mem)[i];
+ return total;
+}
+
+#define TEST_LOOP(expr) \
+ n = 0; \
+ getnstimeofday(&start); \
+ for (i = 0; i < NUM_LOCAL_RUNS; i++) { \
+ expr; \
+ n += STRIDE; \
+ n %= NUM_LOCAL_TEST; \
+ } \
+ getnstimeofday(&end);
+
+/* This doesn't test cache effects at all */
+#define NUM_PERCPU_VARS 16
+DEFINE_PER_CPU(struct local1[NUM_PERCPU_VARS], local1_test);
+DEFINE_PER_CPU(struct local2[NUM_PERCPU_VARS], local2_test);
+DEFINE_PER_CPU(struct local3[NUM_PERCPU_VARS], local3_test);
+DEFINE_PER_CPU(local_t[NUM_PERCPU_VARS], local4_test);
+
+static void test_local_variants(void)
+{
+ struct timespec start, end;
+ unsigned int i, n;
+ unsigned long total, warm_total = 0;
+ struct local1 *l1;
+ struct local2 *l2;
+ struct local3 *l3;
+ local_t *l4;
+
+ if (!test_local_variants_mem)
+ return;
+
+ printk("Running local_t variant benchmarks\n");
+ l1 = test_local_variants_mem;
+ l2 = test_local_variants_mem;
+ l3 = test_local_variants_mem;
+ l4 = test_local_variants_mem;
+
+ printk("atomic_long: ");
+ memset(l1, 0, sizeof(*l1)*NUM_LOCAL_TEST);
+ TEST_LOOP(atomic_long_inc(&l1[n].v));
+ print_result("local_inc", start, end);
+
+ warm_total += warm_local_test_cache(l1, sizeof(*l1)*NUM_LOCAL_TEST);
+ TEST_LOOP(atomic_long_add(1234, &l1[n].v));
+ print_result("local_add", start, end);
+
+ warm_total += warm_local_test_cache(l1, sizeof(*l1)*NUM_LOCAL_TEST);
+ TEST_LOOP(atomic_long_inc(&__get_cpu_var(local1_test)[n%NUM_PERCPU_VARS].v));
+ print_result("cpu_local_inc", start, end);
+
+ warm_total += warm_local_test_cache(l1, sizeof(*l1)*NUM_LOCAL_TEST);
+ total = 0;
+ TEST_LOOP(total += atomic_long_read(&l1[n].v));
+ print_result("local_read", start, end);
+
+ printk("(total was %lu)\n", total);
+
+ printk("irqsave/restore: ");
+ memset(l2, 0, sizeof(*l2)*NUM_LOCAL_TEST);
+ TEST_LOOP(unsigned long flags;
+ local_irq_save(flags);
+ l2[n].v++;
+ local_irq_restore(flags));
+ print_result("local_inc", start, end);
+
+ warm_total += warm_local_test_cache(l2, sizeof(*l2)*NUM_LOCAL_TEST);
+ TEST_LOOP(unsigned long flags;
+ local_irq_save(flags);
+ l2[n].v += 1234;
+ local_irq_restore(flags));
+ print_result("local_add", start, end);
+
+ warm_total += warm_local_test_cache(l2, sizeof(*l2)*NUM_LOCAL_TEST);
+ TEST_LOOP(unsigned long flags;
+ local_irq_save(flags);
+ __get_cpu_var(local2_test)[n%NUM_PERCPU_VARS].v++;
+ local_irq_restore(flags));
+ print_result("cpu_local_inc", start, end);
+
+ warm_total += warm_local_test_cache(l2, sizeof(*l2)*NUM_LOCAL_TEST);
+ total = 0;
+ TEST_LOOP(total += l2[n].v);
+ print_result("local_read", start, end);
+ printk("(total was %lu)\n", total);
+
+ printk("trivalue: ");
+ memset(l3, 0, sizeof(*l3)*NUM_LOCAL_TEST);
+ TEST_LOOP(unsigned int idx
+ = !(preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK)) +
+ !(preempt_count() & HARDIRQ_MASK);
+ l3[n].v[idx]++);
+ print_result("local_inc", start, end);
+
+ warm_total += warm_local_test_cache(l3, sizeof(*l3)*NUM_LOCAL_TEST);
+ TEST_LOOP(unsigned int idx
+ = !(preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK)) +
+ !(preempt_count() & HARDIRQ_MASK);
+ l3[n].v[idx] += 1234);
+ print_result("local_add", start, end);
+
+ warm_total += warm_local_test_cache(l3, sizeof(*l3)*NUM_LOCAL_TEST);
+ TEST_LOOP(unsigned int idx
+ = !(preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK)) +
+ !(preempt_count() & HARDIRQ_MASK);
+ get_cpu_var(local3_test)[n%NUM_PERCPU_VARS].v[idx]++;
+ put_cpu_var());
+ print_result("cpu_local_inc", start, end);
+
+ warm_total += warm_local_test_cache(l3, sizeof(*l3)*NUM_LOCAL_TEST);
+ total = 0;
+ TEST_LOOP(total += l3[n].v[0] + l3[n].v[1] + l3[n].v[2]);
+ print_result("local_read", start, end);
+ printk("(total was %lu)\n", total);
+
+ printk("local_t: ");
+ memset(l4, 0, sizeof(*l4)*NUM_LOCAL_TEST);
+ TEST_LOOP(local_inc(&l4[n]));
+ print_result("local_inc", start, end);
+
+ warm_total += warm_local_test_cache(l4, sizeof(*l4)*NUM_LOCAL_TEST);
+ TEST_LOOP(local_add(1234, &l4[n]));
+ print_result("local_add", start, end);
+
+ warm_total += warm_local_test_cache(l4, sizeof(*l4)*NUM_LOCAL_TEST);
+ TEST_LOOP(cpu_local_inc(local4_test[n%NUM_PERCPU_VARS]));
+ print_result("cpu_local_inc", start, end);
+
+ warm_total += warm_local_test_cache(l4, sizeof(*l4)*NUM_LOCAL_TEST);
+ total = 0;
+ TEST_LOOP(total += local_read(&l4[n]));
+ print_result("local_read", start, end);
+ printk("(total was %lu, warm_total %lu)\n", total, warm_total);
+}
+
asmlinkage void __init start_kernel(void)
{
char * command_line;
@@ -635,6 +829,8 @@ asmlinkage void __init start_kernel(void
*/
locking_selftest();

+ init_test_local_variants();
+
#ifdef CONFIG_BLK_DEV_INITRD
if (initrd_start && !initrd_below_start_ok &&
page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) {
@@ -692,6 +888,8 @@ asmlinkage void __init start_kernel(void
acpi_early_init(); /* before LAPIC and SMP init */

ftrace_init();
+
+ test_local_variants();

/* Do the rest non-__init'ed, we're now alive */
rest_init();