Using __get_cpu_var(obj) is slightly faster than per_cpu_ptr(obj,
raw_smp_processor_id()).
1) Smaller code and memory use
For static and small objects, DEFINE_PER_CPU(type, object) is preferred over a
alloc_percpu() : Better and smaller code to access them, and no extra memory
(storing the pointer, and the percpu array of pointers)
x86_64 code before patch
mov 1237577(%rip),%rax # ffffffff803e5990 <rt_cache_stat>
not %rax # part of per_cpu machinery
mov %gs:0x3c,%edx # get cpu number
movslq %edx,%rdx # extend 32 bits cpu number to 64 bits
mov (%rax,%rdx,8),%rax # get the pointer for this cpu
incl 0x38(%rax)
x86_64 code after patch
mov $per_cpu__rt_cache_stat,%rdx
mov %gs:0x48,%rax # get percpu data offset
incl 0x38(%rax,%rdx,1)
2) False sharing avoidance for SMP :
For a small NR_CPUS, the array of per cpu pointers allocated in alloc_percpu()
can be <= 32 bytes. This let slab code gives a part of a cache line. If the
other part of this 64 bytes (or 128 bytes) cache line is used by a mostly
written object, we can have false sharing and expensive per_cpu_ptr() operations.
Size of rt_cache_stat is 64 bytes, so this patch is not a danger of a too big
increase of bss (in UP mode) or static per_cpu data for SMP
(PERCPU_ENOUGH_ROOM is currently 32768 bytes)
Signed-off-by: Eric Dumazet <[EMAIL PROTECTED]>
--- net-2.6/net/ipv4/route.c 2006-01-17 10:51:24.000000000 +0100
+++ net-2.6-ed/net/ipv4/route.c 2006-01-17 11:25:33.000000000 +0100
@@ -240,9 +240,8 @@
static int rt_hash_log;
static unsigned int rt_hash_rnd;
-static struct rt_cache_stat *rt_cache_stat;
-#define RT_CACHE_STAT_INC(field) \
- (per_cpu_ptr(rt_cache_stat, raw_smp_processor_id())->field++)
+static DEFINE_PER_CPU(struct rt_cache_stat, rt_cache_stat);
+#define RT_CACHE_STAT_INC(field) (__get_cpu_var(rt_cache_stat).field++)
static int rt_intern_hash(unsigned hash, struct rtable *rth,
struct rtable **res);
@@ -401,7 +400,7 @@
if (!cpu_possible(cpu))
continue;
*pos = cpu+1;
- return per_cpu_ptr(rt_cache_stat, cpu);
+ return &per_cpu(rt_cache_stat, cpu);
}
return NULL;
}
@@ -414,7 +413,7 @@
if (!cpu_possible(cpu))
continue;
*pos = cpu+1;
- return per_cpu_ptr(rt_cache_stat, cpu);
+ return &per_cpu(rt_cache_stat, cpu);
}
return NULL;
@@ -3160,10 +3159,6 @@
ipv4_dst_ops.gc_thresh = (rt_hash_mask + 1);
ip_rt_max_size = (rt_hash_mask + 1) * 16;
- rt_cache_stat = alloc_percpu(struct rt_cache_stat);
- if (!rt_cache_stat)
- return -ENOMEM;
-
devinet_init();
ip_fib_init();
@@ -3191,7 +3186,6 @@
if (!proc_net_fops_create("rt_cache", S_IRUGO, &rt_cache_seq_fops) ||
!(rtstat_pde = create_proc_entry("rt_cache", S_IRUGO,
proc_net_stat))) {
- free_percpu(rt_cache_stat);
return -ENOMEM;
}
rtstat_pde->proc_fops = &rt_cpu_seq_fops;
