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[IPV4]: rt_cache_stat can be statically defined

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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 <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2006-01-17 02:54:36 -08:00 committed by David S. Miller
parent 8243126c5e
commit 2f970d8357
1 changed files with 4 additions and 10 deletions
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14
net/ipv4/route.c
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@ -240,9 +240,8 @@ static unsigned rt_hash_mask;
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 @@ static void *rt_cpu_seq_start(struct seq_file *seq, loff_t *pos)
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 @@ static void *rt_cpu_seq_next(struct seq_file *seq, void *v, loff_t *pos)
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 @@ int __init ip_rt_init(void)
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 @@ int __init ip_rt_init(void)
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;