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    Subject[thisops uV2 01/10] percpucounter: Optimize __percpu_counter_add a bit through the use of this_cpu() options.
    The this_cpu_* options can be used to optimize __percpu_counter_add a bit. Avoids
    some address arithmetic and saves 12 bytes.

    Before:


    00000000000001d3 <__percpu_counter_add>:
    1d3: 55 push %rbp
    1d4: 48 89 e5 mov %rsp,%rbp
    1d7: 41 55 push %r13
    1d9: 41 54 push %r12
    1db: 53 push %rbx
    1dc: 48 89 fb mov %rdi,%rbx
    1df: 48 83 ec 08 sub $0x8,%rsp
    1e3: 4c 8b 67 30 mov 0x30(%rdi),%r12
    1e7: 65 4c 03 24 25 00 00 add %gs:0x0,%r12
    1ee: 00 00
    1f0: 4d 63 2c 24 movslq (%r12),%r13
    1f4: 48 63 c2 movslq %edx,%rax
    1f7: 49 01 f5 add %rsi,%r13
    1fa: 49 39 c5 cmp %rax,%r13
    1fd: 7d 0a jge 209 <__percpu_counter_add+0x36>
    1ff: f7 da neg %edx
    201: 48 63 d2 movslq %edx,%rdx
    204: 49 39 d5 cmp %rdx,%r13
    207: 7f 1e jg 227 <__percpu_counter_add+0x54>
    209: 48 89 df mov %rbx,%rdi
    20c: e8 00 00 00 00 callq 211 <__percpu_counter_add+0x3e>
    211: 4c 01 6b 18 add %r13,0x18(%rbx)
    215: 48 89 df mov %rbx,%rdi
    218: 41 c7 04 24 00 00 00 movl $0x0,(%r12)
    21f: 00
    220: e8 00 00 00 00 callq 225 <__percpu_counter_add+0x52>
    225: eb 04 jmp 22b <__percpu_counter_add+0x58>
    227: 45 89 2c 24 mov %r13d,(%r12)
    22b: 5b pop %rbx
    22c: 5b pop %rbx
    22d: 41 5c pop %r12
    22f: 41 5d pop %r13
    231: c9 leaveq
    232: c3 retq


    After:

    00000000000001d3 <__percpu_counter_add>:
    1d3: 55 push %rbp
    1d4: 48 63 ca movslq %edx,%rcx
    1d7: 48 89 e5 mov %rsp,%rbp
    1da: 41 54 push %r12
    1dc: 53 push %rbx
    1dd: 48 89 fb mov %rdi,%rbx
    1e0: 48 8b 47 30 mov 0x30(%rdi),%rax
    1e4: 65 44 8b 20 mov %gs:(%rax),%r12d
    1e8: 4d 63 e4 movslq %r12d,%r12
    1eb: 49 01 f4 add %rsi,%r12
    1ee: 49 39 cc cmp %rcx,%r12
    1f1: 7d 0a jge 1fd <__percpu_counter_add+0x2a>
    1f3: f7 da neg %edx
    1f5: 48 63 d2 movslq %edx,%rdx
    1f8: 49 39 d4 cmp %rdx,%r12
    1fb: 7f 21 jg 21e <__percpu_counter_add+0x4b>
    1fd: 48 89 df mov %rbx,%rdi
    200: e8 00 00 00 00 callq 205 <__percpu_counter_add+0x32>
    205: 4c 01 63 18 add %r12,0x18(%rbx)
    209: 48 8b 43 30 mov 0x30(%rbx),%rax
    20d: 48 89 df mov %rbx,%rdi
    210: 65 c7 00 00 00 00 00 movl $0x0,%gs:(%rax)
    217: e8 00 00 00 00 callq 21c <__percpu_counter_add+0x49>
    21c: eb 04 jmp 222 <__percpu_counter_add+0x4f>
    21e: 65 44 89 20 mov %r12d,%gs:(%rax)
    222: 5b pop %rbx
    223: 41 5c pop %r12
    225: c9 leaveq
    226: c3 retq

    Reviewed-by: Pekka Enberg <penberg@kernel.org>
    Reviewed-by: Tejun Heo <tj@kernel.org>
    Signed-off-by: Christoph Lameter <cl@linux.com>

    ---
    lib/percpu_counter.c | 8 +++-----
    1 file changed, 3 insertions(+), 5 deletions(-)

    Index: linux-2.6/lib/percpu_counter.c
    ===================================================================
    --- linux-2.6.orig/lib/percpu_counter.c 2010-11-03 12:25:37.000000000 -0500
    +++ linux-2.6/lib/percpu_counter.c 2010-11-03 12:27:27.000000000 -0500
    @@ -72,18 +72,16 @@ EXPORT_SYMBOL(percpu_counter_set);
    void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
    {
    s64 count;
    - s32 *pcount;

    preempt_disable();
    - pcount = this_cpu_ptr(fbc->counters);
    - count = *pcount + amount;
    + count = __this_cpu_read(*fbc->counters) + amount;
    if (count >= batch || count <= -batch) {
    spin_lock(&fbc->lock);
    fbc->count += count;
    - *pcount = 0;
    + __this_cpu_write(*fbc->counters, 0);
    spin_unlock(&fbc->lock);
    } else {
    - *pcount = count;
    + __this_cpu_write(*fbc->counters, count);
    }
    preempt_enable();
    }


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