On 2017/05/17 11:39:41 +0800, Junchang Wang wrote:
> On Wed, May 17, 2017 at 1:33 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> On Tue, May 16, 2017 at 11:44:12PM +0800, Junchang Wang wrote:
>>> In function eventual(), if the value of stopflag become larger than zero (the
>>> ''if'' branch is taken), then only the "eventual( )" thread updates stopflag.
>>> So, the READ_ONCE() within the WRITE_ONCE() is not necessary. Remove it.
>>>
>>> In function count_cleanup(), there is no need to run smp_mb( ) in between
>>> statement writing to and statement reading from the same variable, stopflag.
>>> Remove smp_mb().
>>>
>>> Suggested-by: Akira Yokosawa <akiyks@xxxxxxxxx>
>>> Signed-off-by: Junchang Wang <junchangwang@xxxxxxxxx>
>>
>> Removing the memory barrier is good. Removing the stopflag_l local
>> variable is presumably intended to remove one load instruction per pass
>> through the outer loop, assuming that the stopflag_l variable is kept
>> in a register.
>>
>> Is the performance benefit actually visible? My guess is "no".
>> If the performance is not substantial, my thought would be to keep
>> the code simpler, given that this is a textbook.
>>
>
> Hi Paul,
>
> Agree. Thanks for the note. Anyway, let's submit a new patch on
> smp_mb(), which makes the code correct. For simplifying stopflag, we
> can consider submitting a new patch later. How does that sound like?
Hi Jason & Paul,
For a material in Chapter 5, I agree that removing excess READ_ONCE()
is not necessary.
Such optimizations could be discussed somewhere (maybe a Quick Quiz)
in Chapter 14, but I'm not sure.
So, the revised patch just removing a smp_mb() looks good to me.
Thanks, Akira.
>
>
> Thanks,
> --Jason
>
>
>> And yes, there might be other performance-neutral simplifications possible,
>> and I would welcome patches to that effect.
>>
>> Thanx, Paul
>>
>>> ---
>>> CodeSamples/count/count_stat_eventual.c | 8 ++++----
>>> 1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/CodeSamples/count/count_stat_eventual.c b/CodeSamples/count/count_stat_eventual.c
>>> index 464de30..1365168 100644
>>> --- a/CodeSamples/count/count_stat_eventual.c
>>> +++ b/CodeSamples/count/count_stat_eventual.c
>>> @@ -40,16 +40,17 @@ void *eventual(void *arg)
>>> {
>>> int t;
>>> unsigned long sum;
>>> + int stopflag_l = 0;
>>>
>>> - while (READ_ONCE(stopflag) < 3) {
>>> + while (stopflag_l < 3) {
>>> sum = 0;
>>> for_each_thread(t)
>>> sum += READ_ONCE(per_thread(counter, t));
>>> WRITE_ONCE(global_count, sum);
>>> poll(NULL, 0, 1);
>>> - if (READ_ONCE(stopflag)) {
>>> + if ((stopflag_l = READ_ONCE(stopflag)) != 0) {
>>> smp_mb();
>>> - WRITE_ONCE(stopflag, READ_ONCE(stopflag) + 1);
>>> + WRITE_ONCE(stopflag, ++stopflag_l);
>>> }
>>> }
>>> return NULL;
>>> @@ -68,7 +69,6 @@ void count_init(void)
>>> void count_cleanup(void)
>>> {
>>> WRITE_ONCE(stopflag, 1);
>>> - smp_mb();
>>> while (READ_ONCE(stopflag) < 3)
>>> poll(NULL, 0, 1);
>>> smp_mb();
>>> --
>>> 2.7.4
>>>
>>
>
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