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?
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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