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.
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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