2016-04-21 12:28:41

by Minfei Huang

Subject: [PATCH] Remove useless test during exchanging values

The value delta is correct as well, although calc_load_idle[idx] is
equal to 0. Remove this useless test to improve performance, since this
function is called more frequently.

Signed-off-by: Minfei Huang <[email protected]>
---
kernel/sched/loadavg.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
index ef71590..5a5d7ae 100644
--- a/kernel/sched/loadavg.c
+++ b/kernel/sched/loadavg.c
@@ -216,10 +216,9 @@ void calc_load_exit_idle(void)
static long calc_load_fold_idle(void)
{
int idx = calc_load_read_idx();
- long delta = 0;
+ long delta;

- if (atomic_long_read(&calc_load_idle[idx]))
- delta = atomic_long_xchg(&calc_load_idle[idx], 0);
+ delta = atomic_long_xchg(&calc_load_idle[idx], 0);

return delta;
}
--
2.6.3

2016-04-21 13:27:53

by Peter Zijlstra

Subject: Re: [PATCH] Remove useless test during exchanging values

On Thu, Apr 21, 2016 at 08:28:28PM +0800, Minfei Huang wrote:
> The value delta is correct as well, although calc_load_idle[idx] is
> equal to 0. Remove this useless test to improve performance, since this
> function is called more frequently.
>
> Signed-off-by: Minfei Huang <[email protected]>
> ---
> kernel/sched/loadavg.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
> index ef71590..5a5d7ae 100644
> --- a/kernel/sched/loadavg.c
> +++ b/kernel/sched/loadavg.c
> @@ -216,10 +216,9 @@ void calc_load_exit_idle(void)
> static long calc_load_fold_idle(void)
> {
> int idx = calc_load_read_idx();
> - long delta = 0;
> + long delta;
>
> - if (atomic_long_read(&calc_load_idle[idx]))
> - delta = atomic_long_xchg(&calc_load_idle[idx], 0);
> + delta = atomic_long_xchg(&calc_load_idle[idx], 0);

This will actually degrade performance.

The read can have the cacheline in shared mode, while the xchg will
force it into exclusive mode. Also, the xchg atomic operation is a
really expensive instruction.