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.
