[PATCH] sched/fair: remove redundant test_idle_cores for non-smt

[Vincent Guittot]

On Sat, 20 Mar 2021 at 23:21, Barry Song <song.bao.hua at hisilicon.com> wrote:
>
> update_idle_core() is only done for the case of sched_smt_present.
> but test_idle_cores() is done for all machines even those without
> smt.
> this could contribute to up 8%+ hackbench performance loss on a
> machine like kunpeng 920 which has no smt. this patch removes the
> redundant test_idle_cores() for non-smt machines.
>
> we run the below hackbench with different -g parameter from 2 to
> 14, for each different g, we run the command 10 times and get the
> average time:
> $ numactl -N 0 hackbench -p -T -l 20000 -g $1
>
> hackbench will report the time which is needed to complete a certain
> number of messages transmissions between a certain number of tasks,
> for example:
> $ numactl -N 0 hackbench -p -T -l 20000 -g 10
> Running in threaded mode with 10 groups using 40 file descriptors each
> (== 400 tasks)
> Each sender will pass 20000 messages of 100 bytes
>
> The below is the result of hackbench w/ and w/o this patch:
> g=    2      4     6       8      10     12      14
> w/o: 1.8151 3.8499 5.5142 7.2491 9.0340 10.7345 12.0929
> w/ : 1.8428 3.7436 5.4501 6.9522 8.2882  9.9535 11.3367
>                           +4.1%  +8.3%  +7.3%   +6.3%
>
> Signed-off-by: Barry Song <song.bao.hua at hisilicon.com>

Reviewed-by: Vincent Guittot <vincent.guittot at linaro.org>

> ---
>  kernel/sched/fair.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2e2ab1e..de42a32 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6038,9 +6038,11 @@ static inline bool test_idle_cores(int cpu, bool def)
>  {
>         struct sched_domain_shared *sds;
>
> -       sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> -       if (sds)
> -               return READ_ONCE(sds->has_idle_cores);
> +       if (static_branch_likely(&sched_smt_present)) {
> +               sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> +               if (sds)
> +                       return READ_ONCE(sds->has_idle_cores);
> +       }
>
>         return def;
>  }
> --
> 1.8.3.1
>