Hi Barry,
On 2021/3/21 6:14, Barry Song 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.
The patch looks good to me.
May I know for what case we need to keep CONFIG_SCHED_SMT for non-smt
machines?
Thanks,
-Aubrey
> 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....@hisilicon.com>
> ---
> 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;
> }
>
