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

[Mel Gorman <mgorman@suse.de>]

On Sun, Mar 21, 2021 at 11:14:32AM +1300, 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.
> 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@hisilicon.com>

Acked-by: Mel Gorman <mgorman@suse.de>

That said, the numa_idle_core() function then becomes slightly
redundant.  A possible follow up is to move the "idle_core >= 0" check
in numa_idle_core() to its caller in update_numa_stats() and then remove
the redundant check in !static_branch_likely(&sched_smt_present) check
in numa_idle_core.

-- 
Mel Gorman
SUSE Labs