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

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Li, Aubrey [mailto:aubrey.li at linux.intel.com]
> Sent: Monday, March 22, 2021 5:37 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua at hisilicon.com>;
> vincent.guittot at linaro.org; mingo at redhat.com; peterz at infradead.org;
> juri.lelli at redhat.com; dietmar.eggemann at arm.com; rostedt at goodmis.org;
> bsegall at google.com; mgorman at suse.de
> Cc: valentin.schneider at arm.com; linux-arm-kernel at lists.infradead.org;
> linux-kernel at vger.kernel.org; xuwei (O) <xuwei5 at huawei.com>; Zengtao (B)
> <prime.zeng at hisilicon.com>; guodong.xu at linaro.org; yangyicong
> <yangyicong at huawei.com>; Liguozhu (Kenneth) <liguozhu at hisilicon.com>;
> linuxarm at openeuler.org
> Subject: [Linuxarm] Re: [PATCH] sched/fair: remove redundant test_idle_cores
> for non-smt
> 
> 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?


Hi Aubrey,

I think the defconfig of arm64 has always enabled
CONFIG_SCHED_SMT:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/configs/defconfig

it is probably true for x86 as well.

I don't think Linux distribution will build a separate kernel
for machines without smt. so basically the kernel depends on
runtime topology parse to figure out if smt is present rather
than depending on a rebuild.


> 
> 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.hua at 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;
> >  }

Thanks
Barry