linux-kernel - RE: [Linuxarm] Re: [PATCH] sched/fair: remove redundant test_idle

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Date:   Mon, 22 Mar 2021 05:08:18 +0000
From:   "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>
CC:     "valentin.schneider@....com" <valentin.schneider@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xuwei (O)" <xuwei5@...wei.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        "guodong.xu@...aro.org" <guodong.xu@...aro.org>,
        yangyicong <yangyicong@...wei.com>,
        "Liguozhu (Kenneth)" <liguozhu@...ilicon.com>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>
Subject: RE: [Linuxarm]  Re: [PATCH] sched/fair: remove redundant
 test_idle_cores for non-smt



> -----Original Message-----
> From: Li, Aubrey [mailto:aubrey.li@...ux.intel.com]
> Sent: Monday, March 22, 2021 5:37 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@...ilicon.com>;
> vincent.guittot@...aro.org; mingo@...hat.com; peterz@...radead.org;
> juri.lelli@...hat.com; dietmar.eggemann@....com; rostedt@...dmis.org;
> bsegall@...gle.com; mgorman@...e.de
> Cc: valentin.schneider@....com; linux-arm-kernel@...ts.infradead.org;
> linux-kernel@...r.kernel.org; xuwei (O) <xuwei5@...wei.com>; Zengtao (B)
> <prime.zeng@...ilicon.com>; guodong.xu@...aro.org; yangyicong
> <yangyicong@...wei.com>; Liguozhu (Kenneth) <liguozhu@...ilicon.com>;
> linuxarm@...neuler.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@...ilicon.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