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

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

[Barry Song]

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>
---
 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

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

[Li, Aubrey]

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.hua@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;
>  }
> 

RE: [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@linux.intel.com]
> Sent: Monday, March 22, 2021 5:37 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>;
> vincent.guittot@linaro.org; mingo@redhat.com; peterz@infradead.org;
> juri.lelli@redhat.com; dietmar.eggemann@arm.com; rostedt@goodmis.org;
> bsegall@google.com; mgorman@suse.de
> Cc: valentin.schneider@arm.com; linux-arm-kernel@lists.infradead.org;
> linux-kernel@vger.kernel.org; xuwei (O) <xuwei5@huawei.com>; Zengtao (B)
> <prime.zeng@hisilicon.com>; guodong.xu@linaro.org; yangyicong
> <yangyicong@huawei.com>; Liguozhu (Kenneth) <liguozhu@hisilicon.com>;
> linuxarm@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@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

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

[Mel Gorman]

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

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

[Peter Zijlstra]

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%
> 

The patch looks obvious, but the Changelog and Subject needed a lot of
help.

I've changed it like so:

---
Subject: sched/fair: Optimize test_idle_cores() for !SMT
From: Barry Song <song.bao.hua@hisilicon.com>
Date: Sun, 21 Mar 2021 11:14:32 +1300

From: Barry Song <song.bao.hua@hisilicon.com>

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 can 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 !SMT machines.

Hackbench is ran with -g {2..14}, for each g it is ran 10 times to get
an average.

  $ numactl -N 0 hackbench -p -T -l 20000 -g $1

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20210320221432.924-1-song.bao.hua@hisilicon.com
---
 kernel/sched/fair.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6038,9 +6038,11 @@ static inline bool test_idle_cores(int c
 {
 	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;
 }

Re: [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@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@hisilicon.com>

Reviewed-by: Vincent Guittot <vincent.guittot@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
>

[tip: sched/core] sched/fair: Optimize test_idle_cores() for !SMT

[tip-bot2 for Barry Song]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     c8987ae5af793a73e2c0d6ce804d8ff454ea377c
Gitweb:        https://git.kernel.org/tip/c8987ae5af793a73e2c0d6ce804d8ff454ea377c
Author:        Barry Song <song.bao.hua@hisilicon.com>
AuthorDate:    Sun, 21 Mar 2021 11:14:32 +13:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 23 Mar 2021 16:01:59 +01:00

sched/fair: Optimize test_idle_cores() for !SMT

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 can 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 !SMT machines.

Hackbench is ran with -g {2..14}, for each g it is ran 10 times to get
an average.

  $ numactl -N 0 hackbench -p -T -l 20000 -g $1

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20210320221432.924-1-song.bao.hua@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 6aad028..aaa0dfa 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;
 }