2023-10-30 17:33:17

by Keisuke Nishimura

Subject: [PATCH v2] sched/fair: Fix the decision for load balance

should_we_balance is called for the decision to do load-balancing.
When sched ticks invoke this function, only one CPU should return
true. However, in the current code, two CPUs can return true. The
following situation, where b means busy and i means idle, is an
example, because CPU 0 and CPU 2 return true.

[0, 1] [2, 3]
b b i b

This fix checks if there exists an idle CPU with busy sibling(s)
after looking for a CPU on an idle core. If some idle CPUs with busy
siblings are found, just the first one should do load-balancing.

Fixes: b1bfeab9b002 ("sched/fair: Consider the idle state of the whole core for load balance")
Signed-off-by: Keisuke Nishimura <[email protected]>
---
kernel/sched/fair.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2048138ce54b..69d63fae34f4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11079,12 +11079,16 @@ static int should_we_balance(struct lb_env *env)
continue;
}

- /* Are we the first idle CPU? */
+ /*
+ * Are we the first idle core in a MC or higher domain
+ * or the first idle CPU in a SMT domain?
+ */
return cpu == env->dst_cpu;
}

- if (idle_smt == env->dst_cpu)
- return true;
+ /* Are we the first idle CPU with busy siblings? */
+ if (idle_smt != -1)
+ return idle_smt == env->dst_cpu;

/* Are we the first CPU of this group ? */
return group_balance_cpu(sg) == env->dst_cpu;
--
2.34.1

2023-10-31 06:17:19

by Chen Yu

Subject: Re: [PATCH v2] sched/fair: Fix the decision for load balance

On 2023-10-30 at 18:29:46 +0100, Keisuke Nishimura wrote:
> should_we_balance is called for the decision to do load-balancing.
> When sched ticks invoke this function, only one CPU should return
> true. However, in the current code, two CPUs can return true. The
> following situation, where b means busy and i means idle, is an
> example, because CPU 0 and CPU 2 return true.
>
> [0, 1] [2, 3]
> b b i b
>
> This fix checks if there exists an idle CPU with busy sibling(s)
> after looking for a CPU on an idle core. If some idle CPUs with busy
> siblings are found, just the first one should do load-balancing.
>
> Fixes: b1bfeab9b002 ("sched/fair: Consider the idle state of the whole core for load balance")
> Signed-off-by: Keisuke Nishimura <[email protected]>
> ---
> kernel/sched/fair.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2048138ce54b..69d63fae34f4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11079,12 +11079,16 @@ static int should_we_balance(struct lb_env *env)
> continue;
> }
>
> - /* Are we the first idle CPU? */
> + /*
> + * Are we the first idle core in a MC or higher domain

It is possible that the Cluster domain is lower than a MC.
cluser domain: CPUs share the same L2
MC domain: CPUs share the same LLC

grep . domain*/{name,flags}
domain0/name:CLS
domain1/name:MC
domain2/name:NUMA
domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING
domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING
domain2/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA

So, maybe:
Are we the first idle core in a non-SMT domain or higher,

thanks,
Chenyu

> + * or the first idle CPU in a SMT domain?
> + */
> return cpu == env->dst_cpu;
> }
>
> - if (idle_smt == env->dst_cpu)
> - return true;
> + /* Are we the first idle CPU with busy siblings? */
> + if (idle_smt != -1)
> + return idle_smt == env->dst_cpu;
>
> /* Are we the first CPU of this group ? */
> return group_balance_cpu(sg) == env->dst_cpu;
> --
> 2.34.1
>

2023-10-31 06:19:07

by Chen Yu

Subject: Re: [PATCH v2] sched/fair: Fix the decision for load balance

On 2023-10-31 at 13:59:30 +0800, Chen Yu wrote:
> On 2023-10-30 at 18:29:46 +0100, Keisuke Nishimura wrote:
> > should_we_balance is called for the decision to do load-balancing.
> > When sched ticks invoke this function, only one CPU should return
> > true. However, in the current code, two CPUs can return true. The
> > following situation, where b means busy and i means idle, is an
> > example, because CPU 0 and CPU 2 return true.
> >
> > [0, 1] [2, 3]
> > b b i b
> >
> > This fix checks if there exists an idle CPU with busy sibling(s)
> > after looking for a CPU on an idle core. If some idle CPUs with busy
> > siblings are found, just the first one should do load-balancing.
> >
> > Fixes: b1bfeab9b002 ("sched/fair: Consider the idle state of the whole core for load balance")
> > Signed-off-by: Keisuke Nishimura <[email protected]>
> > ---
> > kernel/sched/fair.c | 10 +++++++---
> > 1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 2048138ce54b..69d63fae34f4 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -11079,12 +11079,16 @@ static int should_we_balance(struct lb_env *env)
> > continue;
> > }
> >
> > - /* Are we the first idle CPU? */
> > + /*
> > + * Are we the first idle core in a MC or higher domain
>
> It is possible that the Cluster domain is lower than a MC.
> cluser domain: CPUs share the same L2
> MC domain: CPUs share the same LLC
>
> grep . domain*/{name,flags}
> domain0/name:CLS
> domain1/name:MC
> domain2/name:NUMA
> domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING
> domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING
> domain2/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA
>
> So, maybe:
> Are we the first idle core in a non-SMT domain or higher,
>

I suppose you can also carry the Reviewed-by tags in V1(Shrikanth and Vincent's)
as there is no code change.

Reviewed-by: Chen Yu <[email protected]>

thanks,
Chenyu

2023-10-31 07:47:38

by Shrikanth Hegde

Subject: Re: [PATCH v2] sched/fair: Fix the decision for load balance

On 10/31/23 11:29 AM, Chen Yu wrote:
> On 2023-10-30 at 18:29:46 +0100, Keisuke Nishimura wrote:
>> should_we_balance is called for the decision to do load-balancing.
>> When sched ticks invoke this function, only one CPU should return
>> true. However, in the current code, two CPUs can return true. The
>> following situation, where b means busy and i means idle, is an
>> example, because CPU 0 and CPU 2 return true.
>>
>> [0, 1] [2, 3]
>> b b i b
>>
>> This fix checks if there exists an idle CPU with busy sibling(s)
>> after looking for a CPU on an idle core. If some idle CPUs with busy
>> siblings are found, just the first one should do load-balancing.
>>

As Chen indicated, it would be better to carry reviewed by tags.

>> Fixes: b1bfeab9b002 ("sched/fair: Consider the idle state of the whole core for load balance")
>> Signed-off-by: Keisuke Nishimura <[email protected]>
>> ---
>> kernel/sched/fair.c | 10 +++++++---
>> 1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 2048138ce54b..69d63fae34f4 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -11079,12 +11079,16 @@ static int should_we_balance(struct lb_env *env)
>> continue;
>> }
>>
>> - /* Are we the first idle CPU? */
>> + /*
>> + * Are we the first idle core in a MC or higher domain
>
> It is possible that the Cluster domain is lower than a MC.
> cluser domain: CPUs share the same L2
> MC domain: CPUs share the same LLC
>
> grep . domain*/{name,flags}
> domain0/name:CLS
> domain1/name:MC
> domain2/name:NUMA
> domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING
> domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING
> domain2/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA
>
> So, maybe:
> Are we the first idle core in a non-SMT domain or higher,

Yes. That makes sense. Forgot about recent cluster addition.

>
> thanks,
> Chenyu
>
>> + * or the first idle CPU in a SMT domain?
>> + */
>> return cpu == env->dst_cpu;
>> }
>>
>> - if (idle_smt == env->dst_cpu)
>> - return true;
>> + /* Are we the first idle CPU with busy siblings? */
>> + if (idle_smt != -1)
>> + return idle_smt == env->dst_cpu;
>>
>> /* Are we the first CPU of this group ? */
>> return group_balance_cpu(sg) == env->dst_cpu;
>> --
>> 2.34.1
>>