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Message-ID: <2428a381-1587-2beb-711f-3fab6d773975@linux.vnet.ibm.com>
Date: Tue, 5 Sep 2023 13:33:57 +0530
From: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
To: Tim Chen <tim.c.chen@...ux.intel.com>, peterz@...radead.org
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Subject: Re: [PATCH] sched/fair: Add SMT4 group_smt_balance handling
On 8/22/23 12:49 AM, Tim Chen wrote:
> On Mon, 2023-08-07 at 15:06 +0530, Shrikanth Hegde wrote:
>>>
>>> From: Tim Chen <tim.c.chen@...ux.intel.com>
>>> Date: Fri, 14 Jul 2023 16:09:30 -0700
>>> Subject: [PATCH] sched/fair: Add SMT4 group_smt_balance handling
>>>
>>> For SMT4, any group with more than 2 tasks will be marked as
>>> group_smt_balance. Retain the behaviour of group_has_spare by marking
>>> the busiest group as the group which has the least number of idle_cpus.
>>>
>>> Also, handle rounding effect of adding (ncores_local + ncores_busy)
>>> when the local is fully idle and busy group has more than 2 tasks.
>>> Local group should try to pull at least 1 task in this case.
>>>
>>> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
>>> ---
>>> kernel/sched/fair.c | 18 ++++++++++++++++--
>>> 1 file changed, 16 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index a87988327f88..566686c5f2bd 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -9563,7 +9563,7 @@ static inline long sibling_imbalance(struct lb_env *env,
>>> imbalance /= ncores_local + ncores_busiest;
>>>
>>> /* Take advantage of resource in an empty sched group */
>>> - if (imbalance == 0 && local->sum_nr_running == 0 &&
>>> + if (imbalance <= 1 && local->sum_nr_running == 0 &&
>>> busiest->sum_nr_running > 1)
>>> imbalance = 2;
>>>
>>> @@ -9751,6 +9751,20 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>>> break;
>>>
>>> case group_smt_balance:
>>> + /* no idle cpus on both groups handled by group_fully_busy below */
>>> + if (sgs->idle_cpus != 0 || busiest->idle_cpus != 0) {
>>> + if (sgs->idle_cpus > busiest->idle_cpus)
>>> + return false;
>>> + if (sgs->idle_cpus < busiest->idle_cpus)
>>> + return true;
>>> + if (sgs->sum_nr_running <= busiest->sum_nr_running)
>>> + return false;
>>> + else
>>> + return true;
>>> + }
>>> + goto fully_busy;
>>> + break;
>>> +
>>> case group_fully_busy:
>>> /*
>>> * Select the fully busy group with highest avg_load. In
>>> @@ -9763,7 +9777,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>>> * select the 1st one, except if @sg is composed of SMT
>>> * siblings.
>>> */
>>> -
>>> +fully_busy:
>>> if (sgs->avg_load < busiest->avg_load)
>>> return false;
>>>
>>
>> Hi Tim, Peter.
>>
>> group_smt_balance(cluster scheduling), patches are in tip/sched/core. I dont
>> see this above patch there yet. Currently as is, this can cause function difference
>> in SMT4 systems( such as Power10).
>>
>> Can we please have the above patch as well in tip/sched/core?
>>
>> Acked-by: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
>
> Hi Peter,
>
> Just back from my long vacation. Wonder if you have any comments on the above patch
> for fixing the SMT4 case?
>
> Tim
Hi Tim, Peter.
are there any concerns with the above patch for fixing the SMT4 case.
Currently the behavior is group_smt_balance is set for having even 2 tasks in
SMT4, ideally it should be same as the group_has_spare.
The above patch copies the same behavior to group_smt_balance.
>
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