linux-kernel - Re: [PATCH 1/3] sched/isolation: Add HK_FLAG_SCHED to nohz

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Message-ID: <7fa3dbd5-7c2e-4614-a5f4-258546cb090b@redhat.com>
Date: Tue, 3 Sep 2024 09:24:08 -0400
From: Waiman Long <longman@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sched/isolation: Add HK_FLAG_SCHED to nohz_full


On 9/3/24 09:10, Frederic Weisbecker wrote:
> Le Sun, Aug 18, 2024 at 07:45:18PM -0400, Waiman Long a écrit :
>> The HK_FLAG_SCHED/HK_TYPE_SCHED flag is defined and is also used
>> in kernel/sched/fair.c since commit de201559df87 ("sched/isolation:
>> Introduce housekeeping flags"). However, the corresponding cpumask isn't
>> currently updated anywhere. So the mask is always cpu_possible_mask.
>>
>> Add it in nohz_full setup so that nohz_full CPUs will now be removed
>> from HK_TYPE_SCHED cpumask.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   kernel/sched/isolation.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
>> index 5891e715f00d..a514994af319 100644
>> --- a/kernel/sched/isolation.c
>> +++ b/kernel/sched/isolation.c
>> @@ -196,7 +196,7 @@ static int __init housekeeping_nohz_full_setup(char *str)
>>   	unsigned long flags;
>>   
>>   	flags = HK_FLAG_TICK | HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU |
>> -		HK_FLAG_MISC | HK_FLAG_KTHREAD;
>> +		HK_FLAG_MISC | HK_FLAG_KTHREAD | HK_FLAG_SCHED;
>>   
>>   	return housekeeping_setup(str, flags);
>>   }
> find_new_ilb() already has HK_FLAG_MISC to prevent an isolated CPU
> from being elected as an ilb. So I think we should simply remove HK_FLAG_SCHED.

There is a check for HK_TYPE_SCHED in nohz_balance_enter_idle() and 
nohz_newidle_balance(), though it is essentially a no-op as the cpumask 
has all the CPUs. If we remove HK_TYPE_SCHED, the question now will be 
whether we should remove the checks at these 2 functions or change them 
to HK_TYPE_MISC.

Cheers,
Longman

>
> Thanks.
>