linux-kernel - Re: [PATCH 2/5] sched/topology: Wrappers for sched_domains

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Message-ID: <0d7a9d03-3eb7-42c7-bc6b-20283c2ceee5@redhat.com>
Date: Fri, 7 Mar 2025 09:49:21 -0500
From: Waiman Long <llong@...hat.com>
To: Juri Lelli <juri.lelli@...hat.com>, Waiman Long <llong@...hat.com>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 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>,
 Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Qais Yousef <qyousef@...alina.io>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Swapnil Sapkal <swapnil.sapkal@....com>,
 Shrikanth Hegde <sshegde@...ux.ibm.com>, Phil Auld <pauld@...hat.com>,
 luca.abeni@...tannapisa.it, tommaso.cucinotta@...tannapisa.it,
 Jon Hunter <jonathanh@...dia.com>
Subject: Re: [PATCH 2/5] sched/topology: Wrappers for sched_domains_mutex

On 3/7/25 4:32 AM, Juri Lelli wrote:
> On 05/03/25 10:52, Juri Lelli wrote:
>> On 04/03/25 11:01, Waiman Long wrote:
>>> On 3/4/25 10:05 AM, Waiman Long wrote:
>>>>> --- a/kernel/sched/topology.c
>>>>> +++ b/kernel/sched/topology.c
>>>>> @@ -6,6 +6,19 @@
>>>>>    #include <linux/bsearch.h>
>>>>>      DEFINE_MUTEX(sched_domains_mutex);
>>>>> +#ifdef CONFIG_SMP
>>>>> +void sched_domains_mutex_lock(void)
>>>>> +{
>>>>> +    mutex_lock(&sched_domains_mutex);
>>>>> +}
>>>>> +void sched_domains_mutex_unlock(void)
>>>>> +{
>>>>> +    mutex_unlock(&sched_domains_mutex);
>>>>> +}
>>>>> +#else
>>>>> +void sched_domains_mutex_lock(void) { }
>>>>> +void sched_domains_mutex_unlock(void) { }
>>>>> +#endif
>>>>>      /* Protected by sched_domains_mutex: */
>>>>>    static cpumask_var_t sched_domains_tmpmask;
>>>>> @@ -2791,7 +2804,7 @@ void partition_sched_domains_locked(int
>>>>> ndoms_new, cpumask_var_t doms_new[],
>>>>>    void partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
>>>>>                     struct sched_domain_attr *dattr_new)
>>>>>    {
>>>>> -    mutex_lock(&sched_domains_mutex);
>>>>> +    sched_domains_mutex_lock();
>>>>>        partition_sched_domains_locked(ndoms_new, doms_new, dattr_new);
>>>>> -    mutex_unlock(&sched_domains_mutex);
>>>>> +    sched_domains_mutex_unlock();
>>>>>    }
>>>> There are two "lockdep_assert_held(&sched_domains_mutex);" statements in
>>>> topology.c file and one in cpuset.c. That can be problematic in the
>>>> non-SMP case. Maybe another wrapper to do the assert?
>>> Ignore that as both topology.c and cpuset.c will only be compiled if
>>> CONFIG_SMP is defined. IOW, you don't need the the "#ifdef CONFIG_SMP"
>>> above.
>> Indeed!
> Ah, actually I believe next patch (3/5) introduce usage for the !SMP
> case in sched_rt_handler()
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 4b8e33c615b1..8cebe71d2bb1 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2910,6 +2910,7 @@ static int sched_rt_handler(const struct ctl_table *table, int write, void *buff
>          int ret;
>
>          mutex_lock(&mutex);
> +       sched_domains_mutex_lock();
>          old_period = sysctl_sched_rt_period;
>          old_runtime = sysctl_sched_rt_runtime;
>
> @@ -2936,6 +2937,7 @@ static int sched_rt_handler(const struct ctl_table *table, int write, void *buff
>                  sysctl_sched_rt_period = old_period;
>                  sysctl_sched_rt_runtime = old_runtime;
>          }
> +       sched_domains_mutex_unlock();
>          mutex_unlock(&mutex);
>
>          return ret;
>
> So, I will need to add the ifdef back I guess (I removed it on v2). Do
> you agree?

You are right. That change does introduce the need to define the 
!CONFIG_SMP case. I was looking the existing uses o domains_mutex when 
making that suggestions.

It is a good catch.

Cheers,
Longman