linux-kernel - Re: [PATCH v8 1/7] sched/fair: Provide u64 read for 32-bits arch helper

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Date:   Thu, 5 May 2022 20:44:16 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        vdonnefort@...il.com
Cc:     linux-kernel@...r.kernel.org, morten.rasmussen@....com,
        chris.redpath@....com, qperret@...gle.com, tao.zhou@...ux.dev
Subject: Re: [PATCH v8 1/7] sched/fair: Provide u64 read for 32-bits arch
 helper

+ vdonnefort@...il.com
- vincent.donnefort@....com

On 05/05/2022 20:25, Dietmar Eggemann wrote:
> On 29/04/2022 16:11, Vincent Donnefort wrote:
>> Introducing macro helpers u64_u32_{store,load}() to factorize lockless
>> accesses to u64 variables for 32-bits architectures.
>>
>> Users are for now cfs_rq.min_vruntime and sched_avg.last_update_time. To
>> accommodate the later where the copy lies outside of the structure
>> (cfs_rq.last_udpate_time_copy instead of sched_avg.last_update_time_copy),
>> use the _copy() version of those helpers.
>>
>> Those new helpers encapsulate smp_rmb() and smp_wmb() synchronization and
>> therefore, have a small penalty in set_task_rq_fair() and init_cfs_rq().
> 
> ... but obviously only on 32bit machines. And for set_task_rq_fair() we
> now do one smp_rmb per cfs_rq (prev and next), like we do one smp_wmb()
> per cfs_rq in update_cfs_rq_load_avg().
> 
> [...]
> 
>> @@ -3786,8 +3770,9 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
>>  	decayed |= __update_load_avg_cfs_rq(now, cfs_rq);
>>  
>>  #ifndef CONFIG_64BIT
> 
> Can we not get rid of this last CONFIG_64BIT here?
> 
>> -	smp_wmb();
>> -	cfs_rq->load_last_update_time_copy = sa->last_update_time;
>> +	u64_u32_store_copy(sa->last_update_time,
>> +			   cfs_rq->last_update_time_copy,
>> +			   sa->last_update_time);
> 
> (sa->last_update_time = sa->last_update_time); should dissolve on 64bit.
> 
>>  #endif
> 
> [...]
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>