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Message-ID: <251d4cd4-7a28-af7b-942e-4e9f762fc05f@arm.com>
Date:   Thu, 5 May 2022 20:25:56 +0200
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Subject: Re: [PATCH v8 1/7] sched/fair: Provide u64 read for 32-bits arch
 helper
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To:     Vincent Donnefort <vincent.donnefort@arm.com>,
        peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org
Cc:     linux-kernel@vger.kernel.org, morten.rasmussen@arm.com,
        chris.redpath@arm.com, qperret@google.com, tao.zhou@linux.dev
References: <20220429141148.181816-1-vincent.donnefort@arm.com>
 <20220429141148.181816-2-vincent.donnefort@arm.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
In-Reply-To: <20220429141148.181816-2-vincent.donnefort@arm.com>
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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@arm.com>