linux-kernel - Re: [PATCH] sched/fair: provide u64 read for 32-bits arch helper

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Message-ID: <20200727123801.GJ119549@hirez.programming.kicks-ass.net>
Date:   Mon, 27 Jul 2020 14:38:01 +0200
From:   peterz@...radead.org
To:     vincent.donnefort@....com
Cc:     mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        lukasz.luba@....com, valentin.schneider@....com
Subject: Re: [PATCH] sched/fair: provide u64 read for 32-bits arch helper

On Mon, Jul 27, 2020 at 11:59:24AM +0100, vincent.donnefort@....com wrote:
> From: Vincent Donnefort <vincent.donnefort@....com>
> 
> Introducing two macro helpers u64_32read() and u64_32read_set_copy() to
> factorize the u64 vminruntime and last_update_time read on a 32-bits
> architecture. 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().
> 
> The choice of using a macro over an inline function is driven by the
> conditional u64 variable copy declarations.
> 
>   #ifndef CONFIG_64BIT
>      u64 [vminruntime|last_update_time]_copy;
>   #endif

This lacks a *why*... why did you get up this morning and wrote us this
patch.