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

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Message-ID: <20200727120520.GA257620@e120877-lin.cambridge.arm.com>
Date:   Mon, 27 Jul 2020 13:05:21 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     mingo@...hat.com, peterz@...radead.org, 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

Hi,

On Mon, Jul 27, 2020 at 01:24:54PM +0200, Ingo Molnar wrote:
> 
> * vincent.donnefort@....com <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.
> 
> Could you please explain how "conditional u64 variable copy 
> declarations" prevents us to use an inline function for this

I meant, as the "copy" variable [vminruntime|last_update_time]_copy, is
declared only in the !CONFIG_64BIT case, a function call would fail when
CONFIG_64BIT is set.

   u64_32read(cfs_rq->min_vruntime, cfs_rq->min_vruntime_copy); 
						^
					  not declared

I could rephrase this part to something more understandable ?

> 
> > +/*
> > + * u64_32read() / u64_32read_set_copy()
> > + *
> > + * Use the copied u64 value to protect against data race. This is only
> > + * applicable for 32-bits architectures.
> > + */
> > +#if !defined(CONFIG_64BIT) && defined(CONFIG_SMP)
> > +# define u64_32read(val, copy)						\
> > +({									\
> > +	u64 _val;							\
> > +	u64 _val_copy;							\
> > +									\
> > +	do {								\
> > +		_val_copy = copy;					\
> > +		/*							\
> > +		 * paired with u64_32read_set_copy, ordering access	\
> > +		 * to val and copy.					\
> > +		 */							\
> > +		smp_rmb();						\
> > +		_val = val;						\
> > +	} while (_val != _val_copy);					\
> > +									\
> > +	_val;								\
> > +})
> > +# define u64_32read_set_copy(val, copy)					\
> > +do {									\
> > +	/* paired with u64_32read, ordering access to val and copy */	\
> > +	smp_wmb();							\
> > +	copy = val;							\
> > +} while (0)
> > +#else
> > +# define u64_32read(val, copy) (val)
> > +# define u64_32read_set_copy(val, copy) do { } while (0)
> > +#endif
> > +
> 
> Thanks,
> 
> 	Ingo

-- 
Vincent