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

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Message-ID: <20200818181120.GA297937@e120877-lin.cambridge.arm.com>
Date:   Tue, 18 Aug 2020 19:11:20 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     peterz@...radead.org
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

Hi Peter,

[...]

> > > 
> > > How about something like:
> > > 
> > > #ifdef CONFIG_64BIT
> > > 
> > > #define DEFINE_U64_U32(name)	u64 name
> > > #define u64_u32_load(name)	name
> > > #define u64_u32_store(name, val)name = val
> > > 
> > > #else
> > > 
> > > #define DEFINE_U64_U32(name)			\
> > > 	struct {				\
> > > 		u64 name;			\
> > > 		u64 name##_copy;		\
> > > 	}
> > > 
> > > #define u64_u32_load(name)			\
> > > 	({					\
> > > 		u64 val, copy;			\
> > > 		do {				\
> > > 			val = name;		\
> > > 			smb_rmb();		\
> > > 			copy = name##_copy;	\
> > > 		} while (val != copy);		\
> > 
> > wrong order there; we should first read _copy and then the regular one
> > of course.
> > 
> > > 		val;
> > > 	})
> > > 
> > > #define u64_u32_store(name, val)		\
> > > 	do {					\
> > > 		typeof(val) __val = (val);	\
> > > 		name = __val;			\
> > > 		smp_wmb();			\
> > > 		name##_copy = __val;		\
> > > 	} while (0)
> > > 
> > > #endif
> > 
> > The other approach is making it a full type and inline functions I
> > suppose.
> 
> I didn't pick this approach originally, as I thought it would be way too
> invasive. If the API looks way cleaner, it nonetheless doesn't match
> last_update_time usage. The variable is declared in sched_avg while its copy
> is in struct cfs_rq.
> 
> Moving the copy in sched_avg would mean:
> 
>  * Setting the copy for all struct sched_avg in ___update_load_sum(), instead
>    of just the cfs_rq.avg in update_cfs_rq_load_avg().
> 
>  * Having the DEFINE_U64_U32 declaration in include/linux/sched.h to cover
>    struct sched_avg.

Gentle ping