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

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Message-ID: <20200728195320.GA426859@e120877-lin.cambridge.arm.com>
Date:   Tue, 28 Jul 2020 20:53:21 +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,

On Tue, Jul 28, 2020 at 02:00:27PM +0200, peterz@...radead.org wrote:
> On Tue, Jul 28, 2020 at 01:13:02PM +0200, peterz@...radead.org wrote:
> > On Mon, Jul 27, 2020 at 04:23:03PM +0100, Vincent Donnefort wrote:
> > 
> > > For 32-bit architectures, both min_vruntime and last_update_time are using
> > > similar access. This patch is simply an attempt to unify their usage by
> > > introducing two macros to rely on when accessing those. At the same time, it
> > > brings a comment regarding the barriers usage, as per the kernel policy. So
> > > overall this is just a clean-up without any functional changes.
> > 
> > Ah, I though there was perhaps the idea to make use of armv7-lpae
> > instructions.
> > 
> > Aside of that, I think we need to spend a little time bike-shedding the
> > API/naming here:
> > 
> > > +# define u64_32read(val, copy) (val)
> > > +# define u64_32read_set_copy(val, copy) do { } while (0)
> > 
> > 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.

-- 
Vincent