Hi,
On Tue, Jul 28, 2020 at 02:00:27PM +0200, pet...@infradead.org wrote:
> On Tue, Jul 28, 2020 at 01:13:02PM +0200, pet...@infradead.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
