Re: [Intel-gfx] [PATCH] iosys-map: Add word-sized reads

["Christian König" <christian.koenig@amd.com>]

Am 10.06.22 um 01:20 schrieb Lucas De Marchi:
> Instead of always falling back to memcpy_fromio() for any size, prefer
> using read{b,w,l}(). When reading struct members it's common to read
> individual integer variables individually. Going through memcpy_fromio()
> for each of them poses a high penalty.
>
> Employ a similar trick as __seqprop() by using _Generic() to generate
> only the specific call based on a type-compatible variable.
>
> For a pariticular i915 workload producing GPU context switches,
> __get_engine_usage_record() is particularly hot since the engine usage
> is read from device local memory with dgfx, possibly multiple times
> since it's racy. Test execution time for this test shows a ~12.5%
> improvement with DG2:
>
> Before:
> 	nrepeats = 1000; min = 7.63243e+06; max = 1.01817e+07;
> 	median = 9.52548e+06; var = 526149;
> After:
> 	nrepeats = 1000; min = 7.03402e+06; max = 8.8832e+06;
> 	median = 8.33955e+06; var = 333113;
>
> Other things attempted that didn't prove very useful:
> 1) Change the _Generic() on x86 to just dereference the memory address
> 2) Change __get_engine_usage_record() to do just 1 read per loop,
>     comparing with the previous value read
> 3) Change __get_engine_usage_record() to access the fields directly as it
>     was before the conversion to iosys-map
>
> (3) did gave a small improvement (~3%), but doesn't seem to scale well
> to other similar cases in the driver.
>
> Additional test by Chris Wilson using gem_create from igt with some
> changes to track object creation time. This happens to accidentaly
> stress this code path:
>
> 	Pre iosys_map conversion of engine busyness:
> 	lmem0: Creating    262144 4KiB objects took 59274.2ms
>
> 	Unpatched:
> 	lmem0: Creating    262144 4KiB objects took 108830.2ms
>
> 	With readl (this patch):
> 	lmem0: Creating    262144 4KiB objects took 61348.6ms
>
> 	s/readl/READ_ONCE/
> 	lmem0: Creating    262144 4KiB objects took 61333.2ms
>
> So we do take a little bit more time than before the conversion, but
> that is due to other factors: bringing the READ_ONCE back would be as
> good as just doing this conversion.
>
> Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
> ---
>
> If this is acceptable we should probably add the write counterpart, too.
> Sending here only the read for now since this fixes the issue we are
> seeing and to gather feedback.

As far as I can see looks sane to me, but the kernel test robot tears 
the patch apart.

Probably just a typo somewhere in the 32bit handling.

Apart from that looks good to me.

Regards,
Christian.

>
>   include/linux/iosys-map.h | 26 ++++++++++++++++++++++----
>   1 file changed, 22 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/iosys-map.h b/include/linux/iosys-map.h
> index e69a002d5aa4..4ae3e459419e 100644
> --- a/include/linux/iosys-map.h
> +++ b/include/linux/iosys-map.h
> @@ -333,6 +333,20 @@ static inline void iosys_map_memset(struct iosys_map *dst, size_t offset,
>   		memset(dst->vaddr + offset, value, len);
>   }
>   
> +#ifdef CONFIG_64BIT
> +#define __iosys_map_rd_io_u64_case(val_, vaddr_iomem_)			\
> +	u64: val_ = readq(vaddr_iomem_),
> +#else
> +#define __iosys_map_u64_case(val_, vaddr_iomem_)
> +#endif
> +
> +#define __iosys_map_rd_io(val__, vaddr_iomem__, type__) _Generic(val__,	\
> +	u8: val__ = readb(vaddr_iomem__),				\
> +	u16: val__ = readw(vaddr_iomem__),				\
> +	u32: val__ = readl(vaddr_iomem__),				\
> +	__iosys_map_rd_io_u64_case(val__, vaddr_iomem__)		\
> +	default: memcpy_fromio(&(val__), vaddr_iomem__, sizeof(val__)))
> +
>   /**
>    * iosys_map_rd - Read a C-type value from the iosys_map
>    *
> @@ -346,10 +360,14 @@ static inline void iosys_map_memset(struct iosys_map *dst, size_t offset,
>    * Returns:
>    * The value read from the mapping.
>    */
> -#define iosys_map_rd(map__, offset__, type__) ({			\
> -	type__ val;							\
> -	iosys_map_memcpy_from(&val, map__, offset__, sizeof(val));	\
> -	val;								\
> +#define iosys_map_rd(map__, offset__, type__) ({				\
> +	type__ val;								\
> +	if ((map__)->is_iomem) {						\
> +		__iosys_map_rd_io(val, (map__)->vaddr_iomem + offset__, type__);\
> +	} else {								\
> +		memcpy(&val, (map__)->vaddr + offset__, sizeof(val));		\
> +	}									\
> +	val;									\
>   })
>   
>   /**