RE: [PATCH] x86: introduce memcpy_flushcache_clflushopt

[David Laight]

From: Mikulas Patocka
> Sent: 17 April 2020 13:47
...
> Index: linux-2.6/drivers/md/dm-writecache.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-writecache.c	2020-04-17 14:06:35.139999000 +0200
> +++ linux-2.6/drivers/md/dm-writecache.c	2020-04-17 14:06:35.129999000 +0200
> @@ -1166,7 +1166,10 @@ static void bio_copy_block(struct dm_wri
>  			}
>  		} else {
>  			flush_dcache_page(bio_page(bio));
> -			memcpy_flushcache(data, buf, size);
> +			if (likely(size > 512))
> +				memcpy_flushcache_clflushopt(data, buf, size);
> +			else
> +				memcpy_flushcache(data, buf, size);

Hmmm... have you looked at how long clflush actually takes?
It isn't too bad if you just do a small number, but using it
to flush large buffers can be very slow.

I've an Ivy bridge system where the X-server process requests the
frame buffer be flushed out every 10 seconds (no idea why).
With my 2560x1440 monitor this takes over 3ms.

This really needs a cond_resched() every few clflush instructions.

	David

-
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