linux-kernel - RE: [PATCH] x86: introduce memcpy_flushcache

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Date:   Sat, 18 Apr 2020 13:27:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mikulas Patocka' <mpatocka@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra" <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        device-mapper development <dm-devel@...hat.com>
Subject: RE: [PATCH] x86: introduce memcpy_flushcache_clflushopt

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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