[PATCH 08/21] udl-kms: avoid prefetch

[Mikulas Patocka]

On Wed, 6 Jun 2018, Alexey Brodkin wrote:

> Hi Mikulas,
> 
> On Tue, 2018-06-05 at 11:30 -0400, Mikulas Patocka wrote:
> > 
> > On Tue, 5 Jun 2018, Alexey Brodkin wrote:
> > 
> > > Hi Mikulas,
> > > 
> > > On Sun, 2018-06-03 at 16:41 +0200, Mikulas Patocka wrote:
> > > > Modern processors can detect linear memory accesses and prefetch data
> > > > automatically, so there's no need to use prefetch.
> > > 
> > > Not each and every CPU that's capable of running Linux has prefetch
> > > functionality :)
> > > 
> > > Still read-on...
> > > 
> > > > Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
> > > > 
> > > > ---
> > > >  drivers/gpu/drm/udl/udl_transfer.c |    7 -------
> > > >  1 file changed, 7 deletions(-)
> > > > 
> > > > Index: linux-4.16.12/drivers/gpu/drm/udl/udl_transfer.c
> > > > ===================================================================
> > > > --- linux-4.16.12.orig/drivers/gpu/drm/udl/udl_transfer.c	2018-05-31 14:48:12.000000000 +0200
> > > > +++ linux-4.16.12/drivers/gpu/drm/udl/udl_transfer.c	2018-05-31 14:48:12.000000000 +0200
> > > > @@ -13,7 +13,6 @@
> > > >  #include <linux/module.h>
> > > >  #include <linux/slab.h>
> > > >  #include <linux/fb.h>
> > > > -#include <linux/prefetch.h>
> > > >  #include <asm/unaligned.h>
> > > >  
> > > >  #include <drm/drmP.h>
> > > > @@ -51,9 +50,6 @@ static int udl_trim_hline(const u8 *bbac
> > > >  	int start = width;
> > > >  	int end = width;
> > > >  
> > > > -	prefetch((void *) front);
> > > > -	prefetch((void *) back);
> > > 
> > > AFAIK prefetcher fetches new data according to a known history... i.e. based on previously
> > > used pattern we'll trying to get the next batch of data.
> > > 
> > > But the code above is in the very beginning of the data processing routine where
> > > prefetcher doesn't yet have any history to know what and where to prefetch.
> > > 
> > > So I'd say this particular usage is good.
> > > At least those prefetches shouldn't hurt because typically it
> > > would be just 1 instruction if those exist or nothing if CPU/compiler doesn't
> > > support it.
> > 
> > See this post https://urldefense.proofpoint.com/v2/url?u=https-3A__lwn.net_Articles_444336_&d=DwIBAg&c=DPL6_X_6JkXFx7AXWqB0tg&r=lqdeeSSEes0GFDDl656e
> > ViXO7breS55ytWkhpk5R81I&m=a5RaqJtvajFkM1hL7bOKD5jV7cpFfTvG2Y1cYCdBPd0&s=w0W8wFtAgENp8TE6RzdPGhdKRasJc_otIn08V0EkgrY&e= where they measured that 
> > prefetch hurts performance. Prefetch shouldn't be used unless you have a 
> > proof that it improves performance.
> > 
> > The problem is that the prefetch instruction causes stalls in the pipeline 
> > when it encounters TLB miss and the automatic prefetcher doesn't.
> 
> Wow, thanks for the link.
> I didn't know about that subtle issue with prefetch instructions on ARM and x86.
> 
> So OK in case of UDL these prefetches anyways make not not much sense I guess and there's
> something worse still, see what I've got from WandBoard Quad running kmscube [1] application
> with help of perf utility:
> --------------------------->8-------------------------
> # Overhead  Command  Shared Object            Symbol 
> # ........  .......  .......................  ........................................
> #
>     92.93%  kmscube  [kernel.kallsyms]        [k] udl_render_hline
>      2.51%  kmscube  [kernel.kallsyms]        [k] __divsi3
>      0.33%  kmscube  [kernel.kallsyms]        [k] _raw_spin_unlock_irqrestore
>      0.22%  kmscube  [kernel.kallsyms]        [k] lock_acquire
>      0.19%  kmscube  [kernel.kallsyms]        [k] _raw_spin_unlock_irq
>      0.17%  kmscube  [kernel.kallsyms]        [k] udl_handle_damage
>      0.12%  kmscube  [kernel.kallsyms]        [k] v7_dma_clean_range
>      0.11%  kmscube  [kernel.kallsyms]        [k] l2c210_clean_range
>      0.06%  kmscube  [kernel.kallsyms]        [k] __memzero
> --------------------------->8-------------------------
> 
> That said it's not even USB 2.0 which is a bottle-neck but
> computations in the udl_render_hline().
> 
> 
> [1] https://cgit.freedesktop.org/mesa/kmscube/
> 
> -Alexey

Try this patch 
http://people.redhat.com/~mpatocka/patches/kernel/udl/udlkms-avoid-division.patch

It is doing a lot of divisions - and WandBoard has Cortex-A9, that doesn't 
have division instruction.

BTW. the framebuffer UDL driver (not the modesetting driver) has 
performance counters in sysfs. Their location depends on the system, you 
can find them with find /sys -name "*metrics*"

The file "metrics_reset" resets the counters, so you can measure if the 
prefetch instructions improve performance or not.

Mikulas