Re: [PATCH 2/4] drm/cache: Try to be smarter about clflushing on x86

[Ben Widawsky <benjamin.widawsky@intel.com>]

On Sat, Dec 13, 2014 at 08:15:22PM -0800, Matt Turner wrote:
> On Sat, Dec 13, 2014 at 7:08 PM, Ben Widawsky
> <benjamin.widawsky@intel.com> wrote:
> > Any GEM driver which has very large objects and a slow CPU is subject to very
> > long waits simply for clflushing incoherent objects. Generally, each individual
> > object is not a problem, but if you have very large objects, or very many
> > objects, the flushing begins to show up in profiles. Because on x86 we know the
> > cache size, we can easily determine when an object will use all the cache, and
> > forego iterating over each cacheline.
> >
> > We need to be careful when using wbinvd. wbinvd() is itself potentially slow
> > because it requires synchronizing the flush across all CPUs so they have a
> > coherent view of memory. This can result in either stalling work being done on
> > other CPUs, or this call itself stalling while waiting for a CPU to accept the
> > interrupt. Also, wbinvd() also has the downside of invalidating all cachelines,
> > so we don't want to use it unless we're sure we already own most of the
> > cachelines.
> >
> > The current algorithm is very naive. I think it can be tweaked more, and it
> > would be good if someone else gave it some thought. I am pretty confident in
> > i915, we can even skip the IPI in the execbuf path with minimal code change (or
> > perhaps just some verifying of the existing code). It would be nice to hear what
> > other developers who depend on this code think.
> >
> > Cc: Intel GFX <intel-gfx@lists.freedesktop.org>
> > Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
> > ---
> >  drivers/gpu/drm/drm_cache.c | 20 +++++++++++++++++---
> >  1 file changed, 17 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
> > index d7797e8..6009c2d 100644
> > --- a/drivers/gpu/drm/drm_cache.c
> > +++ b/drivers/gpu/drm/drm_cache.c
> > @@ -64,6 +64,20 @@ static void drm_cache_flush_clflush(struct page *pages[],
> >                 drm_clflush_page(*pages++);
> >         mb();
> >  }
> > +
> > +static bool
> > +drm_cache_should_clflush(unsigned long num_pages)
> > +{
> > +       const int cache_size = boot_cpu_data.x86_cache_size;
> > +
> > +       /* For now the algorithm simply checks if the number of pages to be
> > +        * flushed is greater than the entire system cache. One could make the
> > +        * function more aware of the actual system (ie. if SMP, how large is
> > +        * the cache, CPU freq. etc. All those help to determine when to
> > +        * wbinvd() */
> > +       WARN_ON_ONCE(!cache_size);
> > +       return !cache_size || num_pages < (cache_size >> 2);
> > +}
> >  #endif
> >
> >  void
> > @@ -71,7 +85,7 @@ drm_clflush_pages(struct page *pages[], unsigned long num_pages)
> >  {
> >
> >  #if defined(CONFIG_X86)
> > -       if (cpu_has_clflush) {
> > +       if (cpu_has_clflush && drm_cache_should_clflush(num_pages)) {
> >                 drm_cache_flush_clflush(pages, num_pages);
> >                 return;
> >         }
> > @@ -104,7 +118,7 @@ void
> >  drm_clflush_sg(struct sg_table *st)
> >  {
> >  #if defined(CONFIG_X86)
> > -       if (cpu_has_clflush) {
> > +       if (cpu_has_clflush && drm_cache_should_clflush(st->nents)) {
> >                 struct sg_page_iter sg_iter;
> >
> >                 mb();
> > @@ -128,7 +142,7 @@ void
> >  drm_clflush_virt_range(void *addr, unsigned long length)
> >  {
> >  #if defined(CONFIG_X86)
> > -       if (cpu_has_clflush) {
> > +       if (cpu_has_clflush && drm_cache_should_clflush(length / PAGE_SIZE)) {
> 
> If length isn't a multiple of page size, isn't this ignoring the
> remainder? Should it be rounding length up to the next multiple of
> PAGE_SIZE, like ROUND_UP_TO?

Yeah, we could round_up. In practice it probably won't matter. I actually think
it would be better to pass a size to drm_cache_should_clflush(), and let that
round it up. It sounds like people don't want this patch anyway, so I'll make
the equivalent change in the i915 only patch.

-- 
Ben Widawsky, Intel Open Source Technology Center
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