On Tue, 19 Jun 2012 09:22:03 +0100 Chris Wilson <chris at chris-wilson.co.uk> wrote: > On Mon, 18 Jun 2012 20:38:15 -0700, Ben Widawsky <ben at bwidawsk.net> wrote: > > The history on this patch goes back quite a way. This time around, the > > patch builds on top of the map_unsynchronized that Eric pushed. Eric's > > patch attempted only to solve the problem for LLC machines. Unlike > > my earlier versions of this patch (with the help from Daniel Vetter), we > > do not attempt to cpu map objects in a unsynchronized manner. > > > > The concept is fairly simple - once a buffer is moved into the GTT > > domain, we can assume it remains there unless we tell it otherwise (via > > cpu map). It therefore stands to reason that as long as we can keep the > > object in the GTT domain, and don't ever count on reading back contents, > > things might just work. I believe as long as we are doing GTT mappings > > only, we get to avoid worry about clflushing the dirtied cachelines, but > > that could use some fact checking. > > > > The patch makes some assumptions about how the kernel does buffer > > tracking, this could be conceived as an ABI dependency, but actually the > > behavior is pretty confined. It exploits the fact the BOs are only moved > > into the CPU domain under certain circumstances, and daintily dances > > around those conditions. The main thing here is we assume MADV_WILLNEED > > prevents the object from getting evicted. > > > > I am not aware of a good way to test it's effectiveness > > performance-wise; but it introduces no regressions with piglit on my > > ILK, or SNB. > > This is broken wrt to cache invalidation if I want to rewrite part of > the buffer that already has been read by the GPU. > -Chris > Well if you're talking about what I think you're talking about (ie. not clflushing, but simply dealing with the GPUs internal caching). It's a problem that has existed with all of the non-LLC non-blocking map patches; and sort of the point of non-blocking maps. Play it fast and loose, submit pipe controls if you get nervous. Did I catch your meaning, or were you just talking about clflushing stuff (we also miss chipset flush on really old platforms; I was thinking of restricting this to ILK only)? -- Ben Widawsky, Intel Open Source Technology Center
