On 18 October 2016 at 07:25, Dan Williams <dan.j.williams at intel.com> wrote: > On Sun, Oct 16, 2016 at 1:53 PM, Dave Airlie <airlied at gmail.com> wrote: >> On 17 October 2016 at 04:41, Marek Olšák <maraeo at gmail.com> wrote: >>> On Fri, Oct 14, 2016 at 3:33 AM, Michel Dänzer <michel at daenzer.net> wrote: >>>> >>>> [ Adding Dan Williams and dri-devel ] >>>> >>>> On 14/10/16 03:28 AM, Shawn Starr wrote: >>>>> Hello AMD folks, >>>>> >>>>> I have discovered a problem in Linus master that affects AMDGPU, nobody would >>>>> notice this in drm-next-4.9-wip since its not in this repo. >>>> >>>> [...] >>>> >>>>> 87744ab3832b83ba71b931f86f9cfdb000d07da5 is the first bad commit >>>>> commit 87744ab3832b83ba71b931f86f9cfdb000d07da5 >>>>> Author: Dan Williams <dan.j.williams at intel.com> >>>>> Date: Fri Oct 7 17:00:18 2016 -0700 >>>>> >>>>> mm: fix cache mode tracking in vm_insert_mixed() >>>>> >>>>> vm_insert_mixed() unlike vm_insert_pfn_prot() and vmf_insert_pfn_pmd(), >>>>> fails to check the pgprot_t it uses for the mapping against the one >>>>> recorded in the memtype tracking tree. Add the missing call to >>>>> track_pfn_insert() to preclude cases where incompatible aliased mappings >>>>> are established for a given physical address range. >>>>> >>>>> Link: http://lkml.kernel.org/r/ >>>>> 147328717909.35069.14256589123570653697.stgit at dwillia2- >>>>> desk3.amr.corp.intel.com >>>>> Signed-off-by: Dan Williams <dan.j.williams at intel.com> >>>>> Cc: David Airlie <airlied at linux.ie> >>>>> Cc: Matthew Wilcox <mawilcox at microsoft.com> >>>>> Cc: Ross Zwisler <ross.zwisler at linux.intel.com> >>>>> Signed-off-by: Andrew Morton <akpm at linux-foundation.org> >>>>> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org> >>>>> >>>>> :040000 040000 7517c0019fe49c1830b5a1d81f1dc099c5aab98a >>>>> fd497a604a2af5995db2b8ed1e9c640bede6adf3 M mm >>>>> >>>>> >>>>> Removal of this patch stops graphics stalls. >>>> >>>> Thanks for bisecting this Shawn. >>>> >>>> >>>>> A friend of mine mentions, >>>>> >>>>> "looks like a graphics thingy you depend on is requesting a mapping with a >>>>> not-allowed cache mode, and now you are (rightfully) getting errors?" >>>> >>>> It would be nice to get some more specific pointers what amdgpu (or >>>> maybe ttm, since that calls vm_insert_mixed in ttm_bo_vm_fault) might be >>>> doing wrong. >> >> /* >> * We'd like to use VM_PFNMAP on shared mappings, where >> * (vma->vm_flags & VM_SHARED) != 0, for performance reasons, >> * but for some reason VM_PFNMAP + x86 PAT + write-combine is very >> * bad for performance. Until that has been sorted out, use >> * VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719 >> */ >> vma->vm_flags |= VM_MIXEDMAP; >> >> We have that comment in the ttm code, which to me implies that mixed is >> doing the right thing now, but that is slow, as the interface we >> should be using. >> > > Aren't there only 2 possibilities for this regression? > > 1/ a memtype entry was never made so track_pfn_insert() returns an > uncached mapping > > 2/ a conflicting memtype entry exists and undefined behavior due to > mixed mapping types is avoided with the change. 3/ The CPU usage through this path goes up, and slows things down, though I suspect you it's more an uncached mapping showing up when we don't expect it. Dave.
