On Wed, 27 Sep 2017 22:03:15 +0200 Boris Brezillon <[email protected]> wrote:
> On Wed, 27 Sep 2017 12:41:52 -0700 > Eric Anholt <[email protected]> wrote: > > > Boris Brezillon <[email protected]> writes: > > > > > On Wed, 27 Sep 2017 10:15:23 -0700 > > > Eric Anholt <[email protected]> wrote: > > > > > >> Boris Brezillon <[email protected]> writes: > > >> > > >> > On Wed, 27 Sep 2017 15:24:16 +0100 > > >> > Chris Wilson <[email protected]> wrote: > > >> > > > >> >> Quoting Boris Brezillon (2017-09-27 15:06:53) > > >> >> > On Wed, 27 Sep 2017 14:50:10 +0100 > > >> >> > Chris Wilson <[email protected]> wrote: > > >> >> > > > >> >> > > Quoting Boris Brezillon (2017-09-27 14:45:17) > > >> >> > > > static struct vc4_bo * > > >> >> > > > vc4_bo_from_cache(struct vc4_screen *screen, uint32_t size, > > >> >> > > > const char *name) > > >> >> > > > { > > >> >> > > > @@ -111,6 +121,11 @@ vc4_bo_from_cache(struct vc4_screen > > >> >> > > > *screen, uint32_t size, const char *name) > > >> >> > > > return NULL; > > >> >> > > > } > > >> >> > > > > > >> >> > > > + if (vc4_bo_purgeable(bo, false)) { > > >> >> > > > + mtx_unlock(&cache->lock); > > >> >> > > > + return NULL; > > >> >> > > > > >> >> > > So this would just mean that the bo was purged in the meantime. > > >> >> > > Why not > > >> >> > > just try to use the next one in the cache or allocate afresh? > > >> >> > > > > >> >> > > > >> >> > No, this means the BO was purged and the kernel failed to allocate > > >> >> > the > > >> >> > memory back. We don't care about the retained status here, because > > >> >> > we > > >> >> > don't need to restore BO's content, that's why we're not checking > > >> >> > arg.retained in vc4_bo_purgeable(). Allocating a fresh BO is likely > > >> >> > to > > >> >> > fail with the same ENOMEM error because both path use the CMA mem. > > >> >> > > > >> >> > > >> >> Hmm, you don't treat purging as permanent. But you do track the lose > > >> >> of > > >> >> contents, so retained is false? > > >> > > > >> > vc4_bo_purgeable() is not reporting the retained status, it just > > >> > reports whether the BO can be used or not. I can change > > >> > vc4_bo_purgeable() semantic to return 1 if the BO content was retained, > > >> > 0 if it was purged and -1 if you the ioctl returned an error (ENOMEM) > > >> > if you prefer, but in the end, all I'll check here is > > >> > 'vc4_bo_purgeable() >= 0' because I don't don't care about the retained > > >> > status in this specific use case, all I care about is whether the BO > > >> > can > > >> > be re-used or not (IOW, is there a valid CMA region attached to the > > >> > BO). > > >> > > > >> >> > > >> >> I took a harder line, and said that userspace should recreate the > > >> >> object > > >> >> from scratch after it was purged. I thought that would be easier > > >> >> overall. But maybe not.:) > > >> > > > >> > Well, maybe I'm wrong in how I implemented this > > >> > DRM_IOCTL_VC4_GEM_MADVISE ioctl, but right now, when the BO has been > > >> > purged and someone marks it back as unpurgeable I'm trying to > > >> > re-allocate BO's buffer in the ioctl path, and if the CMA allocation > > >> > fails I return -ENOMEM. I could move the allocation in the fault > > >> > handler, but this would result in pretty much the same behavior except > > >> > it would require an extra page-fault to realize the memory is not > > >> > available or force us to check the retained status and decide to > > >> > release the BO object from the BO cache. > > >> > > >> Hmm. The downside I see to this plan is if we eventually decide to have > > >> the purge operation not clear all the BOs, then we would probably rather > > >> have userspace freeing objects that had been purged until it finds one > > >> in the cache that hadn't been purged, rather than forcing reallocation > > >> of this BO now (and possibly then purging something from elsewhere in > > >> the cache). > > > > > > Okay, that's a good reason to move dma_alloc_wc() in the page-fault > > > path. I need to change a bit the implementation to check cma_gem->vaddr > > > value instead of checking bo->madv != __VC4_MADV_PURGED, otherwise we > > > might pass a non-allocated BO to the GPU/Display-Engine. > > > > Huh, allocation in the page-fault path? We would need the storage to be > > definitely be available at the point that we've set it back to WILLNEED. > > Otherwise I'll "allocate" the BO from the cache, go to fill it through > > my mapping, and sigbus when CMA says we're out of memory. > > Yep, I find that weird too, but that's unfortunately the only way we can > achieve what you want to do. > > The only solution to know the ->retained status is by asking the the DRM > driver to put the BO in WILLNEED or DONTNEED state. If you send ->madv > = DONTNEED, and the kernel returns ->retained = true, this ->retained > state may not be valid anymore when you get back to the application, > because someone else may have triggered a purge. If you send ->madv = > WILLNEED then the ->retained state is guaranteed to be valid until you > explicitly switch back to DONTNEED, but that also means the driver has > already allocated the memory if ->retained is false, so it's already > too late to do what you were suggesting (evict the BO from the > userspace cache to avoid purging other purgeable BOs). Another solution to this problem would be to add a new WILLNEED_IF_NOT_PURGED state that would instruct the driver to only flag the BO as WILLNEED if it's not been purged already. _______________________________________________ mesa-dev mailing list [email protected] https://lists.freedesktop.org/mailman/listinfo/mesa-dev
