[Intel-gfx] [PATCH] drm/i915/gtt: Pull global wc page stash under its own locking
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Wed Jul 4 12:48:18 UTC 2018
On 04/07/2018 12:39, Chris Wilson wrote:
> Currently, the wc-stash used for providing flushed WC pages ready for
> constructing the page directories is assumed to be protected by the
> struct_mutex. However, we want to remove this global lock and so must
> install a replacement global lock for accessing the global wc-stash (the
> per-vm stash continues to be guarded by the vm).
>
> We need to push ahead on this patch due to an oversight in hastily
> removing the struct_mutex guard around the igt_ppgtt_alloc selftest. No
> matter, it will prove very useful (i.e. will be required) in the near
> future.
>
> Fixes: 1f6f00238abf ("drm/i915/selftests: Drop struct_mutex around lowlevel pggtt allocation")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
> drivers/gpu/drm/i915/i915_drv.h | 5 ++
> drivers/gpu/drm/i915/i915_gem_gtt.c | 83 ++++++++++++++++-------------
> 2 files changed, 52 insertions(+), 36 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 2cefe4c30f88..696c0b36f81e 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -954,6 +954,11 @@ struct i915_gem_mm {
> */
> struct pagevec wc_stash;
>
> + /**
> + * Lock for the small stash of WC pages.
> + */
> + spinlock_t wc_lock;
> +
> /**
> * tmpfs instance used for shmem backed objects
> */
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index c6aa761ca085..e0e89e3ae43b 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -377,25 +377,28 @@ static gen6_pte_t iris_pte_encode(dma_addr_t addr,
>
> static struct page *vm_alloc_page(struct i915_address_space *vm, gfp_t gfp)
> {
> - struct pagevec *pvec = &vm->free_pages;
> - struct pagevec stash;
> + struct pagevec *stash = &vm->free_pages;
> + struct pagevec *pvec;
> + struct page *page;
>
> if (I915_SELFTEST_ONLY(should_fail(&vm->fault_attr, 1)))
> i915_gem_shrink_all(vm->i915);
>
> - if (likely(pvec->nr))
> - return pvec->pages[--pvec->nr];
> + if (likely(stash->nr))
> + return stash->pages[--stash->nr];
This is still covered by the mutex?
>
> if (!vm->pt_kmap_wc)
> return alloc_page(gfp);
>
> - /* A placeholder for a specific mutex to guard the WC stash */
> - lockdep_assert_held(&vm->i915->drm.struct_mutex);
> -
> /* Look in our global stash of WC pages... */
> pvec = &vm->i915->mm.wc_stash;
> + page = NULL;
> + spin_lock(&vm->i915->mm.wc_lock);
> if (likely(pvec->nr))
> - return pvec->pages[--pvec->nr];
> + page = pvec->pages[--pvec->nr];
> + spin_unlock(&vm->i915->mm.wc_lock);
> + if (page)
> + return page;
>
> /*
> * Otherwise batch allocate pages to amoritize cost of set_pages_wc.
> @@ -405,7 +408,6 @@ static struct page *vm_alloc_page(struct i915_address_space *vm, gfp_t gfp)
> * So we add our WB pages into a temporary pvec on the stack and merge
> * them into the WC stash after all the allocations are complete.
> */
> - pagevec_init(&stash);
> do {
> struct page *page;
>
> @@ -413,28 +415,30 @@ static struct page *vm_alloc_page(struct i915_address_space *vm, gfp_t gfp)
> if (unlikely(!page))
> break;
>
> - stash.pages[stash.nr++] = page;
> - } while (stash.nr < pagevec_space(pvec));
> + stash->pages[stash->nr++] = page;
> + } while (pagevec_space(stash));
Comment is talking about a temporary stack stash but now that's not the
case any more. As minimum comment needs updating, but I don't understand
ATM if the new approach is safe, or in other words what was going wrong
before. If there is another path to the stash then stash->nr++ is
obviously not safe.
>
> - if (stash.nr) {
> - int nr = min_t(int, stash.nr, pagevec_space(pvec));
> - struct page **pages = stash.pages + stash.nr - nr;
> + if (stash->nr && !set_pages_array_wc(stash->pages, stash->nr)) {
Previously the test was for pages which obviously the local thread
owned. Now I am not sure if the condition says.
> + int nr;
>
> - if (nr && !set_pages_array_wc(pages, nr)) {
> - memcpy(pvec->pages + pvec->nr,
> - pages, sizeof(pages[0]) * nr);
> - pvec->nr += nr;
> - stash.nr -= nr;
> - }
> + /* Merge spare WC pages to the global stash */
> + spin_lock(&vm->i915->mm.wc_lock);
> + nr = min_t(int, stash->nr - 1, pagevec_space(pvec));
> + memcpy(pvec->pages + pvec->nr,
> + stash->pages + stash->nr - nr,
> + sizeof(stash->pages[0]) * nr);
> + pvec->nr += nr;
> + spin_unlock(&vm->i915->mm.wc_lock);
>
> - pagevec_release(&stash);
> + stash->nr -= nr;
> + page = stash->pages[--stash->nr];
> }
>
> - return likely(pvec->nr) ? pvec->pages[--pvec->nr] : NULL;
> + return page;
> }
>
> static void vm_free_pages_release(struct i915_address_space *vm,
> - bool immediate)
> + unsigned int immediate)
> {
> struct pagevec *pvec = &vm->free_pages;
>
> @@ -442,28 +446,32 @@ static void vm_free_pages_release(struct i915_address_space *vm,
>
> if (vm->pt_kmap_wc) {
> struct pagevec *stash = &vm->i915->mm.wc_stash;
> + spinlock_t *lock = &vm->i915->mm.wc_lock;
>
> - /* When we use WC, first fill up the global stash and then
> + /*
> + * When we use WC, first fill up the global stash and then
> * only if full immediately free the overflow.
> */
>
> - lockdep_assert_held(&vm->i915->drm.struct_mutex);
> + spin_lock(lock);
> if (pagevec_space(stash)) {
> do {
> stash->pages[stash->nr++] =
> pvec->pages[--pvec->nr];
> if (!pvec->nr)
> - return;
> + break;
> } while (pagevec_space(stash));
> -
> - /* As we have made some room in the VM's free_pages,
> - * we can wait for it to fill again. Unless we are
> - * inside i915_address_space_fini() and must
> - * immediately release the pages!
> - */
> - if (!immediate)
> - return;
> }
> + spin_unlock(lock);
> +
> + /*
> + * As we have made some room in the VM's free_pages,
> + * we can wait for it to fill again. Unless we are
> + * inside i915_address_space_fini() and must
> + * immediately release the pages!
> + */
> + if (pvec->nr <= immediate)
> + return;
>
> set_pages_array_wb(pvec->pages, pvec->nr);
> }
> @@ -482,7 +490,7 @@ static void vm_free_page(struct i915_address_space *vm, struct page *page)
> */
> might_sleep();
> if (!pagevec_add(&vm->free_pages, page))
> - vm_free_pages_release(vm, false);
> + vm_free_pages_release(vm, PAGEVEC_SIZE - 1);
I suggest keeping the immediate parameter as boolean to go one change at
a time.
> }
>
> static int __setup_page_dma(struct i915_address_space *vm,
> @@ -2123,7 +2131,7 @@ static void i915_address_space_init(struct i915_address_space *vm,
> static void i915_address_space_fini(struct i915_address_space *vm)
> {
> if (pagevec_count(&vm->free_pages))
> - vm_free_pages_release(vm, true);
> + vm_free_pages_release(vm, 0);
>
> drm_mm_takedown(&vm->mm);
> list_del(&vm->global_link);
> @@ -3518,6 +3526,9 @@ int i915_ggtt_init_hw(struct drm_i915_private *dev_priv)
> struct i915_ggtt *ggtt = &dev_priv->ggtt;
> int ret;
>
> + spin_lock_init(&dev_priv->mm.wc_lock);
> + pagevec_init(&dev_priv->mm.wc_stash);
> +
> INIT_LIST_HEAD(&dev_priv->vm_list);
>
> /* Note that we use page colouring to enforce a guard page at the
>
Regards,
Tvrtko
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