On 07/12/2021 13:10, Tvrtko Ursulin wrote:
On 18/10/2021 10:10, Matthew Auld wrote:
For cached objects we can allocate our pages directly in shmem. This
should make it possible(in a later patch) to utilise the existing
i915-gem shrinker code for such objects. For now this is still disabled.
v2(Thomas):
- Add optional try_to_writeback hook for objects. Importantly we need
to check if the object is even still shrinkable; in between us
dropping the shrinker LRU lock and acquiring the object lock it
could for
example have been moved. Also we need to differentiate between
"lazy" shrinking and the immediate writeback mode. Also later we
need to
handle objects which don't even have mm.pages, so bundling this into
put_pages() would require somehow handling that edge case, hence
just letting the ttm backend handle everything in try_to_writeback
doesn't seem too bad.
v3(Thomas):
- Likely a bad idea to touch the object from the unpopulate hook,
since it's not possible to hold a reference, without also creating
circular dependency, so likely this is too fragile. For now just
ensure we at least mark the pages as dirty/accessed when called
from the
shrinker on WILLNEED objects.
- s/try_to_writeback/shrinker_release_pages, since this can do more
than just writeback.
- Get rid of do_backup boolean and just set the SWAPPED flag prior to
calling unpopulate.
- Keep shmem_tt as lowest priority for the TTM LRU bo_swapout walk,
since
these just get skipped anyway. We can try to come up with something
better later.
v4(Thomas):
- s/PCI_DMA/DMA/. Also drop NO_KERNEL_MAPPING and NO_WARN, which
apparently doesn't do anything with streaming mappings.
- Just pass along the error for ->truncate, and assume nothing.
Signed-off-by: Matthew Auld <matthew.auld@xxxxxxxxx>
Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Cc: Oak Zeng <oak.zeng@xxxxxxxxx>
Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Acked-by: Oak Zeng <oak.zeng@xxxxxxxxx>
[snip]
-static void try_to_writeback(struct drm_i915_gem_object *obj,
- unsigned int flags)
+static int try_to_writeback(struct drm_i915_gem_object *obj, unsigned
int flags)
{
+ if (obj->ops->shrinker_release_pages)
+ return obj->ops->shrinker_release_pages(obj,
+ flags & I915_SHRINK_WRITEBACK);
I have a high level question about how does this new vfunc fit with the
existing code.
Because I notice in the implementation (i915_ttm_shrinker_release_pages)
it ends up doing:
...
switch (obj->mm.madv) {
case I915_MADV_DONTNEED:
return i915_ttm_purge(obj);
case __I915_MADV_PURGED:
return 0;
}
... and then...
if (should_writeback)
__shmem_writeback(obj->base.size,
i915_tt->filp->f_mapping);
Which on a glance looks like a possible conceptual duplication of the
concepts in this very function (try_to_writeback):
+
switch (obj->mm.madv) {
case I915_MADV_DONTNEED:
i915_gem_object_truncate(obj);
- return;
+ return 0;
case __I915_MADV_PURGED:
- return;
+ return 0;
}
if (flags & I915_SHRINK_WRITEBACK)
i915_gem_object_writeback(obj);
So question is this the final design or some futher tidy is
possible/planned?
It seems ok to me, all things considered. The TTM version needs to check
if the object is still shrinkable at the start(plus some other stuff),
upon acquiring the object lock. If that succeeds we can do the above
madv checks and potentially just call truncate. Otherwise we can proceed
with shrinking, but again TTM is special here, and we have to call
ttm_bo_validate() underneath(we might not even have mm.pages here). And
then if that all works we might be able to also perform the writeback,
if needed. So I suppose we could add all that directly in
try_to_writeback(), and make it conditional for TTM devices, or I guess
we need two separate hooks, one to do some pre-checking and another do
the actual swap step. Not sure if that is better or worse though.
Background to my question is that I will float a patch which proposes to
remove writeback altogether. There are reports from the fields that it
causes deadlocks and looking at 2d6692e642e7 ("drm/i915: Start writeback
from the shrinker") and its history it seems like it was a known risk.
Apart from the code organisation questions, on the practical level - do
you need writeback from the TTM backend or while I am proposing to
remove it from the "legacy" paths, I can propose removing it from the
TTM flow as well?
Yeah, if that is somehow busted then we should remove from TTM backend also.
Regards,
Tvrtko
