On 9/15/25 14:48, Christian König wrote:
On 15.09.25 14:36, Natalie Vock wrote:
Hi all,
I've been looking into some cases where dmem protection fails to prevent
allocations from ending up in GTT when VRAM gets scarce and apps start
competing hard.
In short, this is because other (unprotected) applications end up
filling VRAM before protected applications do. This causes TTM to back
off and try allocating in GTT before anything else, and that is where
the allocation is placed in the end. The existing eviction protection
cannot prevent this, because no attempt at evicting is ever made
(although you could consider the backing-off as an immediate eviction to
GTT).
Well depending on what you gave as GEM flags from userspace that is expected
behavior.
For applications using RADV we usually give GTT|VRAM as placement which
basically tells the kernel that it shouldn't evict at all and immediately
fallback to GTT.
Yeah, in general this behavior is completely expected - though I'd argue
that protecting VRAM via dmemcg influences the semantics a little here.
Giving GTT|VRAM as placement from userspace essentially says "ok, please
try allocating this in VRAM, but it's ok to fall back to GTT" - whereas
specifying VRAM only essentially says "ok, please allocate this in VRAM,
and really try hard to keep it in VRAM whatever the cost".
Usually, resource allocation failing is good enough of an indicator that
it's not possible to allocate in VRAM. However, when the application's
memory is protected by dmemcg, it essentially says that it actually
should be possible to allocate up to that amount of memory - the cgroup
is entitled to that memory, and the other unprotected cgroups have to
make do with the rest.
I think it's a justifiable tradeoff between the indended function of
VRAM|GTT and the intended function of dmem memory protection to evict
these unprotected cgroups for only as long as the usage doesn't exceed
the awarded protection - this is what this series implements (dropping
the GTT flag in userspace would have negative implications in the case
the app uses more memory than the protection afforded to it, and as I
described, just letting protected memory allocations fall back to GTT is
insufficient too).
Thanks,
Natalie
Regards,
Christian.
This series tries to alleviate this by adding a special case when the
allocation is protected by cgroups: Instead of backing off immediately,
TTM will try evicting unprotected buffers from the domain to make space
for the protected one. This ensures that applications can actually use
all the memory protection awarded to them by the system, without being
prone to ping-ponging (only protected allocations can evict unprotected
ones, never the other way around).
The first two patches just add a few small utilities needed to implement
this to the dmem controller. The second two patches are the TTM
implementation:
"drm/ttm: Be more aggressive..." decouples cgroup charging from resource
allocation to allow us to hold on to the charge even if allocation fails
on first try, and adds a path to call ttm_bo_evict_alloc when the
charged allocation falls within min/low protection limits.
"drm/ttm: Use common ancestor..." is a more general improvement in
correctly implementing cgroup protection semantics. With recursive
protection rules, unused memory protection afforded to a parent node is
transferred to children recursively, which helps protect entire
subtrees from stealing each others' memory without needing to protect
each cgroup individually. This doesn't apply when considering direct
siblings inside the same subtree, so in order to not break
prioritization between these siblings, we need to consider the
relationship of evictor and evictee when calculating protection.
In practice, this fixes cases where a protected cgroup cannot steal
memory from unprotected siblings (which, in turn, leads to eviction
failures and new allocations being placed in GTT).
Thanks,
Natalie
Signed-off-by: Natalie Vock <[email protected]>
---
Natalie Vock (4):
cgroup/dmem: Add queries for protection values
cgroup/dmem: Add dmem_cgroup_common_ancestor helper
drm/ttm: Be more aggressive when allocating below protection limit
drm/ttm: Use common ancestor of evictor and evictee as limit pool
drivers/gpu/drm/ttm/ttm_bo.c | 79 ++++++++++++++++++++++++++++++++------
drivers/gpu/drm/ttm/ttm_resource.c | 48 ++++++++++++++++-------
include/drm/ttm/ttm_resource.h | 6 ++-
include/linux/cgroup_dmem.h | 25 ++++++++++++
kernel/cgroup/dmem.c | 73 +++++++++++++++++++++++++++++++++++
5 files changed, 205 insertions(+), 26 deletions(-)
---
base-commit: f3e82936857b3bd77b824ecd2fa7839dd99ec0c6
change-id: 20250915-dmemcg-aggressive-protect-5cf37f717cdb
Best regards,
