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[Bugfix][ROCm] Route non-divisor small-head MLA decode to Gluon on gfx950 - #51403
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…x950 Fixes vllm-project#51232. Kimi-K3 at TP8 gives 12 local heads per rank. 12 is not a divisor of 16, so use_gluon_decode returned False in "auto" mode, routing single-token decode to the padded ASM persistent path. That path pads to 16 heads and asks AITER for a {bf16, fp8, gqa=16, qseqlen=1} kernel which does not exist on gfx950 — every rank crashes with "cannot get heuristic kernel" before the server starts. Prefer Gluon for ALL small-head counts on gfx950 in "auto" mode, not just divisors of 16. Gluon handles any head count natively without padding. On gfx942 (no Gluon build) the ASM path is still used — no behavior change. Signed-off-by: Zvi Grinberg <zgrinber@redhat.com>
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njhill
added
the
mrv1-only
Fixes #51232.
Kimi-K3 has 96 MLA query heads; at TP8 each rank gets 12 local heads.
12 is not a divisor of 16, so
use_gluon_decodereturnedFalsein"auto" mode, routing single-token decode to the padded ASM persistent
path. That path pads 12 heads to 16 and asks AITER for a
{bf16, fp8, gqa=16, qseqlen=1}kernel, which does not exist on gfx950 — everyrank crashes with
cannot get heuristic kernelbefore the serverstarts.
The Gluon MLA decode kernel handles any head count natively (no
padding), so the fix is to prefer Gluon for ALL small-head counts on
gfx950 in "auto" mode, not just divisors of 16. On gfx942 (no Gluon
build) the ASM path is still used for all counts — no behavior change.
VLLM_ROCM_AITER_MLA_ASM_PADDING=asmstill forces the padded ASMpath if needed.
Changes
rocm_aiter_mla.py: remove them % num_heads == 0gate from the"auto" return in
use_gluon_decodetest_rocm_aiter_mla_head_padding.py: update 3 assertions sonon-divisor counts (12, 6) assert
is on_gfx950instead ofnot