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Fix: Skip invalid GQA tile candidates with stepQ < headsQPerKv. #2383

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KungYork wants to merge 1 commit into from
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Fix: Skip invalid GQA tile candidates with stepQ < headsQPerKv. #2383

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7 changes: 7 additions & 0 deletions include/flashinfer/trtllm/fmha/fmhaKernels.cuh
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Original file line number Diff line number Diff line change
Expand Up @@ -626,6 +626,13 @@ class TllmGenFmhaKernel {
// Load the kernel.
std::tie(func, kernelMeta) = loadKernel(params, selectKernelParamsCopy);

// Handle the exception where computeCtaAndClusterConfig results in numTokensPerCtaQ == 0.
if (kernelMeta.mGroupsTokensHeadsQ) {
if (kernelMeta.mStepQ < params.mNumHeadsQPerKv) {
continue; // Skip this candidate as it's invalid
}
}
Comment on lines +629 to +634

@gemini-code-assist gemini-code-assist Bot Jan 20, 2026

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critical

This check is crucial for preventing a division-by-zero error. However, the fix appears to be incomplete. A similar crash can occur earlier in the selectTileSizeQForGqaGeneration function.

The computeCtaAndClusterConfig function is also called on line 591, which is before this loop. If the initial kernel parameters result in kernelMeta.mStepQ < params.mNumHeadsQPerKv, the program will crash at line 591, as the check is not present there.

To fully resolve the issue, a similar check should be added before the call to computeCtaAndClusterConfig on line 591.

Comment on lines +630 to +634

@gemini-code-assist gemini-code-assist Bot Jan 20, 2026

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medium

For improved readability and conciseness, you can combine these nested if statements into a single condition using the logical AND (&&) operator.

      if (kernelMeta.mGroupsTokensHeadsQ && kernelMeta.mStepQ < params.mNumHeadsQPerKv) {
        continue; // Skip this candidate as it's invalid
      }


// Compute the number of CTAs.
computeCtaAndClusterConfig(ctaLaunchParams, params, kernelMeta, selectKernelParamsCopy);

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