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[fused_rmsnorm] Avoid querying device inside forward #301

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[fused_rmsnorm] Avoid querying device inside forward #301

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9 changes: 8 additions & 1 deletion torchtitan/models/norms.py
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Original file line number Diff line number Diff line change
Expand Up @@ -213,6 +213,13 @@ def _rms_norm_bwd_kernel_sm(
tl.store(DW + row_block_id * N + cols, dw, mask=mask)


def get_sm_count():
# TODO(whc) initializing sm_count here prevents the need to query device during tracing using meta-device.
# But it also forces us to pick a favorite device.
# possibly, we could build a dict of device_id: sm_count here, and then index it during forward instead.
return torch.cuda.get_device_properties(0).multi_processor_count


class TritonFusedRMSNorm(torch.autograd.Function):
@staticmethod
def forward(ctx, x, weight, eps):
Expand Down Expand Up @@ -268,7 +275,7 @@ def backward(ctx, dy):
dx = torch.empty_like(x)
dw = torch.empty_like(weight)

sm_count = torch.cuda.get_device_properties(x.device).multi_processor_count
sm_count = get_sm_count()
_dw = torch.empty((sm_count, N), dtype=torch.float32, device=weight.device)

max_size = 65536 // x.element_size()
Expand Down