[LoRA]Disable linear LoRA kernel PDL

[jeejeelee]

Purpose

Currently, we found that there is a base model GEMM operator between the two LoRA operators in linear LoRA. This causes PDL to be ineffective and actually negatively impacts LoRA performance. Therefore, this PR temporarily disables PDL.

• main branch

============ Serving Benchmark Result ============
Successful requests:                     100       
Failed requests:                         0         
Benchmark duration (s):                  40.08     
Total input tokens:                      204700    
Total generated tokens:                  102400    
Request throughput (req/s):              2.49      
Output token throughput (tok/s):         2554.87   
Peak output token throughput (tok/s):    3900.00   
Peak concurrent requests:                100.00    
Total token throughput (tok/s):          7662.13   
---------------Time to First Token----------------
Mean TTFT (ms):                          5831.36   
Median TTFT (ms):                        5925.33   
P90 TTFT (ms):                           10330.85  
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          33.29     
Median TPOT (ms):                        33.23     
P90 TPOT (ms):                           37.24     
---------------Inter-token Latency----------------
Mean ITL (ms):                           33.29     
Median ITL (ms):                         28.30     
P90 ITL (ms):                            31.06     
==================================================

• This PR

============ Serving Benchmark Result ============
Successful requests:                     100       
Failed requests:                         0         
Benchmark duration (s):                  39.72     
Total input tokens:                      204700    
Total generated tokens:                  102400    
Request throughput (req/s):              2.52      
Output token throughput (tok/s):         2577.76   
Peak output token throughput (tok/s):    3905.00   
Peak concurrent requests:                100.00    
Total token throughput (tok/s):          7730.75   
---------------Time to First Token----------------
Mean TTFT (ms):                          5826.86   
Median TTFT (ms):                        5710.47   
P90 TTFT (ms):                           10110.86  
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          32.95     
Median TPOT (ms):                        33.09     
P90 TPOT (ms):                           37.10     
---------------Inter-token Latency----------------
Mean ITL (ms):                           32.95     
Median ITL (ms):                         27.81     
P90 ITL (ms):                            30.99     
==================================================

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

Documentation preview: https://vllm--31777.org.readthedocs.build/en/31777/

[gemini-code-assist]

Code Review

This pull request disables Programmatic Dependent Launch (PDL) for the linear LoRA kernels (lora_expand and lora_shrink) by hardcoding use_gdc to False. This is a reasonable change as the comment explains that these kernels are not launched back-to-back, which is a prerequisite for PDL to be effective and can otherwise harm performance. The documentation is also updated to point to a relevant GitHub issue.

The changes are correct and well-justified. I have one suggestion regarding a related potential issue in the MoE LoRA implementation where PDL might also be used incorrectly under certain conditions (fully_sharded=True). Addressing this would improve the robustness of the LoRA implementation.

[jeejeelee]

BTW, add MM LoRA support status here

[danisereb]

@jeejeelee should PDL/GDC be disabled in function _fused_moe_lora (file fused_moe_lora_op.py) ?
In this line:

use_gdc = supports_pdl(device) and not fully_sharded

When I run an MoE model on Blackwell I get this error:

error: 'tt.elementwise_inline_asm' op pipeliner doesn't know how to predicate this op.
            tl.extra.cuda.gdc_wait()

The error is not raised when I set use_gdc = False.

I can open an issue and/or PR for this problem if required.