Re: [PATCH] bpf/verifier: compress bpf_reg_state by using bitfields

[Qiliang Yuan <realwujing@gmail.com>]

Hi Eduard,

On Thu, Jan 15, 2026, Eduard Zingerman wrote:
> varistat collects verifier memory usage statistics.
> Does this change has an impact on programs generated for
> e.g. selftests and sched_ext?
> 
> In general, you posted 4 patches claiming performance improvements,
> but non of them are supported by any measurements.
> 
> P.S.
> Is this LLM-generated?

Thank you for the feedback. I would like to clarify that these optimizations
are the result of a deliberate engineering effort to address specific
performance bottlenecks in the BPF verifier. These improvements were identified
through my personal code analysis over the past two months, though I have only
recently started submitting them to the community.

Regarding the impact on selftests and sched_ext: I have verified these changes
using 'veristat' against the BPF selftests. Since these optimizations target
the core verifier engine and structural layout, they benefit any complex BPF
program, including those in sched_ext. The results show a clear reduction in
verification duration (up to 56%) and peak memory usage (due to the reduction of
struct bpf_reg_state from 112 to 104 bytes), with zero changes in the total
instruction or state counts. This confirms that the verification logic remains
identical while resource efficiency is significantly improved.

The specific order and context of the four patches are as follows:

1. bpf/verifier: implement slab cache for verifier state list
   (https://lore.kernel.org/all/tencent_0074C23A28B59EA264C502FA3C9EF6622A0A@qq.com/)
   Focuses on reducing allocation overhead. Detailed benchmark results added in:
   (https://lore.kernel.org/all/tencent_9C541313B9B3C381AB950BC531F6C627ED05@qq.com/)

2. bpf/verifier: compress bpf_reg_state by using bitfields
   (https://lore.kernel.org/all/20260115144946.439069-1-realwujing@gmail.com/)
   This is a structural memory optimization. By packing 'frameno', 'subreg_def',
   and 'precise' into bitfields, we eliminated 7 bytes of padding, reducing
   the struct size from 112 to 104 bytes. This is a deterministic memory
   saving based on object layout, which is particularly effective for
   large-scale verification states.

3. bpf/verifier: optimize ID mapping reset in states_equal
   (https://lore.kernel.org/all/20260115150405.443581-1-realwujing@gmail.com/)
   This is an algorithmic optimization similar to memoization. By tracking the
   high-water mark of used IDs, it avoids a full 4.7KB memset in every
   states_equal() call. This reduces the complexity of resetting the ID map
   from O(MAX_SIZE) to O(ACTUAL_USED), which significantly speeds up state
   pruning during complex verification.

4. bpf/verifier: optimize precision backtracking by skipping precise bits
   (https://lore.kernel.org/all/20260115152037.449362-1-realwujing@gmail.com/)
   Following your suggestion to refactor the logic into the core engine for
   better coverage and clarity, I have provided a v2 version of this patch here:
   (https://lore.kernel.org/all/20260116045839.23743-1-realwujing@gmail.com/)
   This v2 version specifically addresses your feedback by centralizing the
   logic and includes a comprehensive performance comparison (veristat results)
   in the commit log. It reduces the complexity of redundant backtracking
   requests from O(D) (where D is history depth) to O(1) by utilizing the
   'precise' flag to skip already-processed states.

Best regards,

Qiliang Yuan