Re: [patch V2 28/37] rseq: Switch to fast path processing on exit to user

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, Aug 27 2025 at 09:45, Mathieu Desnoyers wrote:
> On 2025-08-26 11:40, Mathieu Desnoyers wrote:
>>>    RSEQ selftests      Before          After              Reduction
>>>
>>>    exit to user:       386281778          387373750
>>>    signal checks:       35661203                  0           100%
>>>    slowpath runs:      140542396 36.38%            100  0.00%    100%
>>>    fastpath runs:                         9509789  2.51%     N/A
>>>    id updates:         176203599 45.62%        9087994  2.35%     95%
>>>    cs checks:          175587856 45.46%        4728394  1.22%     98%
>>>      cs cleared:       172359544   98.16%    1319307   27.90%   99%
>>>      cs fixup:           3228312    1.84%    3409087   72.10%
>
> By the way, you should really not be using the entire rseq selftests
> as a representative workload for profiling the kernel rseq implementation.
>
> Those selftests include "loop injection", "yield injection", "kill
> injection" and "sleep injection" within the relevant userspace code
> paths, which really increase the likelihood of hitting stuff like
> "cs fixup" compared to anything that comes close to a realistic
> use-case. This is really useful for testing correctness, but not
> for profiling. For instance, the "loop injection" introduces busy
> loops within rseq critical sections to significantly increase the
> likelihood of hitting a cs fixup.
>
> Those specific selftests are really just "stress-tests" that don't
> represent any relevant workload.

True, they still tell how much useless work the kernel was doing, no?