Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent().

Linus Torvalds
2025-10-28T16:59:17Z

On Tue, 28 Oct 2025 at 09:42, Kuniyuki Iwashima <kuniyu@google.com> wrote:
>
> This is the Zen 2 platform, so probably the stac/clac cost will be
> more expensive than you expect on Zen 5.

Yeah, clac/stac are horrenously expensive on Zen 2. I think they are
microcoded - and not the fast kind - so effectively serializing.

They got enormously faster and pipelined in later Zen microarchitectures.

Sadly, Agner Fog doesn't do timings on those instructions, and I
haven't found any other place that has some overview of the
performance on different microarchitectures.

Linus