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

On Tue, 28 Oct 2025 09:42:25 -0700
Kuniyuki Iwashima [off-list ref] wrote:

On Tue, Oct 28, 2025 at 2:54 AM David Laight
[off-list ref] wrote:

quoted

On Tue, 28 Oct 2025 05:32:13 +0000
Kuniyuki Iwashima [off-list ref] wrote:

....  

quoted

I rebased on 19ab0a22efbd and tested 4 versions on
AMD EPYC 7B12 machine:  

That is zen5 which I believe has much faster clac/stac than anything else.
(It might also have a faster lfence - not sure.)  

This is the Zen 2 platform, so probably the stac/clac cost will be
more expensive than you expect on Zen 5.

I must has looked the cpu type incorrectly.
AMD haven't made it easy working out the cpu architecture.
I need to get an older zen cpu for my set of test systems
(and some newer Intel ones).

quoted

Getting a 3% change for that diff also seems unlikely.
Even if you halved the execution time of that code the system would have
to be spending 6% of the time in that loop.
Even your original post only shows 1% in ep_try_send_events().

I realised after that you might be showing a 3% change in that 1%.

We saw a similar improvement on the same platform by
1fb0e471611d ("net: remove one stac/clac pair from
move_addr_to_user()").

Certainly removing one could easily be measurable.

	David