linux-kernel - Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put

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Message-ID: <20251028095407.2bb53f85@pumpkin>
Date: Tue, 28 Oct 2025 09:54:07 +0000
From: David Laight <david.laight.linux@...il.com>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: dave.hansen@...el.com, alex@...ti.fr, aou@...s.berkeley.edu,
 axboe@...nel.dk, bp@...en8.de, brauner@...nel.org, catalin.marinas@....com,
 christophe.leroy@...roup.eu, dave.hansen@...ux.intel.com,
 edumazet@...gle.com, hpa@...or.com, kuni1840@...il.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-riscv@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
 maddy@...ux.ibm.com, mingo@...hat.com, mpe@...erman.id.au,
 npiggin@...il.com, palmer@...belt.com, pjw@...nel.org, tglx@...utronix.de,
 torvalds@...ux-foundation.org, will@...nel.org, x86@...nel.org
Subject: Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and
 unsafe_put_user() in epoll_put_uevent().

On Tue, 28 Oct 2025 05:32:13 +0000
Kuniyuki Iwashima <kuniyu@...gle.com> wrote:

....
> 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.)

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().

An 'interesting' test is to replicate the code you are optimising
to see how much slower it goes - you can't gain more than the slowdown.

What is more likely is that breathing on the code changes the cache
line layout and that causes a larger performance change.

A better test for epoll_put_event would be to create 1000 fd (pipes or events).
Then time calls epoll_wait() that return lots of events.

	David