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From: David Laight <david.laight.linux@gmail.com>
To: Kuniyuki Iwashima <kuniyu@google.com>
Cc: dave.hansen@intel.com, alex@ghiti.fr, aou@eecs.berkeley.edu,
	axboe@kernel.dk, bp@alien8.de, brauner@kernel.org,
	catalin.marinas@arm.com, christophe.leroy@csgroup.eu,
	dave.hansen@linux.intel.com, edumazet@google.com, hpa@zytor.com,
	kuni1840@gmail.com, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	linuxppc-dev@lists.ozlabs.org, maddy@linux.ibm.com,
	mingo@redhat.com, mpe@ellerman.id.au, npiggin@gmail.com,
	palmer@dabbelt.com, pjw@kernel.org, tglx@linutronix.de,
	torvalds@linux-foundation.org, will@kernel.org, x86@kernel.org
Subject: Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent().
Date: Tue, 28 Oct 2025 09:54:07 +0000	[thread overview]
Message-ID: <20251028095407.2bb53f85@pumpkin> (raw)
In-Reply-To: <20251028053330.2391078-1-kuniyu@google.com>

On Tue, 28 Oct 2025 05:32:13 +0000
Kuniyuki Iwashima <kuniyu@google.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


WARNING: multiple messages have this Message-ID (diff)
From: David Laight <david.laight.linux@gmail.com>
To: Kuniyuki Iwashima <kuniyu@google.com>
Cc: dave.hansen@intel.com, alex@ghiti.fr, aou@eecs.berkeley.edu,
	axboe@kernel.dk, bp@alien8.de, brauner@kernel.org,
	catalin.marinas@arm.com, christophe.leroy@csgroup.eu,
	dave.hansen@linux.intel.com, edumazet@google.com, hpa@zytor.com,
	kuni1840@gmail.com, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	linuxppc-dev@lists.ozlabs.org, maddy@linux.ibm.com,
	mingo@redhat.com, mpe@ellerman.id.au, npiggin@gmail.com,
	palmer@dabbelt.com, pjw@kernel.org, tglx@linutronix.de,
	torvalds@linux-foundation.org, will@kernel.org, x86@kernel.org
Subject: Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent().
Date: Tue, 28 Oct 2025 09:54:07 +0000	[thread overview]
Message-ID: <20251028095407.2bb53f85@pumpkin> (raw)
In-Reply-To: <20251028053330.2391078-1-kuniyu@google.com>

On Tue, 28 Oct 2025 05:32:13 +0000
Kuniyuki Iwashima <kuniyu@google.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

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

  reply	other threads:[~2025-10-28  9:54 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-10-23  0:04 [PATCH v1 0/2] epoll: Save one stac/clac pair in epoll_put_uevent() Kuniyuki Iwashima
2025-10-23  0:04 ` Kuniyuki Iwashima
2025-10-23  0:04 ` [PATCH v1 1/2] uaccess: Add __user_write_access_begin() Kuniyuki Iwashima
2025-10-23  0:04   ` Kuniyuki Iwashima
2025-10-23  5:37   ` Linus Torvalds
2025-10-23  5:37     ` Linus Torvalds
2025-10-23  8:29     ` David Laight
2025-10-23  8:29       ` David Laight
2025-10-24  5:31       ` Kuniyuki Iwashima
2025-10-24  5:31         ` Kuniyuki Iwashima
2025-10-23  0:04 ` [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent() Kuniyuki Iwashima
2025-10-23  0:04   ` Kuniyuki Iwashima
2025-10-23 19:40   ` Dave Hansen
2025-10-23 19:40     ` Dave Hansen
2025-10-24  5:16     ` Kuniyuki Iwashima
2025-10-24  5:16       ` Kuniyuki Iwashima
2025-10-24 14:05       ` Dave Hansen
2025-10-24 14:05         ` Dave Hansen
2025-10-24 14:47         ` David Laight
2025-10-24 14:47           ` David Laight
2025-10-28  5:32         ` Kuniyuki Iwashima
2025-10-28  5:32           ` Kuniyuki Iwashima
2025-10-28  9:54           ` David Laight [this message]
2025-10-28  9:54             ` David Laight
2025-10-28 16:42             ` Kuniyuki Iwashima
2025-10-28 16:42               ` Kuniyuki Iwashima
2025-10-28 16:58               ` Linus Torvalds
2025-10-28 16:58                 ` Linus Torvalds
2025-10-29  1:42                 ` Andrew Cooper
2025-10-29  1:42                   ` Andrew Cooper
2025-10-28 22:30               ` David Laight
2025-10-28 22:30                 ` David Laight

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