From: Kuniyuki Iwashima <kuniyu@google.com>
To: Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Madhavan Srinivasan <maddy@linux.ibm.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Paul Walmsley <pjw@kernel.org>,
Palmer Dabbelt <palmer@dabbelt.com>,
Albert Ou <aou@eecs.berkeley.edu>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
Jens Axboe <axboe@kernel.dk>,
Christian Brauner <brauner@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nicholas Piggin <npiggin@gmail.com>,
Christophe Leroy <christophe.leroy@csgroup.eu>,
Alexandre Ghiti <alex@ghiti.fr>,
"H. Peter Anvin" <hpa@zytor.com>,
Eric Dumazet <edumazet@google.com>,
Kuniyuki Iwashima <kuniyu@google.com>,
Kuniyuki Iwashima <kuni1840@gmail.com>,
x86@kernel.org, linux-arm-kernel@lists.infradead.org,
linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: [PATCH v1 0/2] epoll: Save one stac/clac pair in epoll_put_uevent().
Date: Thu, 23 Oct 2025 00:04:42 +0000 [thread overview]
Message-ID: <20251023000535.2897002-1-kuniyu@google.com> (raw)
epoll_put_uevent() calls __put_user() twice, which are inlined
to two calls of out-of-line functions, __put_user_nocheck_4()
and __put_user_nocheck_8().
Both functions wrap mov with a stac/clac pair, which is expensive
on an AMD EPYC 7B12 64-Core Processor platform.
__put_user_nocheck_4 /proc/kcore [Percent: local period]
Percent │
89.91 │ stac
0.19 │ mov %eax,(%rcx)
0.15 │ xor %ecx,%ecx
9.69 │ clac
0.06 │ ← retq
This was remarkable while testing neper/tcp_rr with 1000 flows per
thread.
Overhead Shared O Symbol
10.08% [kernel] [k] _copy_to_iter
7.12% [kernel] [k] ip6_output
6.40% [kernel] [k] sock_poll
5.71% [kernel] [k] move_addr_to_user
4.39% [kernel] [k] __put_user_nocheck_4
...
1.06% [kernel] [k] ep_try_send_events
... ^- epoll_put_uevent() was inlined
0.78% [kernel] [k] __put_user_nocheck_8
Patch 1 adds a new uaccess helper that is inlined to a bare stac
without address masking or uaccess_ok(), which is already checked
in ep_check_params().
Patch 2 uses the helper and unsafe_put_user() in epoll_put_uevent().
Kuniyuki Iwashima (2):
uaccess: Add __user_write_access_begin().
epoll: Use __user_write_access_begin() and unsafe_put_user() in
epoll_put_uevent().
arch/arm64/include/asm/uaccess.h | 1 +
arch/powerpc/include/asm/uaccess.h | 13 ++++++++++---
arch/riscv/include/asm/uaccess.h | 1 +
arch/x86/include/asm/uaccess.h | 1 +
include/linux/eventpoll.h | 13 ++++++++-----
include/linux/uaccess.h | 1 +
6 files changed, 22 insertions(+), 8 deletions(-)
--
2.51.1.814.gb8fa24458f-goog
next reply other threads:[~2025-10-23 4:50 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-23 0:04 Kuniyuki Iwashima [this message]
2025-10-23 0:04 ` [PATCH v1 1/2] uaccess: Add __user_write_access_begin() Kuniyuki Iwashima
2025-10-23 5:37 ` Linus Torvalds
2025-10-23 8:29 ` David Laight
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 19:40 ` Dave Hansen
2025-10-24 5:16 ` Kuniyuki Iwashima
2025-10-24 14:05 ` Dave Hansen
2025-10-24 14:47 ` David Laight
2025-10-28 5:32 ` Kuniyuki Iwashima
2025-10-28 9:54 ` David Laight
2025-10-28 16:42 ` Kuniyuki Iwashima
2025-10-28 16:58 ` Linus Torvalds
2025-10-29 1:42 ` Andrew Cooper
2025-10-28 22:30 ` David Laight
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