linux-kernel - [PATCH v2 0/3] entry: inline syscall enter/exit functions

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Date: Mon, 18 Dec 2023 08:45:17 +0100
From: Sven Schnelle <svens@...ux.ibm.com>
To: Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc: linux-kernel@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>
Subject: [PATCH v2 0/3] entry: inline syscall enter/exit functions

Hi List,

looking into the performance of syscall entry/exit after s390 switched
to generic entry showed that there's quite some overhead calling some
of the entry/exit work functions even when there's nothing to do.
This patchset moves the entry and exit function to entry-common.h, so
non inlined code gets only called when there is some work pending.

I wrote a small program that just issues invalid syscalls in a loop.
On an s390 machine, this results in the following numbers:

without this series:

$ ./syscall 1000000000
runtime: 94.886581s / per-syscall 9.488658e-08s

with this series:

$ ./syscall 1000000000
runtime: 84.732391s / per-syscall 8.473239e-08s

so the time required for one syscall dropped from 94.8ns to
84.7ns, which is a drop of about 11%.

Sven Schnelle (3):
  entry: move exit to usermode functions to header file
  entry: move enter_from_user_mode() to header file
  entry: move syscall_enter_from_user_mode() to header file

 include/linux/entry-common.h |  94 +++++++++++++++++++++++++++++--
 kernel/entry/common.c        | 106 ++++-------------------------------
 2 files changed, 100 insertions(+), 100 deletions(-)

Changes in v2:
- don't move of exit_to_user_mode_loop() to header file
-- 
2.40.1