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Message-Id: <20241029-arm-generic-entry-v2-0-573519abef38@linaro.org>
Date: Tue, 29 Oct 2024 11:52:40 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Oleg Nesterov <oleg@...hat.com>, Russell King <linux@...linux.org.uk>,
Kees Cook <kees@...nel.org>, Andy Lutomirski <luto@...capital.net>,
Will Drewry <wad@...omium.org>, Frederic Weisbecker <frederic@...nel.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Jinjie Ruan <ruanjinjie@...wei.com>, Arnd Bergmann <arnd@...db.de>,
Ard Biesheuvel <ardb@...nel.org>, Al Viro <viro@...iv.linux.org.uk>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH RFC v2 00/28] ARM: Switch to generic entry
This patch series converts a slew of ARM assembly into the
corresponding C code, step by step moving the codebase
closer to the expectations of the generic entry code,
and as a last step switches ARM over to the generic
entry code.
This was inspired by Jinjie Ruans similar work for ARM64.
The low-level assembly calls into arch/arm/kernel/syscall.c
to invoke syscalls from userspace, and to the functions listed
in arch/arm/kernel/entry.c for any other transitions to
and from userspace. Looking at these functions and the
call sites in the assembly on the final result should give
a pretty good idea about how this works, and what the
generic entry expects from an architecture.
To test the code the following seccomp patch is needed
on older ARM systems:
https://lore.kernel.org/lkml/20241022-seccomp-compile-error-v2-1-c9f08a4f8ebb@linaro.org/
(patch queued by Kees Cook)
There is a git branch you can pull in and test (v6.12-rc1
based):
https://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-integrator.git/log/?h=b4/arm-generic-entry-v6.12-rc1
Upsides:
- Same code paths as x86, S390, RISCV, Loongarch and probably
soon ARM64 is used for the ARM systems. This includes some
instrumentation stubs helping out with things we haven't
even started to look at such as kmsan and live patching (!).
- By introducing the new callbacks to C, we can move away
from the deprecated (and I think partly unmaintained) context
tracking mechanism for RCU (user_exit_callable(),
user_enter_callable()) in favor of what everyone else
is using, i.e. calling rcu_irq_enter_check_tick() on
IRQ entry. If we do not go with this patch set we can
perhaps look into a separate patch just switching ARM32
to the new context tracking, as tests show the performance
impact appears negligible for this.
- I think also lockdep is now behaving more according to
expectations (the lockdep calls in ARM64 and generic entry
seems different and more fine-granular from the ARM32 code)
but I am no expert in lockdep so I cannot really tell if
this is a real improvement.
Downsides:
- I had to remove the "fast syscall restart" from Al Viro.
I don't know how much it will affect performance, but
if this is something we must have, let's try to make
the solution generic, i.e. add fast syscall restart in
the generic entry code.
- The "superfast return to userspace" using just very
small assembly snippets to get back to userspace on
e.g. IRQs if and only if no instrumentation was compiled
in, is no longer possible, since we unconditionally
call into code written in C. I *think* this accounts
for the majority of the ~3-4% performance impact (see
measurements below).
Testing:
- Booted into Versatile Express QEMU (ARMv7), Ux500 full
graphic UI (PostmarketOS Phosh, ARMv7 on hardware,
Gemini ARMv4 on hardware. No special issues.
- Tested some ptrace/strace obviously, such as issuing
several instances of "ptrace find /" and let this scroll
by in the terminal over some 10 minutes or so.
- Turned on RCU torture tests and ran for a while. Seems
stable and the test outputs look normal.
- Ran stress-ng, which triggers the idle bug below that also
appear during boot.
Performance impact:
The changes were tested using the standard syscall overhead
testing oneliner:
perf bench syscall all
This executes 10,000,000 getppid() in sequence and measures
the time taken for this to complete. The numbers vary a bit
but they are consistent.
In QEMU I tested with Vexpress and two CPU cores (-M vexpress-a15
-m 2G -smp cpus=2). DRM graphics and framebuffer was activated to
give a bit of background IRQ activity (vsync interrupts).
I ran the perf command three times on each configuration, and
picked the one iteration where the original code performed the
best, and the one where the patches kernel performed the worst, to
get a worst-case comparison.
v6.12-rc1 vexpress_defconfig, best invocation:
Total time: 150.723 [sec]
15.072301 usecs/op
66,346 ops/sec
v6.12-rc1 vexpress_defconfig, and this patch set, worst invocation:
Total time: 158.746 [sec]
15.874603 usecs/op
62,993 ops/sec
Here we see a performance degradation of around 4% operations/sec
for a vexpress dualcore defconfig.
Debians stock kernel was noticably faster, so I investigated what
causes this. It turns out that the big performance hog for syscalls
is actually PAN, and I think Debian armhf simply turns this off.
So I re-did the tests with CONFIG_ARM_PAN turned off to emulate the
impact on stock Debian:
v6.12-rc1 vexpress_defconfig, !PAN, best invocation:
Total time: 38.022 [sec]
3.802206 usecs/op
263,005 ops/sec
v6.12-rc1 vexpress_defconfig, !PAN and this patch set, worst
invocation:
Total time: 39.015 [sec]
3.901549 usecs/op
256,308 ops/sec
Again we see around 4% (well just 3% actually). The overhead seems
a little less pronounced when not using PAN.
To conclude if any of this was due to the new context tracking,
I tested to patch back the old context tracking on top of generic
entry. This is hardly something that can be recommended, and anyway
showed no noticeable overhead difference.
Caveats
- This comes up during boot and stress-ng runs:
------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at kernel/context_tracking.c:128
ct_kernel_exit+0xf8/0x100
CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.12.0-rc1+ #31
Hardware name: ARM-Versatile Express
(...)
It is emitted in kernel/context_tracking.c, ct_kernel_exit():
WARN_ON_ONCE(ct_nmi_nesting() != CT_NESTING_IRQ_NONIDLE);
The kernel contains the following comment in ct_irq_enter():
"If your architecture's idle loop does do upcalls to user mode (or
does anything else that results in unbalanced calls to the
irq_enter() and irq_exit() functions), RCU will give you what you
deserve, good and hard. But very infrequently and
irreproducibly.
Use things like work queues to work around this limitation.
You have been warned."
Stern language, some actual help to pinpoint the problem so I
can work on it would be appreciated.
Inserting prints reveals that the ct_nmi_nesting() often
deviates from CT_NESTING_IRQ_NONIDLE at ct_kernel_exit() by
a few multiple of 2 counts up or down.
Open questions:
- Generic entry requires PTRACE_SYSEMU and PTRACE_SYSEMU_SINGLESTEP
to be defined. I added them but don't even know what they
do or if generic entry magically adds support for them
(probably not) so I need help here.
- I need Al Viro's input on how to deal with the "fast syscall
restart" that I bluntly deleted, if we need to reincarnate it
in the generic entry or what we shall do here.
- I need to test with an OABI rootfs.
Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
---
Changes in v2:
- Performance impact measurements have been provided.
- Link to v1: https://lore.kernel.org/r/20241010-arm-generic-entry-v1-0-b94f451d087b@linaro.org
---
Linus Walleij (28):
ARM: Prepare includes for generic entry
ARM: ptrace: Split report_syscall()
ARM: entry: Skip ret_slow_syscall label
ARM: process: Rewrite ret_from_fork i C
ARM: process: Remove local restart
ARM: entry: Invoke syscalls using C
ARM: entry: Rewrite two asm calls in C
ARM: entry: Move trace entry to C function
ARM: entry: save the syscall sp in thread_info
ARM: entry: move all tracing invocation to C
ARM: entry: Merge the common and trace entry code
ARM: entry: Rename syscall invocation
ARM: entry: Create user_mode_enter/exit
ARM: entry: Drop trace argument from usr_entry macro
ARM: entry: Separate call path for syscall SWI entry
ARM: entry: Drop argument to asm_irqentry macros
ARM: entry: Implement syscall_exit_to_user_mode()
ARM: entry: Drop the superfast ret_fast_syscall
ARM: entry: Remove fast and offset register restore
ARM: entry: Untangle ret_fast_syscall/to_user
ARM: entry: Do not double-call exit functions
ARM: entry: Move work processing to C
ARM: entry: Stop exiting syscalls like IRQs
ARM: entry: Complete syscall and IRQ transition to C
ARM: entry: Create irqentry calls from kernel mode
ARM: entry: Move in-kernel hardirq tracing to C
ARM: entry: Add FIQ/NMI C callbacks
ARM: entry: Convert to generic entry
arch/arm/Kconfig | 1 +
arch/arm/include/asm/entry-common.h | 66 ++++++++++++
arch/arm/include/asm/entry.h | 17 +++
arch/arm/include/asm/ptrace.h | 8 +-
arch/arm/include/asm/signal.h | 4 -
arch/arm/include/asm/stacktrace.h | 2 +-
arch/arm/include/asm/switch_to.h | 4 +
arch/arm/include/asm/syscall.h | 7 ++
arch/arm/include/asm/thread_info.h | 18 +---
arch/arm/include/asm/traps.h | 2 +-
arch/arm/include/uapi/asm/ptrace.h | 2 +
arch/arm/kernel/Makefile | 5 +-
arch/arm/kernel/asm-offsets.c | 1 +
arch/arm/kernel/entry-armv.S | 39 +++----
arch/arm/kernel/entry-common.S | 202 ++++++++++++++----------------------
arch/arm/kernel/entry-header.S | 108 +++++--------------
arch/arm/kernel/entry.c | 59 +++++++++++
arch/arm/kernel/process.c | 22 +++-
arch/arm/kernel/ptrace.c | 76 --------------
arch/arm/kernel/signal.c | 57 ++--------
arch/arm/kernel/syscall.c | 31 ++++++
arch/arm/kernel/traps.c | 2 +-
22 files changed, 349 insertions(+), 384 deletions(-)
---
base-commit: 67bc0ffd4658eb91d061a6efa83da65ec838c2ac
change-id: 20240903-arm-generic-entry-ada145378bbe
Best regards,
--
Linus Walleij <linus.walleij@...aro.org>
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