linux-kernel - Re: [PATCH v4 0/5] barrier: Add smp_cond_load_*

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Message-ID: <875xe5sqhg.fsf@oracle.com>
Date: Fri, 29 Aug 2025 15:38:35 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: "Okanovic, Haris" <harisokn@...zon.com>
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Subject: Re: [PATCH v4 0/5] barrier: Add smp_cond_load_*_timewait()


Okanovic, Haris <harisokn@...zon.com> writes:

> On Fri, 2025-08-29 at 01:07 -0700, Ankur Arora wrote:
>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>>
>>
>>
>> Hi,
>>
>> This series adds waited variants of the smp_cond_load() primitives:
>> smp_cond_load_relaxed_timewait(), and smp_cond_load_acquire_timewait().
>>
>> Why?: as the name suggests, the new interfaces are meant for contexts
>> where you want to wait on a condition variable for a finite duration.
>> This is easy enough to do with a loop around cpu_relax(). However,
>> some architectures (ex. arm64) also allow waiting on a cacheline. So,
>> these interfaces handle a mixture of spin/wait with a smp_cond_load()
>> thrown in.
>>
>> There are two known users for these interfaces:
>>
>>  - poll_idle() [1]
>>  - resilient queued spinlocks [2]
>>
>> The interfaces are:
>>    smp_cond_load_relaxed_timewait(ptr, cond_expr, time_check_expr)
>>    smp_cond_load_acquire_spinwait(ptr, cond_expr, time_check_expr)
>>
>> The added parameter, time_check_expr, determines the bail out condition.
>>
>> Changelog:
>>   v3 [3]:
>>     - further interface simplifications (suggested by Catalin Marinas)
>>
>>   v2 [4]:
>>     - simplified the interface (suggested by Catalin Marinas)
>>        - get rid of wait_policy, and a multitude of constants
>>        - adds a slack parameter
>>       This helped remove a fair amount of duplicated code duplication and in hindsight
>>       unnecessary constants.
>>
>>   v1 [5]:
>>      - add wait_policy (coarse and fine)
>>      - derive spin-count etc at runtime instead of using arbitrary
>>        constants.
>>
>> Haris Okanovic had tested an earlier version of this series with
>> poll_idle()/haltpoll patches. [6]
>>
>> Any comments appreciated!
>>
>> Thanks!
>> Ankur
>>
>> [1] https://lore.kernel.org/lkml/20241107190818.522639-3-ankur.a.arora@oracle.com/
>> [2] Uses the smp_cond_load_acquire_timewait() from v1
>>     https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/include/asm/rqspinlock.h
>> [3] https://lore.kernel.org/lkml/20250627044805.945491-1-ankur.a.arora@oracle.com/
>> [4] https://lore.kernel.org/lkml/20250502085223.1316925-1-ankur.a.arora@oracle.com/
>> [5] https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com/
>> [6] https://lore.kernel.org/lkml/f2f5d09e79539754ced085ed89865787fa668695.camel@amazon.com
>>
>> Cc: Arnd Bergmann <arnd@...db.de>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>
>> Cc: Alexei Starovoitov <ast@...nel.org>
>> Cc: linux-arch@...r.kernel.org
>>
>> Ankur Arora (5):
>>   asm-generic: barrier: Add smp_cond_load_relaxed_timewait()
>>   arm64: barrier: Add smp_cond_load_relaxed_timewait()
>>   arm64: rqspinlock: Remove private copy of
>>     smp_cond_load_acquire_timewait
>>   asm-generic: barrier: Add smp_cond_load_acquire_timewait()
>>   rqspinlock: use smp_cond_load_acquire_timewait()
>>
>>  arch/arm64/include/asm/barrier.h    | 22 ++++++++
>>  arch/arm64/include/asm/rqspinlock.h | 84 +----------------------------
>>  include/asm-generic/barrier.h       | 57 ++++++++++++++++++++
>>  include/asm-generic/rqspinlock.h    |  4 ++
>>  kernel/bpf/rqspinlock.c             | 25 ++++-----
>>  5 files changed, 93 insertions(+), 99 deletions(-)
>>
>> --
>> 2.31.1
>>
>
> Tested on AWS Graviton 2, 3, and 4 (ARM64 Neoverse N1, V1, and V2) with
> your V10 haltpoll changes, atop 6.17.0-rc3 (commit 07d9df8008).
> Still seeing between 1.3x and 2.5x speedups in `perf bench sched pipe`
> and `seccomp-notify`; no change in `messaging`.

Great.

> Reviewed-by: Haris Okanovic <harisokn@...zon.com>
> Tested-by: Haris Okanovic <harisokn@...zon.com>

Thank you.

--
ankur