Re: [PATCH v3 0/5] barrier: Add smp_cond_load_*_timewait()

[Ankur Arora <ankur.a.arora@oracle.com>]

Gentle ping for review.

Ankur

Ankur Arora <ankur.a.arora@oracle.com> writes:

> 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_spinwait(ptr, cond_expr,
>                                   time_expr, time_limit, slack)
>    smp_cond_load_acquire_spinwait(ptr, cond_expr,
>                                   time_expr, time_limit, slack)
>
> The added parameters pertain to the timeout checks and a measure of how
> much slack the caller can tolerate in the timeout. The slack is useful
> when in the wait state and thus dependent on an asynchronous event.
>
> Changelog:
>   v2 [3]:
>     - 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 [4]:
>      - 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. [5]
>
> Any comments appreciated!
>
> 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/20250502085223.1316925-1-ankur.a.arora@oracle.com/
> [4] https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com/
> [5] https://lore.kernel.org/lkml/f2f5d09e79539754ced085ed89865787fa668695.camel@amazon.com
>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: linux-arch@vger.kernel.org
>
> Ankur Arora (5):
>   asm-generic: barrier: Add smp_cond_load_relaxed_timewait()
>   asm-generic: barrier: Handle spin-wait in
>     smp_cond_load_relaxed_timewait()
>   asm-generic: barrier: Add smp_cond_load_acquire_timewait()
>   arm64: barrier: Support waiting in smp_cond_load_relaxed_timewait()
>   arm64: barrier: Handle waiting in smp_cond_load_relaxed_timewait()
>
>  arch/arm64/include/asm/barrier.h    |  54 +++++++++++
>  arch/arm64/include/asm/rqspinlock.h |   2 +-
>  include/asm-generic/barrier.h       | 137 ++++++++++++++++++++++++++++
>  3 files changed, 192 insertions(+), 1 deletion(-)


--
ankur