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Date: Fri, 16 May 2025 22:50:58 +0000
From: "Okanovic, Haris" <harisokn@...zon.com>
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Subject: Re: [PATCH v2 0/7] barrier: introduce smp_cond_load_*_timewait()

On Fri, 2025-05-02 at 01:52 -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().
> 
> There are two known users for these interfaces:
> 
>  - poll_idle() [1]
>  - resilient queued spinlocks [2]
> 
> For both of these cases we want to wait on a condition but also want
> to terminate the wait based on a timeout.
> 
> Before describing how v2 implements these interfaces, let me recap the
> problems in v1 (Catalin outlined most of these in [3]):
> 
> smp_cond_load_relaxed_spinwait(ptr, cond_expr, time_expr_ns, time_limit_ns)
> took four arguments, with ptr and cond_expr doing the usual smp_cond_load()
> things and time_expr_ns and time_limit_ns being used to decide the
> terminating condition.
> 
> There were some problems in the timekeeping:
> 
> 1. How often do we do the (relatively expensive) time-check?
> 
>    The choice made was once very 200 spin-wait iterations, with each
>    iteration trying to idle the pipeline by executing cpu_relax().
> 
>    The choice of 200 was, of course, arbitrary and somewhat meaningless
>    across architectures. On recent x86, cpu_relax()/PAUSE takes ~20-30
>    cycles, but on (non-SMT) arm64 cpu_relax()/YIELD is effectively
>    just a NOP.
> 
>    Even if each architecture had its own limit, this will also vary
>    across microarchitectures.
> 
> 2. On arm64, which can do better than just cpu_relax(), for instance,
>    by waiting for a store on an address (WFE), the implementation
>    exclusively used WFE, with the spin-wait only used as a fallback
>    for when the event-stream was disabled.
> 
>    One problem with this was that the worst case timeout overshoot
>    with WFE is ARCH_TIMER_EVT_STREAM_PERIOD_US (100us) and so there's
>    a vast gulf between that and a potentially much smaller granularity
>    with the spin-wait versions. In addition the interface provided
>    no way for the caller to specify or limit the oveshoot.
> 
> Non-timekeeping issues:
> 
> 3. The interface was useful for poll_idle() like users but was not
>    usable if the caller needed to do any work. For instance,
>    rqspinlock uses it thus:
> 
>      smp_cond_load_acquire_timewait(v, c, 0, 1)
> 
>    Here the time-check always evaluates to false and all of the logic
>    (ex. deadlock checking) is folded into the conditional.
> 
> 
> With that foundation, the new interface is:
> 
>    smp_cond_load_relaxed_timewait(ptr, cond_expr, wait_policy,
>                                          time_expr, time_end)
> 
> The added parameter, wait_policy provides a mechanism for the caller
> to apportion time spent spinning or, where supported, in a wait.
> This is somewhat inspired from the queue_poll() mechanism used
> with smp_cond_load() in arm-smmu-v3 [4].
> 
> It addresses (1) by deciding the time-check granularity based on a
> time interval instead of spinning for a fixed number of iterations.
> 
> (2) is addressed by the wait_policy allowing for different slack
> values. The implemented versions of wait_policy allow for a coarse
> or a fine grained slack. A user defined wait_policy could choose
> its own wait parameter. This would also address (3).
> 
> 
> With that, patches 1-5, add the generic and arm64 logic:
> 
>   "asm-generic: barrier: add smp_cond_load_relaxed_timewait()",
>   "asm-generic: barrier: add wait_policy handlers"
> 
>   "arm64: barrier: enable waiting in smp_cond_load_relaxed_timewait()"
>   "arm64: barrier: add coarse wait for smp_cond_load_relaxed_timewait()"
>   "arm64: barrier: add fine wait for smp_cond_load_relaxed_timewait()".
> 
> And, patch 6, adds the acquire variant:
> 
>   "asm-generic: barrier: add smp_cond_load_acquire_timewait()"
> 
> And, finally patch 7 lays out how this could be used for rqspinlock:
> 
>   "bpf: rqspinlock: add rqspinlock policy handler for arm64".
> 
> 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/Z8dRalfxYcJIcLGj@arm.com/
> [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c#n223
> 
> 
> 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 (7):
>   asm-generic: barrier: add smp_cond_load_relaxed_timewait()
>   asm-generic: barrier: add wait_policy handlers
>   arm64: barrier: enable waiting in smp_cond_load_relaxed_timewait()
>   arm64: barrier: add coarse wait for smp_cond_load_relaxed_timewait()
>   arm64: barrier: add fine wait for smp_cond_load_relaxed_timewait()
>   asm-generic: barrier: add smp_cond_load_acquire_timewait()
>   bpf: rqspinlock: add rqspinlock policy handler for arm64
> 
>  arch/arm64/include/asm/barrier.h    |  82 +++++++++++++++
>  arch/arm64/include/asm/rqspinlock.h |  96 ++++--------------
>  include/asm-generic/barrier.h       | 150 ++++++++++++++++++++++++++++
>  3 files changed, 251 insertions(+), 77 deletions(-)
> 
> --
> 2.43.5
> 

Tested on AWS Graviton (ARM64 Neoverse V1) with your V10 haltpoll
changes, atop master 83a896549f.

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

Regards,
Haris Okanovic
AWS Graviton Software

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