Message-ID: <87tt5khw9h.fsf@oracle.com>
Date: Fri, 16 May 2025 18:16:26 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: "Okanovic, Haris" <harisokn@...zon.com>
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Subject: Re: [PATCH v2 0/7] barrier: introduce smp_cond_load_*_timewait()


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

> 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>

Thanks for the review (and the testing)!

--
ankur

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