Re: [RESEND PATCH v7 1/7] asm-generic: barrier: Add smp_cond_load_relaxed_timeout()

Ankur Arora
2025-11-03T21:41:40Z

Arnd Bergmann <arnd@arndb.de> writes:

> On Wed, Oct 29, 2025, at 04:17, Ankur Arora wrote:
>> Arnd Bergmann <arnd@arndb.de> writes:
>>> On Tue, Oct 28, 2025, at 06:31, Ankur Arora wrote:
>>> The FEAT_WFXT version would then look something like
>>>
>>> static inline void __cmpwait_u64_timeout(volatile u64 *ptr, unsigned long val, __u64 ns)
>>> {
>>> unsigned long tmp;
>>> asm volatile ("sev; wfe; ldxr; eor; cbnz; wfet; 1:"
>>> : "=&r" (tmp), "+Q" (*ptr)
>>> : "r" (val), "r" (ns));
>>> }
>>> #define cpu_poll_relax_timeout_wfet(__PTR, VAL, TIMECHECK) \
>>> ({ \
>>> u64 __t = TIMECHECK;
>>> if (__t)
>>> __cmpwait_u64_timeout(__PTR, VAL, __t);
>>> })
>>>
>>> while the 'wfe' version would continue to do the timecheck after the
>>> wait.
>>
>> I think this is a good way to do it if we need the precision
>> at some point in the future.
>
> I'm sorry I wrote this in a confusing way, but I really don't
> care about precision here, but for power savings reasons I
> would like to use a wfe/wfet based wait here (like you do)
> while at the same time being able to turn off the event
> stream entirely as long as FEAR_WFXT is available, saving
> additional power.
>
>>> I have two lesser concerns with the generic definition here:
>>>
>>> - having both a timeout and a spin counter in the same loop
>>> feels redundant and error-prone, as the behavior in practice
>>> would likely depend a lot on the platform. What is the reason
>>> for keeping the counter if we already have a fixed timeout
>>> condition?
>>
>> The main reason was that the time check is expensive in power terms.
>> Which is fine for platforms with a WFE like primitive but others
>> want to do the time check only infrequently. That's why poll_idle()
>> introduced a rate limit on polling (which the generic definition
>> reused here.)
>
> Right, I misunderstood how this works, so this part is fine.
>
>>> - I generally dislike the type-agnostic macros like this one,
>>> it adds a lot of extra complexity here that I feel can be
>>> completely avoided if we make explicitly 32-bit and 64-bit
>>> wide versions of these macros. We probably won't be able
>>> to resolve this as part of your series, but ideally I'd like
>>> have explicitly-typed versions of cmpxchg(), smp_load_acquire()
>>> and all the related ones, the same way we do for atomic_*()
>>> and atomic64_*().
>>
>> Ah. And the caller uses say smp_load_acquire_long() or whatever, and
>> that resolves to whatever makes sense for the arch.
>>
>> The __unqual_scalar_typeof() does look pretty ugly when looking at the
>> preprocesed version but other than that smp_cond_load() etc look
>> pretty straight forward. Just for my curiousity could you elaborate on
>> the complexity?
>
> The nested macros with __unqual_scalar_typeof() make the
> preprocessed version completely unreadable, especially when
> combined with other sets of complex macros like min()/max(),
> tracepoints or arm64 atomics.
>
> If something goes wrong inside of it, it becomes rather
> hard for people to debug or even read the compiler warnings.
> Since I do a lot of build testing, I do tend to run into those
> more than others do.
>
> I've also seen cases where excessively complex macros get
> nested to the point of slowing down the kernel build, which
> can happen once the nesting expands to megabytes of source
> code.

Thanks for the detail. Yeah, looking at the preprocessed code (as I had
to do when working on this) it all really feels quite excessive.
Combined with other complex macro constructs ...

--
ankur