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emacs 27.2 From: Ankur Arora To: Arnd Bergmann Cc: Ankur Arora , linux-kernel@vger.kernel.org, Linux-Arch , linux-arm-kernel@lists.infradead.org, linux-pm@vger.kernel.org, bpf@vger.kernel.org, Catalin Marinas , Will Deacon , Peter Zijlstra , Andrew Morton , Mark Rutland , Haris Okanovic , "Christoph Lameter (Ampere)" , Alexei Starovoitov , "Rafael J . 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linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Arnd Bergmann writes: > On Wed, Oct 29, 2025, at 04:17, Ankur Arora wrote: >> Arnd Bergmann 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