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X-CRM114-Status: GOOD ( 26.33 ) X-BeenThere: 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 Catalin Marinas writes: > On Thu, Sep 18, 2025 at 09:05:22PM +0100, Will Deacon wrote: >> > +/* Re-declared here to avoid include dependency. */ >> > +extern bool arch_timer_evtstrm_available(void); >> > + >> > +#define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_check_expr) \ >> > +({ \ >> > + typeof(ptr) __PTR = (ptr); \ >> > + __unqual_scalar_typeof(*ptr) VAL; \ >> > + bool __wfe = arch_timer_evtstrm_available(); \ >> > + \ >> > + for (;;) { \ >> > + VAL = READ_ONCE(*__PTR); \ >> > + if (cond_expr) \ >> > + break; \ >> > + if (time_check_expr) \ >> > + break; \ >> > + if (likely(__wfe)) \ >> > + __cmpwait_relaxed(__PTR, VAL); \ >> > + else \ >> > + cpu_relax(); \ >> >> It'd be an awful lot nicer if we could just use the generic code if >> wfe isn't available. One option would be to make that available as >> e.g. __smp_cond_load_relaxed_timeout_cpu_relax() and call it from the >> arch code when !arch_timer_evtstrm_available() but a potentially cleaner >> version would be to introduce something like cpu_poll_relax() and use >> that in the core code. >> >> So arm64 would do: >> >> #define SMP_TIMEOUT_SPIN_COUNT 1 >> #define cpu_poll_relax(ptr, val) do { \ >> if (arch_timer_evtstrm_available()) \ >> __cmpwait_relaxed(ptr, val); \ >> else \ >> cpu_relax(); \ >> } while (0) >> >> and then the core code would have: >> >> #ifndef cpu_poll_relax >> #define cpu_poll_relax(p, v) cpu_relax() >> #endif > > A slight problem here is that we have two users that want different spin > counts: poll_idle() uses 200, rqspinlock wants 16K. They've been > empirically chosen but I guess it also depends on what they call in > time_check_expr and the resolution they need. From the discussion on > patch 5, Kumar would like to override the spin count to 16K from the > current one of 200 (or if poll_idle works with 16K, we just set that as > the default; we have yet to hear from the cpuidle folk). > > I guess on arm64 we'd first #undef it and redefine it as 1. I think you mean 16k? I have some (small) misgivings about that code choosing the same value for all platforms but that could easily be addressed if it becomes an issue at some point. > The > non-event stream variant is for debug only really, I'd expect it to > always have it on in production (or go for WFET). > So yeah, I think the above would work. Ankur proposed something similar > in the early versions but I found it too complicated (a spin and wait > policy callback populating the spin variable). Your proposal looks a lot > simpler. Yeah. This looks a much simpler way of abstracting the choice of the mechanism, polling/waiting/some mixture to the architecture without needing any separate policy etc. -- ankur