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X-CRM114-Status: GOOD ( 18.55 ) 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 Mon, Nov 03, 2025 at 01:00:33PM -0800, Ankur Arora wrote: >> /** >> * smp_cond_load_relaxed_timeout() - (Spin) wait for cond with no ordering >> * guarantees until a timeout expires. >> * @ptr: pointer to the variable to wait on >> * @cond: boolean expression to wait for >> * @time_expr: time expression in caller's preferred clock >> * @time_end: end time in nanosecond (compared against time_expr; >> * might also be used for setting up a future event.) >> * >> * Equivalent to using READ_ONCE() on the condition variable. >> * >> * Note that the expiration of the timeout might have an architecture specific >> * delay. >> */ >> #ifndef smp_cond_load_relaxed_timeout >> #define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr, time_end_ns) \ >> ({ \ >> typeof(ptr) __PTR = (ptr); \ >> __unqual_scalar_typeof(*ptr) VAL; \ >> u32 __n = 0, __spin = SMP_TIMEOUT_POLL_COUNT; \ >> u64 __time_end_ns = (time_end_ns); \ >> \ >> for (;;) { \ >> VAL = READ_ONCE(*__PTR); \ >> if (cond_expr) \ >> break; \ >> cpu_poll_relax(__PTR, VAL, __time_end_ns); \ > > With time_end_ns being passed to cpu_poll_relax(), we assume that this > is always the absolute time. Do we still need time_expr in this case? > It works for WFET as long as we can map this time_end_ns onto the > hardware CNTVCT. So I like this idea. Given that we only promise a coarse granularity we should be able to get by with using a coarse clock of our choosing. However, maybe some callers need a globally consistent clock just in case they could migrate and do something stateful in the cond_expr? (For instance rqspinlock wants ktime_mono. Though I don't think these callers can migrate.) > Alternatively, we could pass something like remaining_ns, though not > sure how smp_cond_load_relaxed_timeout() can decide to spin before > checking time_expr again (we probably went over this in the past two > years ;)). I'm sure it is in there somewhere :). This one?: https://lore.kernel.org/lkml/aJy414YufthzC1nv@arm.com/. Though the whole wait_policy thing confused the issue somewhat there. Though that problem exists for both remaining_ns and for time_end_ns with WFE. I think we are fine so long as SMP_TIMEOUT_POLL_COUNT is defined to be 1. For now, I think it makes sense to always pass the absolute deadline even if the caller uses remaining_ns. So: #define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr, remaining_ns) \ ({ \ typeof(ptr) __PTR = (ptr); \ __unqual_scalar_typeof(*ptr) VAL; \ u32 __n = 0, __spin = SMP_TIMEOUT_POLL_COUNT; \ u64 __time_start_ns = (time_expr); \ s64 __time_end_ns = __time_start_ns + (remaining_ns); \ \ for (;;) { \ VAL = READ_ONCE(*__PTR); \ if (cond_expr) \ break; \ cpu_poll_relax(__PTR, VAL, __time_end_ns); \ if (++__n < __spin) \ continue; \ if ((time_expr) >= __time_end_ns) { \ VAL = READ_ONCE(*__PTR); \ break; \ } \ __n = 0; \ } \ (typeof(*ptr))VAL; \ }) -- ankur