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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 Tue, Oct 28, 2025, at 06:31, Ankur Arora wrote: >> Support waiting in smp_cond_load_relaxed_timeout() via >> __cmpwait_relaxed(). Limit this to when the event-stream is enabled, >> to ensure that we wake from WFE periodically and don't block forever >> if there are no stores to the cacheline. >> >> In the unlikely event that the event-stream is unavailable, fallback >> to spin-waiting. >> >> Also set SMP_TIMEOUT_POLL_COUNT to 1 so we do the time-check for each >> iteration in smp_cond_load_relaxed_timeout(). > > After I looked at the entire series again, this one feels like > a missed opportunity. Especially on low-power systems but possibly > on any ARMv9.2+ implementation including Cortex-A320, it would > be nice to be able to both turn off the event stream and also > make this function take fewer wakeups: > >> +/* Re-declared here to avoid include dependency. */ >> +extern bool arch_timer_evtstrm_available(void); >> + >> +#define cpu_poll_relax(ptr, val) \ >> +do { \ >> + if (arch_timer_evtstrm_available()) \ >> + __cmpwait_relaxed(ptr, val); \ >> + else \ >> + cpu_relax(); \ >> +} while (0) >> + > > Since the caller knows exactly how long it wants to wait for, > we should be able to fit a 'wfet' based primitive in here and > pass the timeout as another argument. Per se, I don't disagree with this when it comes to WFET. Handling a timeout, however, is messier when we use other mechanisms. Some problems that came up in my earlier discussions with Catalin: - when using WFE, we also need some notion of slack - and if a caller specifies only a small or no slack, then we need to combine WFE+cpu_relax() - for platforms that only use a polling primitive, we want to check the clock only intermittently for power reasons. Now, this could be done with an architecture specific spin-count. However, if the caller specifies a small slack, then we might need to we check the clock more often as we get closer to the deadline etc. A smaller problem was that different users want different clocks and so folding the timeout in a 'timeout_cond_expr' lets us do away with the interface having to handle any of that. I had earlier versions [v2] [v3] which had rather elaborate policies for handling timeout, slack etc. But, given that the current users of the interface don't actually care about precision, all of that seemed a little overengineered. [v2] https://lore.kernel.org/lkml/20250502085223.1316925-1-ankur.a.arora@oracle.com/#r [v3] https://lore.kernel.org/lkml/20250627044805.945491-1-ankur.a.arora@oracle.com/ -- ankur