On Thu, 19 Nov 2009, Jean Delvare wrote: > > That still does not explain why yield() is necessary _between_ the > > transaction attempts. > > It is not _necessary_. We are just trying to be fair to other kernel > threads, because bit-banging is expensive and this is the only case > where we know we can tolerate a delay. > > Just to clarify things... does (or did) yield() have anything to do > with CONFIG_PREEMPT_VOLUNTARY? No. > > That code is fully preemptible, otherwise you could not call > > yield(). > > How does one know what code is preemtible and what code is not? The > rest of the i2c-algo-bit code should definitely _not_ be preemtible, as > it is highly timing sensitive. Code is preemptible when preempt_count() == 0 and interrupts are enabled. spin_lock() implicitely disables preemption. > > And as I said before nobody even noticed that the yield() > > default implementation was changed to a NOOP by default in the > > scheduler. > > Well, I guess only people monitoring system latency would notice, as > this is the only thing yield() was supposed to help with in the first > place. > > You say "NOOP by default", does this imply there is a way to change > this? There is a sysctl: sysctl_sched_compat_yield > Was yield() turned into NOOP by design, or was it a bug? By design. The semantics of yield and the fairness approach of CFS are not really working well together. Also yield() for SCHED_OTHER is not really specified. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
