On Thu, May 29, 2014 at 10:23:01AM +0100, Lorenzo Pieralisi wrote: > On platforms implementing CPU power management, the CPUidle subsystem > can allow CPUs to enter idle states where local timers logic is lost on power > down. To keep the software timers functional the kernel relies on an > always-on broadcast timer to be present in the platform to relay the > interrupt signalling the timer expiries. > > For platforms implementing CPU core gating that do not implement an always-on > HW timer or implement it in a broken way, this patch adds code to initialize > the kernel software broadcast hrtimer upon boot. It relies on a dynamically > chosen CPU to be always powered-up. This CPU then relays the timer interrupt > to CPUs in deep-idle states through its HW local timer device. > > On systems with power management capabilities but no functional HW broadcast > tick device, the hrtimer based clock event device allows the kernel to > enter high-resolution timer mode, which improves system latencies and saves > dynamic power. > > The side effect of having a CPU always-on has implications on power management > platform capabilities and makes CPUidle suboptimal, since at least a CPU is > kept always in a shallow idle state by the kernel to relay timer interrupts, > but at least leaves the kernel with a functional system with some working > power > management capabilities. > > The hrtimer based clock event device has lowest possible rating so that, > if a platform contains a functional HW clock event device with broadcast > capabilities, that device is always chosen as a tick broadcast device instead > of the software based one, now present by default. > > Cc: Preeti U Murthy <pre...@linux.vnet.ibm.com> > Cc: Will Deacon <will.dea...@arm.com> > Acked-by: Mark Rutland <mark.rutl...@arm.com> > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieral...@arm.com>
Thanks Lorenzo, Acked-by: Will Deacon <will.dea...@arm.com> With this patch applied, cyclictest starts reporting sane numbers and I no longer see latency-related failures in LTP when it does things like test timeouts and nanosleep()s. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/