| Message ID | 1401355381-11446-1-git-send-email-lorenzo.pieralisi@arm.com |
|---|---|
| State | New |
| Headers | show |
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 <preeti@linux.vnet.ibm.com> > Cc: Will Deacon <will.deacon@arm.com> > Acked-by: Mark Rutland <mark.rutland@arm.com> > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> Thanks Lorenzo, Acked-by: Will Deacon <will.deacon@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 majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Hi Lorenzo, On 05/29/2014 02:53 PM, 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 It would be best to use the term "hrtimer based broadcast device" throughout the changelog for uniformity and to avoid confusion instead of mixing it with "software broadcast". > 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. Sorry but I do not understand the above paragraph. What do you mean by "allows the kernel to enter high resolution timer mode" ? And how does it improve system latency? I understand that the hrtimer based clockevent device saves dynamic power since it provides a mechanism in which cpus can enter deeper idle states. > > 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. I think this statement "instead of the software based one, now present by default" is incorrect. The hrtimer based clock event device will come into picture only when the arch calls tick_setup_hrtimer_broadcast() explicitly. Otherwise either the arch should register a real clock device which does broadcast or should disable deep idle states where the local timers stop. So I would suggest skipping the last paragraph as it is not conveying anything in specific. The fact that a clock device with the highest rating will be chosen is already known and need not be mentioned explicitly IMHO. > > Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com> > Cc: Will Deacon <will.deacon@arm.com> > Acked-by: Mark Rutland <mark.rutland@arm.com> > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> > --- > arch/arm64/kernel/time.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/arch/arm64/kernel/time.c b/arch/arm64/kernel/time.c > index 29c39d5..3d43900 100644 > --- a/arch/arm64/kernel/time.c > +++ b/arch/arm64/kernel/time.c > @@ -18,6 +18,7 @@ > * along with this program. If not, see <http://www.gnu.org/licenses/>. > */ > > +#include <linux/clockchips.h> > #include <linux/export.h> > #include <linux/kernel.h> > #include <linux/interrupt.h> > @@ -67,6 +68,8 @@ void __init time_init(void) > > clocksource_of_init(); > > + tick_setup_hrtimer_broadcast(); > + > arch_timer_rate = arch_timer_get_rate(); > if (!arch_timer_rate) > panic("Unable to initialise architected timer.\n"); > You have defined flag "CPUIDLE_FLAG_TIMER_STOP" for your deep idle states in which timer stops right? Regards Preeti U Murthy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Hi Preeti, On Thu, May 29, 2014 at 12:04:36PM +0100, Preeti U Murthy wrote: > Hi Lorenzo, > > On 05/29/2014 02:53 PM, 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 > > It would be best to use the term "hrtimer based broadcast device" > throughout the changelog for uniformity and to avoid confusion instead > of mixing it with "software broadcast". > > > 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. > > Sorry but I do not understand the above paragraph. What do you mean by > "allows the kernel to enter high resolution timer mode" ? And how does > it improve system latency? I understand that the hrtimer based > clockevent device saves dynamic power since it provides a mechanism in > which cpus can enter deeper idle states. When there's no oneshot-capable broadcast device and the CPU-local clock_event_device has the CLK_EVT_FEAT_C3STOP flag, tick_is_oneshot_available will return false. Thus tick_check_oneshot_change will return false, and hrtimer_switch_to_hres will never switch to high resolution mode (and we can also never enter NOHZ mode), leaving us stuck in periodic mode. In periodic mode ticks occur at fixed intervals, and any timer wakeup will occur after the tick following the requested wakeup time, adding some amount of latency over what would be possible with high resolution mode. Additionally, as we can only wake up at said ticks and not between them, it's possible that several timers for intervals shorter than that tick interval will fire at once upon a timer tick. Any tasks which requested these wakeups will fight for CPU time, and some will see additional latency because of this. > > > > 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. > > I think this statement "instead of the software based one, now present > by default" is incorrect. The hrtimer based clock event device will come > into picture only when the arch calls tick_setup_hrtimer_broadcast() > explicitly. Otherwise either the arch should register a real clock > device which does broadcast or should disable deep idle states where the > local timers stop. So I would suggest skipping the last paragraph as it > is not conveying anything in specific. The fact that a clock device with > the highest rating will be chosen is already known and need not be > mentioned explicitly IMHO. I think it is worth keeping the paragraph to allay anyone's fear that the hrtimer based broadcast device might be selected in preference to a real suitable clock. I would otherwise not be aware that the hrtimer based broadcast device had the lowest rating (and would have to go and look that up separately). As the arch code has delegated timer registration to clocksoruce_of_init, it doesn't know whether any of the real devices that may have been registered are suitable as a broadcast source for oneshot events. So we can't conditionally register the hrtimer based broadcast device. Perhaps we could replace "now present by default" with "which is unconditionally registered in case no suitable hardware device is present"? Cheers, Mark. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Hi Preeti, On Thu, May 29, 2014 at 12:04:36PM +0100, Preeti U Murthy wrote: > Hi Lorenzo, > > On 05/29/2014 02:53 PM, 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 > > It would be best to use the term "hrtimer based broadcast device" > throughout the changelog for uniformity and to avoid confusion instead > of mixing it with "software broadcast". Agreed. > > 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. > > Sorry but I do not understand the above paragraph. What do you mean by > "allows the kernel to enter high resolution timer mode" ? And how does > it improve system latency? I understand that the hrtimer based > clockevent device saves dynamic power since it provides a mechanism in > which cpus can enter deeper idle states. See Mark's reply, I have nothing to add. I will remove this paragraph anyway. > > 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. > > I think this statement "instead of the software based one, now present > by default" is incorrect. The hrtimer based clock event device will come > into picture only when the arch calls tick_setup_hrtimer_broadcast() > explicitly. Otherwise either the arch should register a real clock > device which does broadcast or should disable deep idle states where the > local timers stop. So I would suggest skipping the last paragraph as it > is not conveying anything in specific. The fact that a clock device with > the highest rating will be chosen is already known and need not be > mentioned explicitly IMHO. > > > > > Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com> > > Cc: Will Deacon <will.deacon@arm.com> > > Acked-by: Mark Rutland <mark.rutland@arm.com> > > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> > > --- > > arch/arm64/kernel/time.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/arch/arm64/kernel/time.c b/arch/arm64/kernel/time.c > > index 29c39d5..3d43900 100644 > > --- a/arch/arm64/kernel/time.c > > +++ b/arch/arm64/kernel/time.c > > @@ -18,6 +18,7 @@ > > * along with this program. If not, see <http://www.gnu.org/licenses/>. > > */ > > > > +#include <linux/clockchips.h> > > #include <linux/export.h> > > #include <linux/kernel.h> > > #include <linux/interrupt.h> > > @@ -67,6 +68,8 @@ void __init time_init(void) > > > > clocksource_of_init(); > > > > + tick_setup_hrtimer_broadcast(); > > + > > arch_timer_rate = arch_timer_get_rate(); > > if (!arch_timer_rate) > > panic("Unable to initialise architected timer.\n"); > > > > You have defined flag "CPUIDLE_FLAG_TIMER_STOP" for your deep idle > states in which timer stops right? Yes, I would have noticed otherwise =) Thanks, Lorenzo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On 05/29/2014 06:09 PM, Mark Rutland wrote: > Hi Preeti, > > On Thu, May 29, 2014 at 12:04:36PM +0100, Preeti U Murthy wrote: >> Hi Lorenzo, >> >> On 05/29/2014 02:53 PM, 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 >> >> It would be best to use the term "hrtimer based broadcast device" >> throughout the changelog for uniformity and to avoid confusion instead >> of mixing it with "software broadcast". >> >>> 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. >> >> Sorry but I do not understand the above paragraph. What do you mean by >> "allows the kernel to enter high resolution timer mode" ? And how does >> it improve system latency? I understand that the hrtimer based >> clockevent device saves dynamic power since it provides a mechanism in >> which cpus can enter deeper idle states. > > When there's no oneshot-capable broadcast device and the CPU-local > clock_event_device has the CLK_EVT_FEAT_C3STOP flag, > tick_is_oneshot_available will return false. Thus > tick_check_oneshot_change will return false, and hrtimer_switch_to_hres > will never switch to high resolution mode (and we can also never enter > NOHZ mode), leaving us stuck in periodic mode. > > In periodic mode ticks occur at fixed intervals, and any timer wakeup > will occur after the tick following the requested wakeup time, adding > some amount of latency over what would be possible with high resolution > mode. Additionally, as we can only wake up at said ticks and not between > them, it's possible that several timers for intervals shorter than that > tick interval will fire at once upon a timer tick. Any tasks which > requested these wakeups will fight for CPU time, and some will see > additional latency because of this. Ah ok I see now. Thanks for the explanation :) > >>> >>> 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. >> >> I think this statement "instead of the software based one, now present >> by default" is incorrect. The hrtimer based clock event device will come >> into picture only when the arch calls tick_setup_hrtimer_broadcast() >> explicitly. Otherwise either the arch should register a real clock >> device which does broadcast or should disable deep idle states where the >> local timers stop. So I would suggest skipping the last paragraph as it >> is not conveying anything in specific. The fact that a clock device with >> the highest rating will be chosen is already known and need not be >> mentioned explicitly IMHO. > > I think it is worth keeping the paragraph to allay anyone's fear that > the hrtimer based broadcast device might be selected in preference to a > real suitable clock. I would otherwise not be aware that the hrtimer > based broadcast device had the lowest rating (and would have to go and > look that up separately). > > As the arch code has delegated timer registration to > clocksoruce_of_init, it doesn't know whether any of the real devices > that may have been registered are suitable as a broadcast source for > oneshot events. So we can't conditionally register the hrtimer based > broadcast device. > > Perhaps we could replace "now present by default" with "which is > unconditionally registered in case no suitable hardware device is > present"? Yes I would say "which gets registered in case no suitable hardware devices is present." This would make it clearer. Regards Preeti U Murthy > > Cheers, > Mark. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
diff --git a/arch/arm64/kernel/time.c b/arch/arm64/kernel/time.c index 29c39d5..3d43900 100644 --- a/arch/arm64/kernel/time.c +++ b/arch/arm64/kernel/time.c @@ -18,6 +18,7 @@ * along with this program. If not, see <http://www.gnu.org/licenses/>. */ +#include <linux/clockchips.h> #include <linux/export.h> #include <linux/kernel.h> #include <linux/interrupt.h> @@ -67,6 +68,8 @@ void __init time_init(void) clocksource_of_init(); + tick_setup_hrtimer_broadcast(); + arch_timer_rate = arch_timer_get_rate(); if (!arch_timer_rate) panic("Unable to initialise architected timer.\n");