On Tue 2022-04-05 20:53:04, Lecopzer Chen wrote: > > > On Thu 2022-03-24 22:14:05, Lecopzer Chen wrote: > > > With the recent feature added to enable perf events to use pseudo NMIs > > > as interrupts on platforms which support GICv3 or later, its now been > > > possible to enable hard lockup detector (or NMI watchdog) on arm64 > > > platforms. So enable corresponding support. > > > > > > One thing to note here is that normally lockup detector is initialized > > > just after the early initcalls but PMU on arm64 comes up much later as > > > device_initcall(). To cope with that, overriding watchdog_nmi_probe() to > > > let the watchdog framework know PMU not ready, and inform the framework > > > to re-initialize lockup detection once PMU has been initialized. > > > > > > [1]: http://lore.kernel.org/linux-arm-kernel/1610712101-14929-1-git-send-email-sumit.garg@xxxxxxxxxx > > > > > > --- /dev/null > > > +++ b/arch/arm64/kernel/watchdog_hld.c > > > @@ -0,0 +1,37 @@ > > > +// SPDX-License-Identifier: GPL-2.0 > > > +#include <linux/nmi.h> > > > +#include <linux/cpufreq.h> > > > +#include <linux/perf/arm_pmu.h> > > > + > > > +/* > > > + * Safe maximum CPU frequency in case a particular platform doesn't implement > > > + * cpufreq driver. Although, architecture doesn't put any restrictions on > > > + * maximum frequency but 5 GHz seems to be safe maximum given the available > > > + * Arm CPUs in the market which are clocked much less than 5 GHz. On the other > > > + * hand, we can't make it much higher as it would lead to a large hard-lockup > > > + * detection timeout on parts which are running slower (eg. 1GHz on > > > + * Developerbox) and doesn't possess a cpufreq driver. > > > + */ > > > +#define SAFE_MAX_CPU_FREQ 5000000000UL // 5 GHz > > > +u64 hw_nmi_get_sample_period(int watchdog_thresh) > > > +{ > > > + unsigned int cpu = smp_processor_id(); > > > + unsigned long max_cpu_freq; > > > + > > > + max_cpu_freq = cpufreq_get_hw_max_freq(cpu) * 1000UL; > > > + if (!max_cpu_freq) > > > + max_cpu_freq = SAFE_MAX_CPU_FREQ; > > > + > > > + return (u64)max_cpu_freq * watchdog_thresh; > > > +} > > > > This change is not mentioned in the commit message. > > Please, put it into a separate patch. > > > Actully, This cames from > [1]: http://lore.kernel.org/linux-arm-kernel/1610712101-14929-1-git-send-email-sumit.garg@xxxxxxxxxx > And I didn't touch the commit message from the origin patch. > But of course, I could imporve it with proper description if > anyone thinks it's not good enough. I see. > Would you mean put this function hw_nmi_get_sample_period() in patch > 6th? > In the view of "arm64 uses delayed init with all the functionality it need to set up", > IMO, this make sense for me to put into a single patch. Or you could split it in two patches and add hw_nmi_get_sample_period() in the earlier patch. > But if you still think this should put into a separate patch, I'll do it:) It is always better to split the changes whenever possible. It makes the review easier. And it also helps to find the real culprit of a regression using bisection. > > > +int __init watchdog_nmi_probe(void) > > > +{ > > > + if (!allow_lockup_detector_init_retry) > > > + return -EBUSY; > > > > How do you know that you should return -EBUSY > > when retry in not enabled? > > > > I guess that it is an optimization to make it fast > > during the first call. But the logic is far from > > obvious. > > > > Yes, you can see this as an optimization, because arm64 PMU is not ready > during lockup_detector_init(), so the watchdog_nmi_probe() must fail. > > Thus we only want to do watchdog_nmi_probe() in delayed init, > so if not in the state (allow_lockup_detector_init_retry=true), just tell > > if it's unclear Yes, it is far from obvious. > maybe a brief comment can be add like this: > > + /* arm64 is only able to initialize lockup detecor during delayed init */ > + if (!allow_lockup_detector_init_retry) > + return -EBUSY; No, please, remove this optimization. It just makes problems: + it requires a comment here because the logic is far from obvious. + it is the reason why we need another variable to avoid the race in lockup_detector_check(), see the discussion about the 4th patch. > > > + > > > + if (!arm_pmu_irq_is_nmi()) > > > + return -ENODEV; > > > + > > > + return hardlockup_detector_perf_init(); > > > +} > > > For arm_pmu_irq_is_nmi() checking, we do need it, becasue arm64 needs > explictly turns on Pseudo-NMI to support base function for NMI. > > hardlockup_detector_perf_init() will success even if we haven't had > Pseudo-NMI turns on, however, the pmu interrupts will act like a > normal interrupt instead of NMI and the hardlockup detector would be broken. I see. Please, explain this in a comment. It is another thing that is far from obvious. Best Regards, Petr
