> 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. Okay, I'll split this part into another change, thanks. > > > > +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. After some days studying, if I remove this if-condition which means the following hardlockup_detector_perf_init() needs to return -EBUSY. However, the default return value that if pmu is not ready is -ENOENT. The call path for hardlockup_detector_perf_init() is really complicated, I have some approach about this: 1. abstract second variable with Kconfig. a. Add a ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT (the naming is a little bit long, may have better naming) in "lib/Kconfig.debug" if ARCH knew they do need delayed init for lockup detector. + select ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT if HAVE_HARDLOCKUP_DETECTOR_PERF b. and the watchdog_nmi_probe would look like. +int __init watchdog_nmi_probe(void) +{ + int ret; + + /* comment here... */ + if (!arm_pmu_irq_is_nmi()) + return -ENODEV; + + ret = hardlockup_detector_perf_init(); + if (ret && + IS_ENABLED(ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT)) + return -EBUSY; + + return ret; +} and than we can have only one variable (allow_lockup_detector_init_retry) in 4th patch. 2. base on ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT, change inside hardlockup_detector_perf_init(). int __init hardlockup_detector_perf_init(void) { int ret = hardlockup_detector_event_create(); if (ret) { pr_info("Perf NMI watchdog permanently disabled\n"); + + /* comment here... */ + if (IS_ENABLED(ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT)) + ret = -EBUSY; } else { perf_event_release_kernel(this_cpu_read(watchdog_ev)); this_cpu_write(watchdog_ev, NULL); } return ret; } 3. Don't add any other config, try to find a proper location to return -EBUSY in hardlockup_detector_event_create(). IMHO, this may involve the PMU subsys and should be the hardest approach. > > > > + > > > > + 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. > thank you, I'll just add the comment above like this. /* * 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. */ if (!arm_pmu_irq_is_nmi()) return -ENODEV; thanks BRs, Lecopzer
