On Mon 2023-05-01 08:24:46, Douglas Anderson wrote:
> From: Colin Cross <ccr...@android.com>
>
> Implement a hardlockup detector that doesn't doesn't need any extra
> arch-specific support code to detect lockups. Instead of using
> something arch-specific we will use the buddy system, where each CPU
> watches out for another one. Specifically, each CPU will use its
> softlockup hrtimer to check that the next CPU is processing hrtimer
> interrupts by verifying that a counter is increasing.
>
> --- /dev/null
> +++ b/kernel/watchdog_buddy_cpu.c
> +int watchdog_nmi_enable(unsigned int cpu)
> +{
> + /*
> + * The new CPU will be marked online before the first hrtimer interrupt
> + * runs on it.
It does not need to be the first hrtimer interrupt. The CPU might have
been offlined/onlined repeatedly. The counter might have any value.
> + * If another CPU tests for a hardlockup on the new CPU
> + * before it has run its first hrtimer, it will get a false positive.
> + * Touch the watchdog on the new CPU to delay the first check for at
> + * least 3 sampling periods to guarantee one hrtimer has run on the new
> + * CPU.
> + */
> + per_cpu(watchdog_touch, cpu) = true;
We should touch also the next_cpu:
/*
* We are going to check the next CPU. Our watchdog_hrtimer
* need not be zero if the CPU has already been online earlier.
* Touch the watchdog on the next CPU to avoid false positive
* if we try to check it in less then 3 interrupts.
*/
next_cpu = watchdog_next_cpu(cpu);
if (next_cpu < nr_cpu_ids)
per_cpu(watchdog_touch, next_cpu) = true;
Alternative would be to clear watchdog_hrtimer. But it would kind-of
affect also the softlockup detector.
> + /* Match with smp_rmb() in watchdog_check_hardlockup() */
> + smp_wmb();
> + cpumask_set_cpu(cpu, &watchdog_cpus);
> + return 0;
> +}
> +
> +void watchdog_nmi_disable(unsigned int cpu)
> +{
> + unsigned int next_cpu = watchdog_next_cpu(cpu);
> +
> + /*
> + * Offlining this CPU will cause the CPU before this one to start
> + * checking the one after this one. If this CPU just finished checking
> + * the next CPU and updating hrtimer_interrupts_saved, and then the
> + * previous CPU checks it within one sample period, it will trigger a
> + * false positive. Touch the watchdog on the next CPU to prevent it.
> + */
> + if (next_cpu < nr_cpu_ids)
> + per_cpu(watchdog_touch, next_cpu) = true;
> + /* Match with smp_rmb() in watchdog_check_hardlockup() */
> + smp_wmb();
> + cpumask_clear_cpu(cpu, &watchdog_cpus);
> +}
> +
Best Regards,
Petr
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