Messages in this thread | | | Date | Thu, 11 Apr 2024 18:59:36 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] sched/isolation: fix boot crash when the boot CPU is nohz_full |
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Just in case...
This reveals another issue, tick_setup_device() can't use smp_call_function() due to irqs_disabled(), but lets discuss this later/separately too.
On 04/11, Oleg Nesterov wrote: > > Documentation/timers/no_hz.rst states that the "nohz_full=" mask must not > include the boot CPU, this is no longer true after the commit 08ae95f4fd3b > ("nohz_full: Allow the boot CPU to be nohz_full"). > > However after another commit aae17ebb53cd ("workqueue: Avoid using isolated > cpus' timers on queue_delayed_work") the kernel will crash at boot time in > this case; housekeeping_any_cpu() returns an invalid cpu nr until smp_init() > paths bring the 1st housekeeping CPU up. > > Change housekeeping_any_cpu() to check the result of cpumask_any_and() and > return smp_processor_id() in this case. Yes, this is just the simple and > backportable workaround which fixes the symptom, but smp_processor_id() at > boot time should be safe at least for type == HK_TYPE_TIMER, this more or > less matches the tick_do_timer_boot_cpu logic. > > We should not worry about cpu_down(); tick_nohz_cpu_down() will not allow > to offline tick_do_timer_cpu (the 1st online housekeeping CPU). > > Fixes: aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_delayed_work") > Reported-by: Chris von Recklinghausen <crecklin@redhat.com> > Closes: https://lore.kernel.org/all/20240402105847.GA24832@redhat.com/ > Signed-off-by: Oleg Nesterov <oleg@redhat.com> > --- > Documentation/timers/no_hz.rst | 7 ++----- > kernel/sched/isolation.c | 11 ++++++++++- > 2 files changed, 12 insertions(+), 6 deletions(-) > > diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst > index f8786be15183..7fe8ef9718d8 100644 > --- a/Documentation/timers/no_hz.rst > +++ b/Documentation/timers/no_hz.rst > @@ -129,11 +129,8 @@ adaptive-tick CPUs: At least one non-adaptive-tick CPU must remain > online to handle timekeeping tasks in order to ensure that system > calls like gettimeofday() returns accurate values on adaptive-tick CPUs. > (This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running > -user processes to observe slight drifts in clock rate.) Therefore, the > -boot CPU is prohibited from entering adaptive-ticks mode. Specifying a > -"nohz_full=" mask that includes the boot CPU will result in a boot-time > -error message, and the boot CPU will be removed from the mask. Note that > -this means that your system must have at least two CPUs in order for > +user processes to observe slight drifts in clock rate.) Note that this > +means that your system must have at least two CPUs in order for > CONFIG_NO_HZ_FULL=y to do anything for you. > > Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded. > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c > index 373d42c707bc..2a262d3ecb3d 100644 > --- a/kernel/sched/isolation.c > +++ b/kernel/sched/isolation.c > @@ -46,7 +46,16 @@ int housekeeping_any_cpu(enum hk_type type) > if (cpu < nr_cpu_ids) > return cpu; > > - return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask); > + cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask); > + if (likely(cpu < nr_cpu_ids)) > + return cpu; > + /* > + * Unless we have another problem this can only happen > + * at boot time before start_secondary() brings the 1st > + * housekeeping CPU up. > + */ > + WARN_ON_ONCE(system_state == SYSTEM_RUNNING || > + type != HK_TYPE_TIMER); > } > } > return smp_processor_id(); > -- > 2.25.1.362.g51ebf55 >
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