Messages in this thread | | | Date | Wed, 3 Apr 2024 22:38:14 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work |
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Hi Tejun,
On 04/03, Tejun Heo wrote: > > (cc'ing Frederic and quoting whole body) > > Hello, Oleg. > > On Tue, Apr 02, 2024 at 12:58:47PM +0200, Oleg Nesterov wrote: > > This patch was applied as aae17ebb53cd3da but as Chris reports with this > > commit the kernel can crash at boot time because __queue_delayed_work() > > doesn't check that housekeeping_any_cpu() returns a valid cpu < nr_cpu_ids. > > > > Just boot the kernel with nohz_full=mask which includes the boot cpu, say > > nohz_full=0-6 on a machine with 8 CPUs. __queue_delayed_work() will use > > add_timer_on(timer, NR_CPUS /* returned by housekeeping_any_cpu */) until > > start_secondary() brings CPU 7 up. > > > > The problem is simple, but I do not know what should we do, I know nothing > > about CPU isolation. > > > > We can fix __queue_delayed_work(), this is simple, but other callers of > > housekeeping_any_cpu() seem to assume it must always return a valid CPU > > too. So perhaps we should change housekeeping_any_cpu() > > Yeah, patching this up from wq side is easy but housekeeping_any_cpu() > always being able to pick a housekeeping CPU would be better. > > > - return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask); > > + cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask); > > + if (cpu < nr_cpu_ids) > > + return cpu; > > > > ? > > > > But I'm afraid this can hide other problems. May be > > > > - return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask); > > + cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask); > > + if (cpu < nr_cpu_ids) > > + return cpu; > > + > > + WARN_ON(system_state > SYSTEM_BOOTING); > > > > ? > > > > ------------------------------------------------------------------------------- > > OTOH, Documentation/timers/no_hz.rst says > > > > 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. > > > > and this doesn't match the reality. > > Don't some archs allow the boot CPU to go down too tho? If so, this doesn't > really solve the problem, right?
I do not know. But I thought about this too.
In the context of this discussion we do not care if the boot CPU goes down. But we need at least one housekeeping CPU after cpu_down(). The comment in cpu_down_maps_locked() says
Also keep at least one housekeeping cpu onlined
but it checks HK_TYPE_DOMAIN, and I do not know (and it is too late for me to try to read the code ;) if housekeeping.cpumasks[HK_TYPE_TIMER] can get empty or not.
Oleg.
> > So it seems that we should fix housekeeping_setup() ? see the patch below. > > > > In any case the usage of cpu_present_mask doesn't look right to me. > > > > Oleg. > > > > --- a/kernel/sched/isolation.c > > +++ b/kernel/sched/isolation.c > > @@ -129,7 +154,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags) > > cpumask_andnot(housekeeping_staging, > > cpu_possible_mask, non_housekeeping_mask); > > > > - if (!cpumask_intersects(cpu_present_mask, housekeeping_staging)) { > > + if (!cpumask_test_cpu(smp_processor_id(), housekeeping_staging)) { > > __cpumask_set_cpu(smp_processor_id(), housekeeping_staging); > > __cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask); > > if (!housekeeping.flags) { > >
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