Messages in this thread | | | Date | Thu, 18 Apr 2024 10:54:40 -0400 | From | Phil Auld <> | Subject | Re: [PATCH] sched/isolation: fix boot crash when maxcpus < first-housekeeping-cpu |
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On Sat, Apr 13, 2024 at 04:17:46PM +0200 Oleg Nesterov wrote: > housekeeping_setup() checks cpumask_intersects(present, online) to ensure > that the kernel will have at least one housekeeping CPU after smp_init(), > but this doesn't work if the maxcpus= kernel parameter limits the number > of processors available after bootup. > > For example, the kernel with "maxcpus=2 nohz_full=0-2" parameters crashes > at boot time on my virtual machine with 4 CPUs. > > Change housekeeping_setup() to use cpumask_first_and() and check that the > returned cpu number is valid and less than setup_max_cpus. > > Another corner case is "nohz_full=0" on a machine with a single CPU or > with the maxcpus=1 kernel argument. In this case non_housekeeping_mask > is empty and IIUC tick_nohz_full_setup() makes no sense. And indeed, the > kernel hits the WARN_ON(tick_nohz_full_running) in tick_sched_do_timer(). > > And how should the kernel interpret the "nohz_full=" parameter? I think > it should be silently ignored, but currently cpulist_parse() happily > returns the empty cpumask and this leads to the same problem. > > Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask) > and do nothing in this case. > > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Looks good to me. One less footgun.
Reviewed-by: Phil Auld <pauld@redhat.com>
> --- > kernel/sched/isolation.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c > index 2a262d3ecb3d..5891e715f00d 100644 > --- a/kernel/sched/isolation.c > +++ b/kernel/sched/isolation.c > @@ -118,6 +118,7 @@ static void __init housekeeping_setup_type(enum hk_type type, > static int __init housekeeping_setup(char *str, unsigned long flags) > { > cpumask_var_t non_housekeeping_mask, housekeeping_staging; > + unsigned int first_cpu; > int err = 0; > > if ((flags & HK_FLAG_TICK) && !(housekeeping.flags & HK_FLAG_TICK)) { > @@ -138,7 +139,8 @@ 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)) { > + first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging); > + if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) { > __cpumask_set_cpu(smp_processor_id(), housekeeping_staging); > __cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask); > if (!housekeeping.flags) { > @@ -147,6 +149,9 @@ static int __init housekeeping_setup(char *str, unsigned long flags) > } > } > > + if (cpumask_empty(non_housekeeping_mask)) > + goto free_housekeeping_staging; > + > if (!housekeeping.flags) { > /* First setup call ("nohz_full=" or "isolcpus=") */ > enum hk_type type; > -- > 2.25.1.362.g51ebf55 > > >
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