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Message-ID: <20240418145440.GD29188@lorien.usersys.redhat.com>
Date: Thu, 18 Apr 2024 10:54:40 -0400
From: Phil Auld <pauld@...hat.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Frederic Weisbecker <frederic@...nel.org>,
Nicholas Piggin <npiggin@...il.com>,
Leonardo Bras <leobras@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
linux-kernel@...r.kernel.org, Junyao Zhao <junzhao@...hat.com>,
Chris von Recklinghausen <crecklin@...hat.com>
Subject: Re: [PATCH] sched/isolation: fix boot crash when maxcpus <
first-housekeeping-cpu
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@...hat.com>
Looks good to me. One less footgun.
Reviewed-by: Phil Auld <pauld@...hat.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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