Re: [PATCH v2 3/2] sched/deadline: Check bandwidth overflow earlier for hotplug

Jon Hunter
2025-01-10T11:52:49Z

Hi Juri,

On 15/11/2024 11:48, Juri Lelli wrote:
> Currently we check for bandwidth overflow potentially due to hotplug
> operations at the end of sched_cpu_deactivate(), after the cpu going
> offline has already been removed from scheduling, active_mask, etc.
> This can create issues for DEADLINE tasks, as there is a substantial
> race window between the start of sched_cpu_deactivate() and the moment
> we possibly decide to roll-back the operation if dl_bw_deactivate()
> returns failure in cpuset_cpu_inactive(). An example is a throttled
> task that sees its replenishment timer firing while the cpu it was
> previously running on is considered offline, but before
> dl_bw_deactivate() had a chance to say no and roll-back happened.
>
> Fix this by directly calling dl_bw_deactivate() first thing in
> sched_cpu_deactivate() and do the required calculation in the former
> function considering the cpu passed as an argument as offline already.
>
> By doing so we also simplify sched_cpu_deactivate(), as there is no need
> anymore for any kind of roll-back if we fail early.
>
> Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
> ---
> Thanks Waiman and Phil for testing and reviewing the scratch version of
> this change. I think the below might be better, as we end up with a
> clean-up as well.
>
> Please take another look when you/others have time.
> ---
> kernel/sched/core.c | 22 +++++++---------------
> kernel/sched/deadline.c | 12 ++++++++++--
> 2 files changed, 17 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d1049e784510..e2c6eacf793e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8054,19 +8054,14 @@ static void cpuset_cpu_active(void)
> cpuset_update_active_cpus();
> }
>
> -static int cpuset_cpu_inactive(unsigned int cpu)
> +static void cpuset_cpu_inactive(unsigned int cpu)
> {
> if (!cpuhp_tasks_frozen) {
> - int ret = dl_bw_deactivate(cpu);
> -
> - if (ret)
> - return ret;
> cpuset_update_active_cpus();
> } else {
> num_cpus_frozen++;
> partition_sched_domains(1, NULL, NULL);
> }
> - return 0;
> }
>
> static inline void sched_smt_present_inc(int cpu)
> @@ -8128,6 +8123,11 @@ int sched_cpu_deactivate(unsigned int cpu)
> struct rq *rq = cpu_rq(cpu);
> int ret;
>
> + ret = dl_bw_deactivate(cpu);
> +
> + if (ret)
> + return ret;
> +
> /*
> * Remove CPU from nohz.idle_cpus_mask to prevent participating in
> * load balancing when not active
> @@ -8173,15 +8173,7 @@ int sched_cpu_deactivate(unsigned int cpu)
> return 0;
>
> sched_update_numa(cpu, false);
> - ret = cpuset_cpu_inactive(cpu);
> - if (ret) {
> - sched_smt_present_inc(cpu);
> - sched_set_rq_online(rq, cpu);
> - balance_push_set(cpu, false);
> - set_cpu_active(cpu, true);
> - sched_update_numa(cpu, true);
> - return ret;
> - }
> + cpuset_cpu_inactive(cpu);
> sched_domains_numa_masks_clear(cpu);
> return 0;
> }
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 267ea8bacaf6..6e988d4cd787 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -3505,6 +3505,13 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
> }
> break;
> case dl_bw_req_deactivate:
> + /*
> + * cpu is not off yet, but we need to do the math by
> + * considering it off already (i.e., what would happen if we
> + * turn cpu off?).
> + */
> + cap -= arch_scale_cpu_capacity(cpu);
> +
> /*
> * cpu is going offline and NORMAL tasks will be moved away
> * from it. We can thus discount dl_server bandwidth
> @@ -3522,9 +3529,10 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
> if (dl_b->total_bw - fair_server_bw > 0) {
> /*
> * Leaving at least one CPU for DEADLINE tasks seems a
> - * wise thing to do.
> + * wise thing to do. As said above, cpu is not offline
> + * yet, so account for that.
> */
> - if (dl_bw_cpus(cpu))
> + if (dl_bw_cpus(cpu) - 1)
> overflow = __dl_overflow(dl_b, cap, fair_server_bw, 0);
> else
> overflow = 1;

I have noticed a suspend regression on one of our Tegra boards and
bisect is pointing to this commit. If I revert this on top of -next then
I don't see the issue.

The only messages I see when suspend fails are ...

[ 53.905976] Error taking CPU1 down: -16
[ 53.909887] Non-boot CPUs are not disabled

So far this is only happening on Tegra186 (ARM64). Let me know if you
have any thoughts.

Thanks
Jon

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