Re: [PATCH v2 0/2] Fix DEADLINE bandwidth accounting in root domain changes and hotplug

[Phil Auld <pauld@redhat.com>]

On Thu, Nov 14, 2024 at 04:14:00PM +0000 Juri Lelli wrote:
> Thanks Waiman and Phil for the super quick review/test of this v2!
> 
> On 14/11/24 14:28, Juri Lelli wrote:
> 
> ...
> 
> > In all honesty, I still see intermittent issues that seems to however be
> > related to the dance we do in sched_cpu_deactivate(), where we first
> > turn everything related to a cpu/rq off and revert that if
> > cpuset_cpu_inactive() reveals failing DEADLINE checks. But, since these
> > seem to be orthogonal to the original discussion we started from, I
> > wanted to send this out as an hopefully meaningful update/improvement
> > since yesterday. Will continue looking into this.
> 
> About this that I mentioned, it looks like the below cures it (and
> hopefully doesn't regress wrt the other 2 patches).
> 
> What do everybody think?
>

I think that makes sense.  I think it's better not to have that
deadline call buried the cpuset code as well.


Reviewed-by: Phil Auld <pauld@redhat.com>



> ---
> Subject: [PATCH] sched/deadline: Check bandwidth overflow earlier for hotplug
>
> 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.
> 
> Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
> ---
>  kernel/sched/core.c     |  9 +++++----
>  kernel/sched/deadline.c | 12 ++++++++++--
>  2 files changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d1049e784510..43dfb3968eb8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8057,10 +8057,6 @@ static void cpuset_cpu_active(void)
>  static int 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++;
> @@ -8128,6 +8124,11 @@ 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
> 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;
> 

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