Re: [PATCH v9 3/8] cpuset: Rebuild root domain deadline accounting information

[Waiman Long <longman@redhat.com>]

On 12/16/22 18:35, Qais Yousef wrote:
> Hi
>
> On 07/19/19 15:59, Juri Lelli wrote:
>> When the topology of root domains is modified by CPUset or CPUhotplug
>> operations information about the current deadline bandwidth held in the
>> root domain is lost.
>>
>> This patch addresses the issue by recalculating the lost deadline
>> bandwidth information by circling through the deadline tasks held in
>> CPUsets and adding their current load to the root domain they are
>> associated with.
>>
>> Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
>> Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
>> ---
> We see that rebuild_root_domain() can take 10+ ms (I get a max of 20ms quite
> consistently) on suspend/resume.
>
> Do we actually need to rebuild_root_domain() if we're going through
> a suspend/resume cycle?
>
> ie: would something like the below make sense? We'd skip this logic if
> cpuhp_tasks_frozen is set which indicates it's not a real hotplug operation but
> we're suspending/resuming.
>
>
> Cheers
>
> --
> Qais Yousef
>
>
> --->8---
>
>
>  From 4cfd50960ad872c5eb810ad3038eaf840bab5182 Mon Sep 17 00:00:00 2001
> From: Qais Yousef <qyousef@layalina.io>
> Date: Tue, 29 Nov 2022 19:01:52 +0000
> Subject: [PATCH] sched: cpuset: Don't rebuild sched domains on suspend-resume
>
> Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
> enabled rebuilding sched domain on cpuset and hotplug operations to
> correct deadline accounting.
>
> Rebuilding sched domain is a slow operation and we see 10+ ms delays
> on suspend-resume because of that.
>
> Since nothing is expected to change on suspend-resume operation; skip
> rebuilding the sched domains to regain some of the time lost.
>
> Debugged-by: Rick Yiu <rickyiu@google.com>
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
>   kernel/cgroup/cpuset.c  | 6 ++++++
>   kernel/sched/deadline.c | 3 +++
>   2 files changed, 9 insertions(+)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index b474289c15b8..2ff68d625b7b 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1067,6 +1067,9 @@ static void update_tasks_root_domain(struct cpuset *cs)
>   	struct css_task_iter it;
>   	struct task_struct *task;
>   
> +	if (cpuhp_tasks_frozen)
> +		return;
> +
>   	css_task_iter_start(&cs->css, 0, &it);
>   
>   	while ((task = css_task_iter_next(&it)))
> @@ -1084,6 +1087,9 @@ static void rebuild_root_domains(void)
>   	lockdep_assert_cpus_held();
>   	lockdep_assert_held(&sched_domains_mutex);
>   
> +	if (cpuhp_tasks_frozen)
> +		return;
> +
>   	rcu_read_lock();
>   
>   	/*

rebuild_root_domains() is the only caller of update_tasks_root_domain(). 
So the first hunk is redundant as update_tasks_root_domain() won't be 
called when cpuhp_tasks_frozen is set.

Cheers,
Longman