Re: [PATCH v3] sched: cpuset: Don't rebuild root domains on suspend-resume

[Hao Luo]

On Mon, Feb 6, 2023 at 2:15 PM Qais Yousef <qyousef@xxxxxxxxxxx> wrote:
>
> Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
> enabled rebuilding root domain on cpuset and hotplug operations to
> correct deadline accounting.
>
> Rebuilding root domain is a slow operation and we see 10+ of ms delays
> on suspend-resume because of that (worst case captures 20ms which
> happens often).
>
> Since nothing is expected to change on suspend-resume operation; skip
> rebuilding the root domains to regain the some of the time lost.
>
> Achieve this by refactoring the code to pass whether dl accoutning needs
> an update to rebuild_sched_domains(). And while at it, rename
> rebuild_root_domains() to update_dl_rd_accounting() which I believe is
> a more representative name since we are not really rebuilding the root
> domains, but rather updating dl accounting at the root domain.
>
> Some users of rebuild_sched_domains() will skip dl accounting update
> now:
>
>         * Update sched domains when relaxing the domain level in cpuset
>           which only impacts searching level in load balance
>         * update sched domains when cpufreq governor changes and we need
>           to create the perf domains
>
> Users in arch/x86 and arch/s390 are left with the old behavior.
>
> Debugged-by: Rick Yiu <rickyiu@xxxxxxxxxx>
> Signed-off-by: Qais Yousef (Google) <qyousef@xxxxxxxxxxx>
> ---

Hi Qais,

Thank you for reporting this. We observed the same issue in our
production environment. Rebuild_root_domains() is also called under
cpuset_write_resmask, which handles writing to cpuset.cpus. Under
production workloads, on a 4.15 kernel, we observed the median latency
of writing cpuset.cpus at 3ms, p99 at 7ms. Now the median becomes
60ms, p99 at >100ms. Writing cpuset.cpus is a fairly frequent and
critical path in production, but blindly traversing every task in the
system is not scalable. And its cost is really unnecessary for users
who don't use deadline tasks at all.

>
> The better solution that was discussed before is to not iterate through every
> task in the system and let cpuset track when dl tasks are added to it and do
> smarter iteration. ATM even if there are no dl tasks in the system we'll
> blindly go through every task in the hierarchy to update nothing.
>

Great idea. This works. I tried this solution. With that, 98% of
cpuset.cpu writes are now within 7ms. Though there are still long
tails, they are caused by another issue, irrelevant to
rebuild_root_domains().

Your suggestion of update_dl_rd_accounting() also makes sense to me.

Hao