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

[Hao Luo]

On Tue, Mar 7, 2023 at 12:09 PM Waiman Long <longman@xxxxxxxxxx> wrote:
>
> On 3/7/23 14:56, Hao Luo wrote:
> > 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 rebuild_root_domains() function shouldn't be called when updating
> cpuset.cpus unless it is a partition root. Is it?
>

I think it's because we were using the legacy hierarchy. I'm not
familiar with cpuset partition though.

Hao