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

[Waiman Long <longman@redhat.com>]

On 3/7/23 14:56, Hao Luo wrote:
> On Mon, Feb 6, 2023 at 2:15 PM Qais Yousef <qyousef@layalina.io> 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@google.com>
>> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
>> ---
> 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?

Cheers,
Longman