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

[Waiman Long <longman@redhat.com>]

On 3/7/23 16:06, Hao Luo wrote:
> On Tue, Mar 7, 2023 at 12:09 PM Waiman Long <longman@redhat.com> wrote:
>> 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?
>>
> I think it's because we were using the legacy hierarchy. I'm not
> familiar with cpuset partition though.

In legacy hierarchy, changing cpuset.cpus shouldn't lead to the calling 
of rebuild_root_domains() unless you play with cpuset.sched_load_balance 
file by changing it to 0 in the right cpusets. If you are touching 
cpuset.sched_load_balance, you shouldn't change cpuset.cpus that often.

Cheers,
Longman