Messages in this thread | | | Date | Tue, 18 Oct 2022 13:04:40 +0200 | From | Greg Kroah-Hartman <> | Subject | Re: sched/debug: CPU hotplug operation suffers in a large cpu systems |
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On Tue, Oct 18, 2022 at 04:07:06PM +0530, Vishal Chourasia wrote: > On Mon, Oct 17, 2022 at 04:54:11PM +0200, Greg Kroah-Hartman wrote: > > On Mon, Oct 17, 2022 at 04:19:31PM +0200, Peter Zijlstra wrote: > > > > > > +GregKH who actually knows about debugfs. > > > > > > On Mon, Oct 17, 2022 at 06:40:49PM +0530, Vishal Chourasia wrote: > > > > smt=off operation on system with 1920 CPUs is taking approx 59 mins on v5.14 > > > > versus 29 mins on v5.11 measured using: > > > > # time ppc64_cpu --smt=off > > > > > > > > > > > > |--------------------------------+----------------+--------------| > > > > | method | sysctl | debugfs | > > > > |--------------------------------+----------------+--------------| > > > > | unregister_sysctl_table | 0.020050 s | NA | > > > > | build_sched_domains | 3.090563 s | 3.119130 s | > > > > | register_sched_domain_sysctl | 0.065487 s | NA | > > > > | update_sched_domain_debugfs | NA | 2.791232 s | > > > > | partition_sched_domains_locked | 3.195958 s | 5.933254 s | > > > > |--------------------------------+----------------+--------------| > > > > > > > > Note: partition_sched_domains_locked internally calls build_sched_domains > > > > and calls other functions respective to what's being currently used to > > > > export information i.e. sysctl or debugfs > > > > > > > > Above numbers are quoted from the case where we tried offlining 1 cpu in system > > > > with 1920 online cpus. > > > > > > > > From the above table, register_sched_domain_sysctl and > > > > unregister_sysctl_table collectively took ~0.085 secs, whereas > > > > update_sched_domain_debugfs took ~2.79 secs. > > > > > > > > Root cause: > > > > > > > > The observed regression stems from the way these two pseudo-filesystems handle > > > > creation and deletion of files and directories internally. > > > > Yes, debugfs is not optimized for speed or memory usage at all. This > > happens to be the first code path I have seen that cares about this for > > debugfs files. > > > > You can either work on not creating so many debugfs files (do you really > > really need all of them all the time?) Or you can work on moving > > debugfs to use kernfs as the backend logic, which will save you both > > speed and memory usage overall as kernfs is used to being used on > > semi-fast paths. > > > > Maybe do both? > > > > hope this helps, > > > > greg k-h > > Yes, we need to create 7-8 files per domain per CPU, eventually ending up > creating a lot of files.
Why do you need to? What tools require these debugfs files to be present?
And if you only have 7-8 files per CPU, that does not seem like a lot of files overall (14000-16000)? If you only offline 1 cpu, how is removing 7 or 8 files a bottleneck? Do you really offline 1999 cpus for a 2k system?
> Is there a possibility of reverting back to /proc/sys/kernel/sched_domain/?
No, these are debugging-only things, they do not belong in /proc/
If you rely on them for real functionality, that's a different story, but I want to know what tool uses them and for what functionality as debugfs should never be relied on for normal operation of a system.
thanks,
greg k-h
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