Re: CPU excessively long times between frequency scaling driver calls - bisected

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Feb 24, 2022 at 08:29:22AM -0800, Doug Smythies wrote:
> 
> On 2022.02.24 04:08:30 Paul E. McKenney wrote:
> > On Thu, Feb 24, 2022 at 04:08:30PM +0800, Feng Tang wrote:
> >> On Wed, Feb 23, 2022 at 03:23:20PM +0100, Rafael J. Wysocki wrote:
> >>> On Wed, Feb 23, 2022 at 1:40 AM Feng Tang <feng.tang@intel.com> wrote:
> >>> 
> >>> But this is not related to idle as such, but to the fact that idle
> >>> sometimes stops the scheduler tick which otherwise would run the
> >>> cpufreq governor callback on a regular basis.
> >>> 
> >>> It is stopping the tick that gets us into trouble, so I would avoid
> >>> doing it if the current performance state is too aggressive.
> >> 
> >> I've tried to simulate Doug's environment by using his kconfig, and
> >> offline my 36 CPUs Desktop to leave 12 CPUs online, and on it I can
> >> still see Local timer interrupts when there is no active load, with
> >> the longest interval between 2 timer interrupts is 4 seconds, while
> >> idle class's task_tick_idle() will do nothing, and CFS'
> >> task_tick_fair() will in turn call cfs_rq_util_change()
> >
> > Every four seconds?  Could you please post your .config?
> >
> >							Thanx, Paul
> 
> I steal the kernel config from the Ubuntu mainline PPA.
> See also earlier on this thread:
> 
> https://lore.kernel.org/linux-pm/CAAYoRsXkyWf0vmEE2HvjF6pzCC4utxTF=7AFx1PJv4Evh=C+Ow@mail.gmail.com/
> 
> but relevant part copied here:
> 
> > I steal the kernel configuration file from the Ubuntu mainline PPA
> > [1], what they call "lowlatency", or 1000Hz tick. I make these
> > changes before compile:
> >
> > scripts/config --disable DEBUG_INFO
> > scripts/config --disable SYSTEM_TRUSTED_KEYS
> > scripts/config --disable SYSTEM_REVOCATION_KEYS
> >
> > I [will] also send you the config and dmesg files in an off-list email.
> >
> > [1] https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.17-rc3/
> 
> I put the same one I sent Feng on my web site where I was
> sharing stuff with Srinivas (coded to avoid the barrage of bots):
> 
> double u double u double u dot smythies dot com/~doug/linux/s18/hwp/srinivas/long_dur/

Thank you!

I don't see CONFIG_FAST_NO_HZ=y in your .config, so that is not the
reason for your every-four-second timers.  ;-)

(CONFIG_FAST_NO_HZ is being removed, FYI.)

							Thanx, Paul