linux-kernel - Re: [PATCH] cpufreq: schedutil: Don't skip freq update when limits change

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Message-ID: <20190806040059.ty4qhjarx4obomhs@vireshk-i7>
Date:   Tue, 6 Aug 2019 09:30:59 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Doug Smythies <dsmythies@...us.net>,
        "'Rafael J. Wysocki'" <rafael@...nel.org>,
        'Ingo Molnar' <mingo@...hat.com>,
        'Peter Zijlstra' <peterz@...radead.org>,
        'Linux PM' <linux-pm@...r.kernel.org>,
        'Vincent Guittot' <vincent.guittot@...aro.org>,
        'Joel Fernandes' <joel@...lfernandes.org>,
        "'v4 . 18+'" <stable@...r.kernel.org>,
        'Linux Kernel Mailing List' <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: schedutil: Don't skip freq update when limits
 change

On 02-08-19, 11:11, Rafael J. Wysocki wrote:
> On Friday, August 2, 2019 5:48:19 AM CEST Viresh Kumar wrote:
> > On 01-08-19, 10:57, Doug Smythies wrote:
> > > Disagree.
> > > All I did was use a flag where it used to be set to UNIT_MAX, to basically
> > > implement the same thing.
> > 
> > And the earlier code wasn't fully correct as well, that's why we tried
> > to fix it earlier.
> 
> Your argument seems to be "There was an earlier problem related to this, which
> was fixed, so it is fragile and I'd rather avoid it".  Still, you are claiming that the
> code was in fact incorrect and you are not giving convincing arguments to
> support that.
> 
> > So introducing the UINT_MAX thing again would be
> > wrong, even if it fixes the problem for you.
> 
> Would it be wrong, because it would reintroduce the fragile code, or would it
> be wrong, because it would re-introduce a bug?  What bug if so?

There will be two issues here if that patch is reintroduced:

- It will cause the BUG to reappear, which was fixed by the earlier
  commit. The commit log of ecd28842912 explains the bug in detail.

- And overriding next_freq as a flag will make the code fragile and we
  may have similar bugs coming up.

But yeah, lets continue discussion on the intel-pstate patch now.

-- 
viresh