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From: Ionela Voinescu <ionela.voinescu@arm.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [PATCH V3 3/3] arm64: topology: Make AMUs work with modular cpufreq drivers
Date: Fri, 8 Jan 2021 09:44:17 +0000	[thread overview]
Message-ID: <20210108094416.GA19952@arm.com> (raw)
In-Reply-To: <20201217105049.z3aqhl4mo56hhqvk@vireshk-i7>

On Thursday 17 Dec 2020 at 16:20:49 (+0530), Viresh Kumar wrote:
> On 16-12-20, 19:37, Ionela Voinescu wrote:
> > I did not yet test this, but reading this comment made me wonder..
> > 
> > arch_scale_freq_invariant() (or topology_scale_freq_invariant()) is also
> > called from schedutil when obtaining the next frequency.
> > 
> > So if we had a system that only partly supports AMUs but had at some
> > point a cpufreq driver that provided FIE for the other CPUs, when we
> > unregister the driver, the cpufreq_freq_invariance static key is
> > disabled. Therefore, none of the conditions for system invariance is
> > now accomplished and arch_scale_freq_invariant() will return false.
> > This will be broken as utilization is still scaled, but the algorithm
> > for computing the next frequency in schedutil will not take this into
> > account.
> 
> I think the best and the easiest solution for this is:
> 
> bool arch_freq_counters_available(const struct cpumask *cpus)
> {
>         return amu_freq_invariant();
> }
> 
> But we probably need to rename it to something like arch_is_fie().
> 

Now that I think of it again (after spending 30 minutes trying to come
up with a more clear solution) I realised this is not actually a
problem :).

The only location that checks the invariance status is schedutil, but
what a cpufreq governor does becomes irrelevant if you remove the
cpufreq driver. The only potential problem is if one then inmods a
cpufreq driver that's not invariant. But I think that might be on "if"
too many to consider. What do you think?

Thanks,
Ionela.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

WARNING: multiple messages have this Message-ID (diff)
From: Ionela Voinescu <ionela.voinescu@arm.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH V3 3/3] arm64: topology: Make AMUs work with modular cpufreq drivers
Date: Fri, 8 Jan 2021 09:44:17 +0000	[thread overview]
Message-ID: <20210108094416.GA19952@arm.com> (raw)
In-Reply-To: <20201217105049.z3aqhl4mo56hhqvk@vireshk-i7>

On Thursday 17 Dec 2020 at 16:20:49 (+0530), Viresh Kumar wrote:
> On 16-12-20, 19:37, Ionela Voinescu wrote:
> > I did not yet test this, but reading this comment made me wonder..
> > 
> > arch_scale_freq_invariant() (or topology_scale_freq_invariant()) is also
> > called from schedutil when obtaining the next frequency.
> > 
> > So if we had a system that only partly supports AMUs but had at some
> > point a cpufreq driver that provided FIE for the other CPUs, when we
> > unregister the driver, the cpufreq_freq_invariance static key is
> > disabled. Therefore, none of the conditions for system invariance is
> > now accomplished and arch_scale_freq_invariant() will return false.
> > This will be broken as utilization is still scaled, but the algorithm
> > for computing the next frequency in schedutil will not take this into
> > account.
> 
> I think the best and the easiest solution for this is:
> 
> bool arch_freq_counters_available(const struct cpumask *cpus)
> {
>         return amu_freq_invariant();
> }
> 
> But we probably need to rename it to something like arch_is_fie().
> 

Now that I think of it again (after spending 30 minutes trying to come
up with a more clear solution) I realised this is not actually a
problem :).

The only location that checks the invariance status is schedutil, but
what a cpufreq governor does becomes irrelevant if you remove the
cpufreq driver. The only potential problem is if one then inmods a
cpufreq driver that's not invariant. But I think that might be on "if"
too many to consider. What do you think?

Thanks,
Ionela.

  reply	other threads:[~2021-01-08  9:46 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-15  5:34 [PATCH V3 1/3] arm64: topology: Avoid the have_policy check Viresh Kumar
2020-12-15  5:34 ` Viresh Kumar
2020-12-15  5:34 ` [PATCH V3 2/3] arm64: topology: Reorder init_amu_fie() a bit Viresh Kumar
2020-12-15  5:34   ` Viresh Kumar
2020-12-15 11:53   ` Ionela Voinescu
2020-12-15 11:53     ` Ionela Voinescu
2020-12-15  5:34 ` [PATCH V3 3/3] arm64: topology: Make AMUs work with modular cpufreq drivers Viresh Kumar
2020-12-15  5:34   ` Viresh Kumar
2020-12-15 11:56   ` Ionela Voinescu
2020-12-15 11:56     ` Ionela Voinescu
2020-12-16  0:03   ` Ionela Voinescu
2020-12-16  0:03     ` Ionela Voinescu
2020-12-16  4:38     ` Viresh Kumar
2020-12-16  4:38       ` Viresh Kumar
2020-12-16 19:37       ` Ionela Voinescu
2020-12-16 19:37         ` Ionela Voinescu
2020-12-17 10:50         ` Viresh Kumar
2020-12-17 10:50           ` Viresh Kumar
2021-01-08  9:44           ` Ionela Voinescu [this message]
2021-01-08  9:44             ` Ionela Voinescu
2021-01-08 10:42             ` Ionela Voinescu
2021-01-08 10:42               ` Ionela Voinescu
2021-01-08 11:03             ` Viresh Kumar
2021-01-08 11:03               ` Viresh Kumar
2020-12-17  7:57 ` [PATCH V3 1/3] arm64: topology: Avoid the have_policy check Viresh Kumar
2020-12-17  7:57   ` Viresh Kumar
2020-12-17 10:55   ` Catalin Marinas
2020-12-17 10:55     ` Catalin Marinas
2020-12-18  4:26     ` Viresh Kumar
2020-12-18  4:26       ` Viresh Kumar
2020-12-18 11:01       ` Catalin Marinas
2020-12-18 11:01         ` Catalin Marinas
2020-12-18 11:04         ` Viresh Kumar
2020-12-18 11:04           ` Viresh Kumar

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