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From: Ionela Voinescu <ionela.voinescu@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: mark.rutland@arm.com, linux-kernel@vger.kernel.org,
	sudeep.holla@arm.com, will@kernel.org, morten.rasmussen@arm.com,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 3/3] arm64: implement CPPC FFH support using AMUs
Date: Fri, 13 Nov 2020 12:28:46 +0000	[thread overview]
Message-ID: <20201113122846.GA31212@arm.com> (raw)
In-Reply-To: <20201112180045.GO29613@gaia>

Hi,

On Thursday 12 Nov 2020 at 18:00:46 (+0000), Catalin Marinas wrote:
> On Fri, Nov 06, 2020 at 12:53:34PM +0000, Ionela Voinescu wrote:
> > +static inline
> > +int counters_read_on_cpu(int cpu, smp_call_func_t func, u64 *val)
> > +{
> > +	if (!cpu_has_amu_feat(cpu))
> > +		return -EOPNOTSUPP;
> > +
> > +	smp_call_function_single(cpu, func, val, 1);
> > +
> > +	return 0;
> > +}
> 
> I got lost in the cpufreq call chains. Can this function ever be called
> with interrupts disabled?
> 

The short answer is: not with the current implementation of its only
user, the cppc_cpufreq driver (given the current cpufreq implementation).

The long answer is: there is a case where the cpufreq .get function is
called with local interrupts disabled - cpufreq_quick_get(), but there
are a few "if"s in between this becoming a problem:

 1. If cppc_cpufreq ever implements .setpolicy or,
    1.1 Current implementation of cpufreq_quick_get() changes.
 2. If one of the few users of cpufreq_quick_get() is used: cppc_cpufreq
    ends up being used as CPU cooling device(+IPA governor) or
    devfreq/tegra30 is used.

 In this potential case, smp_call_function_single() will warn us of call
 with irqs_disable() and if the stars align there could be a potential
 deadlock if two CPUs try to IPI (get counter reads of) each other.

So it could be called with irqs disabled, but a few code changes are
needed before that happens.

I can bail out of counters_read_on_cpu() if irqs_disabled() to be on the
safe side.

Thanks for the review,
Ionela.


> -- 
> Catalin

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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: Catalin Marinas <catalin.marinas@arm.com>
Cc: mark.rutland@arm.com, sudeep.holla@arm.com, will@kernel.org,
	morten.rasmussen@arm.com, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 3/3] arm64: implement CPPC FFH support using AMUs
Date: Fri, 13 Nov 2020 12:28:46 +0000	[thread overview]
Message-ID: <20201113122846.GA31212@arm.com> (raw)
In-Reply-To: <20201112180045.GO29613@gaia>

Hi,

On Thursday 12 Nov 2020 at 18:00:46 (+0000), Catalin Marinas wrote:
> On Fri, Nov 06, 2020 at 12:53:34PM +0000, Ionela Voinescu wrote:
> > +static inline
> > +int counters_read_on_cpu(int cpu, smp_call_func_t func, u64 *val)
> > +{
> > +	if (!cpu_has_amu_feat(cpu))
> > +		return -EOPNOTSUPP;
> > +
> > +	smp_call_function_single(cpu, func, val, 1);
> > +
> > +	return 0;
> > +}
> 
> I got lost in the cpufreq call chains. Can this function ever be called
> with interrupts disabled?
> 

The short answer is: not with the current implementation of its only
user, the cppc_cpufreq driver (given the current cpufreq implementation).

The long answer is: there is a case where the cpufreq .get function is
called with local interrupts disabled - cpufreq_quick_get(), but there
are a few "if"s in between this becoming a problem:

 1. If cppc_cpufreq ever implements .setpolicy or,
    1.1 Current implementation of cpufreq_quick_get() changes.
 2. If one of the few users of cpufreq_quick_get() is used: cppc_cpufreq
    ends up being used as CPU cooling device(+IPA governor) or
    devfreq/tegra30 is used.

 In this potential case, smp_call_function_single() will warn us of call
 with irqs_disable() and if the stars align there could be a potential
 deadlock if two CPUs try to IPI (get counter reads of) each other.

So it could be called with irqs disabled, but a few code changes are
needed before that happens.

I can bail out of counters_read_on_cpu() if irqs_disabled() to be on the
safe side.

Thanks for the review,
Ionela.


> -- 
> Catalin

  reply	other threads:[~2020-11-13 12:30 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-06 12:53 [PATCH v4 0/3] arm64: cppc: add FFH support using AMUs Ionela Voinescu
2020-11-06 12:53 ` Ionela Voinescu
2020-11-06 12:53 ` [PATCH v4 1/3] arm64: wrap and generalise counter read functions Ionela Voinescu
2020-11-06 12:53   ` Ionela Voinescu
2020-11-13 14:11   ` Sudeep Holla
2020-11-13 14:11     ` Sudeep Holla
2020-11-06 12:53 ` [PATCH v4 2/3] arm64: split counter validation function Ionela Voinescu
2020-11-06 12:53   ` Ionela Voinescu
2020-11-13 14:12   ` Sudeep Holla
2020-11-13 14:12     ` Sudeep Holla
2020-11-06 12:53 ` [PATCH v4 3/3] arm64: implement CPPC FFH support using AMUs Ionela Voinescu
2020-11-06 12:53   ` Ionela Voinescu
2020-11-12 18:00   ` Catalin Marinas
2020-11-12 18:00     ` Catalin Marinas
2020-11-13 12:28     ` Ionela Voinescu [this message]
2020-11-13 12:28       ` Ionela Voinescu
2020-11-13 12:54       ` Catalin Marinas
2020-11-13 12:54         ` Catalin Marinas
2020-11-13 14:16   ` Sudeep Holla
2020-11-13 14:16     ` Sudeep Holla
2020-11-13 16:37     ` Ionela Voinescu
2020-11-13 16:37       ` Ionela Voinescu
2020-11-13 20:03       ` Catalin Marinas
2020-11-13 20:03         ` Catalin Marinas
2020-11-13 15:53 ` [PATCH] arm64: abort counter_read_on_cpu() when irqs_disabled() Ionela Voinescu
2020-11-13 15:53   ` Ionela Voinescu
2020-11-13 16:02   ` Mark Rutland
2020-11-13 16:02     ` Mark Rutland
2020-11-13 16:58     ` Ionela Voinescu
2020-11-13 16:58       ` Ionela Voinescu
2020-11-13 17:30       ` Mark Rutland
2020-11-13 17:30         ` Mark Rutland
2020-11-13 19:55       ` Catalin Marinas
2020-11-13 19:55         ` Catalin Marinas
2020-11-13 20:26 ` [PATCH v4 0/3] arm64: cppc: add FFH support using AMUs Catalin Marinas
2020-11-13 20:26   ` Catalin Marinas

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