linux-kernel - Re: [PATCH v2 6/10] cpufreq: Support for fast frequency switching

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAJZ5v0hnnANO6-CCgaika+FZtoXGf5V1Z8Wmmv+niZx=h7obWQ@mail.gmail.com>
Date:	Fri, 4 Mar 2016 23:58:33 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Juri Lelli <juri.lelli@....com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Michael Turquette <mturquette@...libre.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v2 6/10] cpufreq: Support for fast frequency switching

On Fri, Mar 4, 2016 at 11:32 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> On Fri, Mar 4, 2016 at 11:18 PM, Steve Muckle <steve.muckle@...aro.org> wrote:
>> On 03/03/2016 07:07 PM, Rafael J. Wysocki wrote:
>>> +void cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
>>> +                             unsigned int target_freq, unsigned int relation)
>>> +{
>>> +     unsigned int freq;
>>> +
>>> +     freq = cpufreq_driver->fast_switch(policy, target_freq, relation);
>>> +     if (freq != CPUFREQ_ENTRY_INVALID) {
>>> +             policy->cur = freq;
>>> +             trace_cpu_frequency(freq, smp_processor_id());
>>> +     }
>>> +}
>>
>> Even if there are platforms which may change the CPU frequency behind
>> cpufreq's back, breaking the transition notifiers, I'm worried about the
>> addition of an interface which itself breaks them. The platforms which
>> do change CPU frequency on their own have probably evolved to live with
>> or work around this behavior. As other platforms migrate to fast
>> frequency switching they might be surprised when things don't work as
>> advertised.
>
> Well, intel_pstate doesn't do notifies at all, so anything depending
> on them is already broken when it is used.  Let alone the hardware
> P-states coordination mechanism (HWP) where the frequency is
> controlled by the processor itself entirely.
>
> That said I see your point.
>
>> I'm not sure what the easiest way to deal with this is. I see the
>> transition notifiers are the srcu type, which I understand to be
>> blocking. Going through the tree and reworking everyone's callbacks and
>> changing the type to atomic is obviously not realistic.
>
> Right.
>
>> How about modifying cpufreq_register_notifier to return an error if the
>> driver has a fast_switch callback installed and an attempt to register a
>> transition notifier is made?
>
> That sounds like a good idea.

Transition notifiers may be registered before the driver is
registered, so that won't help in all cases.

Thanks,
Rafael