linux-kernel - Re: [PATCH] cpufreq: conservative: fix requested

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]

Date:	Fri, 8 Nov 2013 19:43:28 +0200
From:	Stratos Karafotis <skarafotis@...il.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Xiaoguang Chen <chenxg@...vell.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xiaoguang Chen <chenxg.marvell@...il.com>
Subject: Re: [PATCH] cpufreq: conservative: fix requested_freq reduction issue

On Fri, Nov 8, 2013 at 6:55 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 8 November 2013 00:36, Stratos Karafotis <skarafotis@...il.com> wrote:
>> I think the existing code already checks if the requested_freq is greater
>> than policy->max in __cpufreq_driver_target.
>
> Yes it does. But the problem is:
> - cs_check_cpu() sets requested_freq above policy->max
> - We execute following code because (requested_freq != policy->max)
>
>     dbs_info->requested_freq += get_freq_target(cs_tuners, policy);
>     __cpufreq_driver_target(policy, dbs_info->requested_freq,
> CPUFREQ_RELATION_H);
> - In __cpufreq_driver_target(), we don't do anything and return early..
> - Above will keep on repeating all the time..
>
> If we change the code as I have suggested it to be:
> - After first loop where requested_freq went over policy->max, we will
> return early from cs_check_cpu(), but we have already set freq to max..
>
>> If we put this check earlier, cpufreq will never reach policy->max.
>
> Can you please explain why do you see that happening?

Please let me rephrase my previous post. In some circumstances (depending
on freq_step and freq_table values) CPU frequency will never reach to
policy->max.

For example suppose that (for simplicity values in MHz):
policy->max = 1000
policy->cur = 800
requested_freq = 800
freq_target = 300

In 'first' iteration, if we return early with this code (because
requested_freq will be
1100):
if (dbs_info->requested_freq >= policy->max)
     return;

CPU freq will never go over 800MHz.

I think the current code works correctly.
- The requested freq will go to 1100 in first iteration.
- __cpufreq_driver_target will change CPU freq to 1000
- dbs_cpufreq_notifier will adjust the requested_freq to 1000

In next iteration the code:
if (dbs_info->requested_freq == policy->max)
        return;

will keep the freq to max and break out early.

So, I think there is no need for an extra check because of
dbs_cpufreq_notifier code.


Thanks,
Stratos Karafotis
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/