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From: Mauro <registosites@hotmail.com>
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To: Viresh Kumar <viresh.kumar@linaro.org>,
        Peter Zijlstra <peterz@infradead.org>
CC: Arjan van de Ven <arjan@linux.intel.com>, Len Brown <lenb@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Eliezer Tamir <eliezer.tamir@linux.intel.com>,
        Zhang Rui <rui.zhang@intel.com>, jacob.jun.pan@linux.intel.com,
        Mike Galbraith <bitbucket@online.de>, Ingo Molnar <mingo@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        John Stultz <john.stultz@linaro.org>,
        Andy Lutomirski <luto@amacapital.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Srivatsa S. Bhat" <srivatsa@mit.edu>
Subject: Re: [PATCH 11/15] x86: Rewrite cyc2ns to avoid the need to disable
 IRQs
References: <20131212140835.729222186@infradead.org>	<20131212141655.238445339@infradead.org>	<CAOh2x=mgeozUMGYQubQ4GKPEB6k_TkcV3CSTzNLqjKok-gROQg@mail.gmail.com>	<20140617121535.GY6758@twins.programming.kicks-ass.net> <CAKohpo=6t-VHgJfrq7wz35y_zKfL+wb8amgj0ZV9cezb39r=uA@mail.gmail.com>
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On 17-06-2014 13:42, Viresh Kumar wrote:
>> So what you can try is force a cyc2ns read before the write in
>> set_cyc2ns_scale(). I think its possible that if we do not do the read,
>> the write will wait for a 'free' slot indefinitely.
> 
> Hmm, I see. Thanks for your quick response.
> 
> Are you suggesting something like this ? :
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 57e5ce1..290ac03 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -249,6 +249,9 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
>         if (!cpu_khz)
>                 goto done;
> 
> +       data = cyc2ns_read_begin();
> +       cyc2ns_read_end(data);
> +
>         data = cyc2ns_write_begin(cpu);
> 
>         rdtscll(tsc_now);
> 
> 
> @Mauro: Please try this once Peter confirms.

I have tried this and it doesn't solve the problem, the machine still
hangs after the second frequency change after cpu offline/online.