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Message-ID: <abb15757-cbe6-037f-e8d3-5df9fbbf6c04@huawei.com>
Date:   Thu, 26 Oct 2023 09:55:39 +0800
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Subject: Re: [PATCH 3/3] cpufreq: CPPC: Eliminate the impact of cpc_read()
 latency error
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To:     Mark Rutland <mark.rutland@arm.com>
CC:     <broonie@kernel.org>, <joey.gouly@arm.com>, <will@kernel.org>,
        <amit.kachhap@arm.com>, <rafael@kernel.org>,
        <catalin.marinas@arm.com>, <james.morse@arm.com>, <maz@kernel.org>,
        <viresh.kumar@linaro.org>, <sumitg@nvidia.com>,
        <yang@os.amperecomputing.com>, <linux-kernel@vger.kernel.org>,
        <linux-pm@vger.kernel.org>, <linux-arm-kernel@lists.infradead.org>,
        <wangxiongfeng2@huawei.com>, <xiexiuqi@huawei.com>,
        Ionela Voinescu <ionela.voinescu@arm.com>
References: <20231025093847.3740104-1-zengheng4@huawei.com>
 <20231025093847.3740104-4-zengheng4@huawei.com>
 <ZTj1fMsMj-Mekfn3@FVFF77S0Q05N>
From:   Zeng Heng <zengheng4@huawei.com>
In-Reply-To: <ZTj1fMsMj-Mekfn3@FVFF77S0Q05N>
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在 2023/10/25 19:01, Mark Rutland 写道:
> On Wed, Oct 25, 2023 at 05:38:47PM +0800, Zeng Heng wrote:
>
> The previous patch added this function, and calls it with smp_call_on_cpu(),
> where it'll run in IRQ context with IRQs disabled...

smp_call_on_cpu() puts the work to the bind-cpu worker.

And this function will be called in task context, and IRQs is certainly enabled.


Zeng Heng

>>   	struct fb_ctr_pair *fb_ctrs = val;
>>   	int cpu = fb_ctrs->cpu;
>>   	int ret;
>> +	unsigned long timeout;
>>   
>>   	ret = cppc_get_perf_ctrs(cpu, &fb_ctrs->fb_ctrs_t0);
>>   	if (ret)
>>   		return ret;
>>   
>> -	udelay(2); /* 2usec delay between sampling */
>> +	if (likely(!irqs_disabled())) {
>> +		/*
>> +		 * Set 1ms as sampling interval, but never schedule
>> +		 * to the idle task to prevent the AMU counters from
>> +		 * stopping working.
>> +		 */
>> +		timeout = jiffies + msecs_to_jiffies(1);
>> +		while (!time_after(jiffies, timeout))
>> +			cond_resched();
>> +
>> +	} else {
> ... so we'll enter this branch of the if-else ...
>
>> +		pr_warn_once("CPU%d: Get rate in atomic context", cpu);
> ... and pr_warn_once() for something that's apparently normal and outside of
> the user's control?
>
> That doesn't make much sense to me.
>
> Mark.
>
>> +		udelay(2); /* 2usec delay between sampling */
>> +	}
>>   
>>   	return cppc_get_perf_ctrs(cpu, &fb_ctrs->fb_ctrs_t1);
>>   }
>> -- 
>> 2.25.1
>>