Re: [PATCH 3/3] cpufreq: CPPC: Eliminate the impact of cpc_read() latency error

From: Mark Rutland <mark.rutland@arm.com>
To: Zeng Heng <zengheng4@huawei.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>
Subject: Re: [PATCH 3/3] cpufreq: CPPC: Eliminate the impact of cpc_read() latency error
Date: Thu, 26 Oct 2023 12:26:20 +0100	[thread overview]
Message-ID: <ZTpM3OXZ8C1OrBIQ@FVFF77S0Q05N> (raw)
In-Reply-To: <abb15757-cbe6-037f-e8d3-5df9fbbf6c04@huawei.com>

On Thu, Oct 26, 2023 at 09:55:39AM +0800, Zeng Heng wrote:
> 
> 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.

Ah, sorry -- I had confused this with the smp_call_function*() family, which do
this in IRQ context.

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

Understood; given that, please ignore my comments below.

Mark.

> 
> 
> 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
> > > 

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