linux-kernel - Re: [PATCH 2/3] x86, perf: Add a separate Arch Perfmon v4 PMI handler

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Date:   Mon, 6 Aug 2018 20:35:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     tglx@...utronix.de, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, eranian@...gle.com,
        ak@...ux.intel.com, alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH 2/3] x86, perf: Add a separate Arch Perfmon v4 PMI handler

On Mon, Aug 06, 2018 at 10:23:42AM -0700, kan.liang@...ux.intel.com wrote:
> @@ -2044,6 +2056,14 @@ static void intel_pmu_disable_event(struct perf_event *event)
>  	if (unlikely(event->attr.precise_ip))
>  		intel_pmu_pebs_disable(event);
>  
> +	/*
> +	 * We could disable freezing here, but doesn't hurt if it's on.
> +	 * perf remembers the state, and someone else will likely
> +	 * reinitialize.
> +	 *
> +	 * This avoids an extra MSR write in many situations.
> +	 */
> +
>  	if (unlikely(hwc->config_base == MSR_ARCH_PERFMON_FIXED_CTR_CTRL)) {
>  		intel_pmu_disable_fixed(hwc);
>  		return;
> @@ -2119,6 +2139,11 @@ static void intel_pmu_enable_event(struct perf_event *event)
>  	if (event->attr.exclude_guest)
>  		cpuc->intel_ctrl_host_mask |= (1ull << hwc->idx);
>  
> +	if (x86_pmu.counter_freezing && !cpuc->frozen_enabled) {
> +		enable_counter_freeze();
> +		cpuc->frozen_enabled = 1;
> +	}
> +
>  	if (unlikely(event_is_checkpointed(event)))
>  		cpuc->intel_cp_status |= (1ull << hwc->idx);
>  

Why here? That doesn't really make sense; should this not be in
intel_pmu_cpu_starting() or something?

> +static bool disable_counter_freezing;
> +module_param(disable_counter_freezing, bool, 0444);
> +MODULE_PARM_DESC(disable_counter_freezing, "Disable counter freezing feature."
> +		"The PMI handler will fall back to generic handler."
> +		"Default is false (enable counter freezing feature).");

Why?

> +	/*
> +	 * Ack the PMU late after the APIC.  This avoids bogus

That doesn't make sense. PMU and APIC do not have order.

> +	 * freezing on Skylake CPUs.  The acking unfreezes the PMU
> +	 */
> +	if (status) {
> +		intel_pmu_ack_status(status);
> +	} else {
> +		/*
> +		 * CPU may issues two PMIs very close to each other.
> +		 * When the PMI handler services the first one, the
> +		 * GLOBAL_STATUS is already updated to reflect both.
> +		 * When it IRETs, the second PMI is immediately
> +		 * handled and it sees clear status. At the meantime,
> +		 * there may be a third PMI, because the freezing bit
> +		 * isn't set since the ack in first PMI handlers.
> +		 * Double check if there is more work to be done.
> +		 */

Urgh... fun fun fun.

> +		status = intel_pmu_get_status();
> +		if (status)
> +			goto again;
> +	}
> +
> +	if (bts)
> +		intel_bts_enable_local();
> +	cpuc->enabled = pmu_enabled;
> +	return handled;
> +}

> @@ -3432,6 +3538,11 @@ static void intel_pmu_cpu_dying(int cpu)
>  	free_excl_cntrs(cpu);
>  
>  	fini_debug_store_on_cpu(cpu);
> +
> +	if (cpuc->frozen_enabled) {
> +		cpuc->frozen_enabled = 0;
> +		disable_counter_freeze();
> +	}
>  }

See, you have the dying thing, so why not the matching starting thing.

> @@ -4442,6 +4555,15 @@ __init int intel_pmu_init(void)
>  		pr_cont("full-width counters, ");
>  	}
>  
> +	/*
> +	 * For arch perfmon 4 use counter freezing to avoid
> +	 * several MSR accesses in the PMI.
> +	 */
> +	if (x86_pmu.counter_freezing) {
> +		x86_pmu.handle_irq = intel_pmu_handle_irq_v4;
> +		pr_cont("counter freezing, ");
> +	}

Lets not print the counter freezing, we already print v4, right?


> @@ -561,6 +566,7 @@ struct x86_pmu {
>  	struct x86_pmu_quirk *quirks;
>  	int		perfctr_second_write;
>  	bool		late_ack;
> +	bool		counter_freezing;

Please make the both of them int or something.

>  	u64		(*limit_period)(struct perf_event *event, u64 l);
>  
>  	/*