Re: [PATCH V5 2/4] drivers/perf: imx_ddr: Add ddr performance counter support

Zhi Li
2019-04-24T19:13:45Z

On Wed, Apr 24, 2019 at 5:58 AM Will Deacon <will.deacon@arm.com> wrote:
>
> Hi Frank,
>
> On Tue, Apr 16, 2019 at 08:39:33PM +0000, Frank Li wrote:
> > Add ddr performance monitor support for iMX8QXP
> >
> > There are 4 counters for ddr perfomance events.
> > counter 0 is dedicated for cycles.
> > you choose any up to 3 no cycles events.
>
> I was about to pick this up, but I still have a few questions/comments
> on the code:
>
> > +static irqreturn_t ddr_perf_irq_handler(int irq, void *p)
> > +{
> > + int i;
> > + u8 reg;
> > + int val;
> > + int counter;
> > + struct ddr_pmu *pmu = (struct ddr_pmu *) p;
> > + struct perf_event *event;
> > +
> > + /* Only cycles counter overflowed can issue irq. all counters will
> > + * be stopped when cycles counter overflow. but other counter don't stop
> > + * when overflow happen. Update all of the local counter values,
> > + * then reset the cycles counter, so the others can continue
> > + * counting. cycles counter is fastest counter in all events. at last
> > + * 4 times than other counters.
> > + */
> > + for (i = 0; i < NUM_COUNTER; i++) {
> > +
> > + if (!pmu->active_events[i])
> > + continue;
> > +
> > + event = pmu->active_events[i];
> > + counter = event->hw.idx;
> > + reg = counter * 4 + COUNTER_CNTL;
> > + val = readl(pmu->base + reg);
>
> Does this read have a side-effect, or can it be removed given that you don't
> use val for anything else?

It can be removed.

>
> > + ddr_perf_event_update(event);
> > +
> > + if (counter == EVENT_CYCLES_COUNTER) {
> > + ddr_perf_event_enable(pmu,
> > + EVENT_CYCLES_ID,
> > + EVENT_CYCLES_COUNTER,
> > + true);
> > + ddr_perf_event_update(event);
> > + }
> > + }
> > +
> > + return IRQ_HANDLED;
> > +}
>
> What stops the IRQ handler running concurrently with perf callbacks? I can't
> see any locking here, or is the IRQ supposed to be affine to the same CPU
> that's handling the perf context?

I don't under your means here.
ddr_perf_event_update is lockless design.

I can't found problem if run ddr_perf_event_enable at both perf
context and IRQ concurrently.

>
> > +static int ddr_perf_offline_cpu(unsigned int cpu, struct hlist_node *node)
> > +{
> > + struct ddr_pmu *pmu = hlist_entry_safe(node, struct ddr_pmu, node);
> > + int target;
> > +
> > + if (!cpumask_test_and_clear_cpu(cpu, &pmu->cpu))
> > + return 0;
> > +
> > + target = cpumask_any_but(cpu_online_mask, cpu);
> > + if (target >= nr_cpu_ids)
> > + return 0;
> > +
> > + perf_pmu_migrate_context(&pmu->pmu, cpu, target);
> > + cpumask_set_cpu(target, &pmu->cpu);
> > +
> > + return 0;
> > +}
> > +
> > +static int ddr_perf_probe(struct platform_device *pdev)
> > +{
> > + struct ddr_pmu *pmu;
> > + struct device_node *np;
> > + void __iomem *base;
> > + struct resource *iomem;
> > + char *name;
> > + char *hpname;
> > + int num;
> > + int ret;
> > + u32 irq;
> > + const struct of_device_id *of_id =
> > + of_match_device(imx_ddr_pmu_dt_ids, &pdev->dev);
> > +
> > + iomem = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > + base = devm_ioremap_resource(&pdev->dev, iomem);
> > + if (IS_ERR(base))
> > + return PTR_ERR(base);
> > +
> > + np = pdev->dev.of_node;
> > +
> > + pmu = devm_kzalloc(&pdev->dev, sizeof(*pmu), GFP_KERNEL);
> > + if (!pmu)
> > + return -ENOMEM;
> > +
> > + num = ddr_perf_init(pmu, base, &pdev->dev);
> > +
> > + name = devm_kasprintf(&pdev->dev, GFP_KERNEL, "ddr%d", num);
> > + if (!name)
> > + return -ENOMEM;
> > +
> > + hpname = devm_kasprintf(&pdev->dev, GFP_KERNEL,
> > + "perf/imx/ddr%d:online", num);
> > + if (!hpname)
> > + return -ENOMEM;
> > +
> > + pmu->flags = (uintptr_t) of_id->data;
> > +
> > + cpumask_set_cpu(raw_smp_processor_id(), &pmu->cpu);
> > + ret = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN, hpname, NULL,
> > + ddr_perf_offline_cpu);
> > +
> > + if (ret < 0) {
> > + dev_err(&pdev->dev, "cpuhp_setup_state_multi failed\n");
> > + goto ddr_perf_err;
> > + }
> > +
> > + pmu->cpuhp_state = ret;
> > +
> > + /* Register the pmu instance for cpu hotplug */
> > + cpuhp_state_add_instance_nocalls(pmu->cpuhp_state, &pmu->node);
> > +
> > + ret = perf_pmu_register(&(pmu->pmu), name, -1);
> > + if (ret)
> > + goto ddr_perf_err;
> > +
> > + /* Request irq */
> > + irq = of_irq_get(np, 0);
> > + if (irq < 0) {
>
> irq is a u32 so this will never execute. You need to make it an int.
>
> > + dev_err(&pdev->dev, "Failed to get irq: %d", irq);
> > + ret = irq;
> > + goto ddr_perf_irq_err;
> > + }
> > +
> > + ret = devm_request_irq(&pdev->dev, irq,
> > + ddr_perf_irq_handler,
> > + IRQF_TRIGGER_RISING | IRQF_ONESHOT,
>
> Can you explain why you need to pass these specific IRQ flags, please? I'm
> struggling to see what the ONESHOT is buying you.

I think default should be okay. I will update at next version
>
> Will