Messages in this thread | | | Date | Wed, 28 Feb 2018 14:17:33 -0800 | From | Saravana Kannan <> | Subject | Re: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support |
| |
On 02/25/2018 06:36 AM, Mark Rutland wrote: > On Fri, Feb 23, 2018 at 04:53:18PM -0800, Saravana Kannan wrote: >> On 01/02/2018 03:25 AM, Suzuki K Poulose wrote: >>> +static void dsu_pmu_event_update(struct perf_event *event) >>> +{ >>> + struct hw_perf_event *hwc = &event->hw; >>> + u64 delta, prev_count, new_count; >>> + >>> + do { >>> + /* We may also be called from the irq handler */ >>> + prev_count = local64_read(&hwc->prev_count); >>> + new_count = dsu_pmu_read_counter(event); >>> + } while (local64_cmpxchg(&hwc->prev_count, prev_count, new_count) != >>> + prev_count); >>> + delta = (new_count - prev_count) & DSU_PMU_COUNTER_MASK(hwc->idx); >>> + local64_add(delta, &event->count); >>> +} >>> + >>> +static void dsu_pmu_read(struct perf_event *event) >>> +{ >>> + dsu_pmu_event_update(event); >>> +} > >> I sent out a patch that'll allow PMUs to set an event flag to avoid >> unnecessary smp calls when the event can be read from any CPU. You could >> just always set that if you can't have multiple DSU's running the kernel (I >> don't know if the current ARM designs support having multiple DSUs in a >> SoC/system) or set it if associated_cpus == cpu_present_mask. > > As-is, that won't be safe, given the read function calls the event_update() > function, which has side-effects on hwc->prec_count and event->count. Those > need to be serialized somehow.
You have to grab the dsu_pmu->pmu_lock spin lock anyway because the system registers are shared across all CPUs. So, just expanding it a bit to lock the hwc->prev_count and event->count updated doesn't seem to be any worse. In fact, it's better than sending pointless IPIs.
The local64_read/cmpxchg/add etc makes sense when you have per-cpu system registers like in the case of the ARM CPU PMU registers. It doesn't really buy us much for registers shared across the CPUs.
Thanks, Saravana
-- Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
| |