Re: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support
From: Saravana Kannan
Date: Fri Mar 02 2018 - 14:20:07 EST
On 03/02/2018 02:42 AM, Mark Rutland wrote:
On Thu, Mar 01, 2018 at 12:35:49PM -0800, Saravana Kannan wrote:
On 03/01/2018 03:49 AM, Mark Rutland wrote:
On Wed, Feb 28, 2018 at 02:17:33PM -0800, Saravana Kannan wrote:
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.
I believe that lock is currently superfluous, because the perf core
ensures operations are cpu-affine, and have interrupts disabled in most
cases (thanks to the context lock).
I don't think it's superfluous. You have a common "event counter" selection
register and a common "event counter value" register. You can two CPUs
racing to read two unrelated event counters and end up causing one of them
to read a bogus value from the wrong event counter.
It's important to note that the DSU PMU's event_init() ensures events
are affine to a single CPU, and the perf core code serializes operations
on those events via the context lock.
Ah, I see that now. Thanks!
Therefore, two CPUs *won't* try to access the registers simultaneously.
Right, and this driver seems to be going through a lot of work to make
sure all events are read in one CPU.
Do you even have an upstream target where there are multiple DSU's in a
system? If not, we can simplify a ton of this code (no hotplug
notifiers, no migrating PMUs, no SMP calls, etc) by just adding a
spinlock and letting any CPU read these DSU counters.
If you need to support a system with multiple DSUs, I think it's still
useful to add CPU mask for events and letting the perf framework read
events on any of those CPUs.
If events could be active on multiple CPUs simultaneously, I agree that
the lock would be necessary. However, there would also be other problems
to deal with in that case.
If we want to allow pmu::read() from arbitrary CPUs the DSU is affine
to, I agree we'd need the lock to serialize accesses to the registers
and data structures.
Agreed.
So, depending on how many DSUs you want to support in the mainline
kernel, we can simplify it a ton. And if not, we can still try to remove
the need for smp calls so that we don't cause power impact when trying
to profile while measuring power.
Thanks,
Saravana
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