On Fri, Nov 07, 2014 at 10:09:59AM +0000, Matt Fleming wrote:
> On Fri, 07 Nov, at 10:08:04AM, Peter Zijlstra wrote:
> >
> > How is that supposed to work? You call __intel_cqm_event_count() on the
> > one cpu per socket, but then you use a local_add, not an atomic_add,
> > even though these adds can happen concurrently as per IPI broadcast.
>
> Ouch, right. That's broken.
>
> > Also, I think smp_call_function_many() ignores the current cpu, if this
> > cpu happens to be the cpu for this socket, you're up some creek without
> > no paddle, right?
>
> OK, I didn't realise that. Yeah that sounds very problematic. I think my
> eyes skipped over the word "other" in the smp_call_function_many() docs,
>
> * smp_call_function_many(): Run a function on a set of other CPUs.
>
> So, the correct way to do this is to iterate over cqm_cpumask and invoke
> smp_call_function_single(), right?
>
> > Thirdly, there is no serialization around calling perf_event_count() [or
> > your pmu::count method] so you cannot temporarily put it to 0.
>
> Urgh, thanks. Good spot. I'm gonna have to think of a suitable
> serialisation mechanism because all the current ones are pretty
> heavy-handed. And of course, there's the added fun that it needs to be
> held across the IPIs.
>
> Perhaps a per-cache-group mutex?
struct rmid_read {
unsigned int rmid;
atomic64_t value;
}
static void __remote_rmid_read(void *arg)
{
struct rmid_read *rr = arg;
u64 val = __rmid_read(rr->rmid);
if (val & (RMID_VAL_ERROR | RMID_VAL_UNAVAIL))
return;
atomic64_add(val, &rr->value);
}
static void task_rmid_read(struct perf_event *event)
{
struct rmid_read rr = {
.rmid = ...,
.value = ATOMIC64_INIT(0),
};
int cpu;
...
cpu = get_cpu();
smp_call_function_many(&cqm_cpumask, __remote_rmid_read, &rr);
if (cpumask_test(cpu, &cqm_cpumask)) {
local_irq_disable();
__remote_rmid_read(&rr);
local_irq_enable();
}
put_cpu();
local64_set(&event->count, atomic64_read(&rr->value));
}
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