On Fri, Sep 16, 2016 at 11:33:39AM -0400, Neil Leeder wrote:
> Hi Mark,
> Thank you for the thorough review. I will post an updated patchset
> which addresses all of your comments. There is just one outstanding
> comment which I have a question about:
[...]
> On 9/1/2016 12:30 PM, Mark Rutland wrote:
> > On Tue, Aug 30, 2016 at 01:01:33PM -0400, Neil Leeder wrote:
> >> + /* Don't allow groups with mixed PMUs, except for s/w events */
> >> + if (event->group_leader->pmu != event->pmu &&
> >> + !is_software_event(event->group_leader)) {
> >> + dev_warn(&l2cache_pmu->pdev->dev,
> >> + "Can't create mixed PMU group\n");
> >> + return -EINVAL;
> >> + }
> >> +
> >> + list_for_each_entry(sibling, &event->group_leader->sibling_list,
> >> + group_entry)
> >> + if (sibling->pmu != event->pmu &&
> >> + !is_software_event(sibling)) {
> >> + dev_warn(&l2cache_pmu->pdev->dev,
> >> + "Can't create mixed PMU group\n");
> >> + return -EINVAL;
> >> + }
> >> +
> >> + hwc->idx = -1;
> >> + hwc->config_base = event->attr.config;
> >> +
> >> + /*
> >> + * Ensure all events are on the same cpu so all events are in the
> >> + * same cpu context, to avoid races on pmu_enable etc.
> >> + */
> >> + slice = get_hml2_pmu(event->cpu);
> >> + event->cpu = slice->on_cpu;
> >
> > This could put an event on a different CPU to its group siblings, which
> > is broken.
>
> This is the same logic as in arm-ccn.c:arm_ccn_pmu_event_init(), where there
> is a single CPU designated as the CPU to be used for all events.
>
> All events for this slice are forced to slice->on_cpu which is the CPU
> set in the cpumask for this slice.
The CCN is a little different. For the CCN, a single CPU is designated
to handle *all* events.
For this driver, a CPU is designated per-slice, judging by the existence
of hml2_pmu::on_cpu (unless that's superfluous). We've only verified
that the events are all for this PMU, not the same slice, and thus each
event->cpu may differ.
> I'm not sure how this can put an event on a different CPU to its group
> siblings?
In practice today, we'll try to schedule the event on it's group
leader's CPU, but accounting and subsequent manipulation could go wrong.
Thanks,
Mark.
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