On Mon, Apr 04, 2016 at 04:42:11PM -0700, Tai Tri Nguyen wrote:
> On Fri, Apr 1, 2016 at 5:18 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >> +static int get_next_avail_cntr(struct xgene_pmu_dev *pmu_dev)
> >> +{
> >> + int shift, cntr, retval;
> >> + unsigned long flags;
> >> +
> >> + raw_spin_lock_irqsave(&pmu_dev->lock, flags);
> >> +
> >> + for (cntr = 0; cntr < pmu_dev->max_counters; cntr++) {
> >> + shift = cntr;
> >> + if (!(pmu_dev->cntr_assign_mask & (1ULL << shift))) {
> >> + pmu_dev->cntr_assign_mask |= (1ULL << shift);
> >> + retval = cntr;
> >> + goto out;
> >> + }
> >> + }
> >> + retval = -ENOSPC;
> > Are the spinlocks necessary?
> >
> > I thought add and del couldn't be called in parallel for the same
> > context, and those are the only users of this mask.
> >
>
> I'm trying to avoid the case where multiple events may claim the same
> available counter.
> There's a race condition here.
I don't think there is, so long as events are all associated with the same CPU,
and hence the same ctx.
As I mentioned, add and del are the only users of this mask. Both of those are
only called with ctx->lock held, so I couldn't see how these could race.
Were you considering events in different cpu contexts racing?
Is there something I've missed?
> >> + hwc->config = config;
> >> + if (config1)
> >> + hwc->extra_reg.config = config1;
> >> + else
> >> + /* Enable all Agents */
> >> + hwc->extra_reg.config = 0xFFFFFFFFFFFFFFFFULL;
> >
> > I'm not sure I follow what's going on here.
> >
> > It would be good to document precisely what this means.
>
> These are X-Gene PMU specific for monitoring performance of a specific
> data path.
> X-Gene PMUs have 2 registers capable of masking the Agents from which
> the request come from. If the bit with the bit number corresponding to
> the AgentID
> is set, the event will be counted only if it is caused by a request
> from that agent.
> Each PMU has different set of Agents. By default, the event will be counted for
> all agent requests.
>
> I'll have it commented better for next revision of the patch.
It might be worth having something under Documentation/ for this, similarly to
what we do for CCN in Documentation/arm/CCN.txt.
How is the user expected to determine agent IDs? Is there a listing somewhere?
Does this change between reivisions? This may be worth documenting.
Thanks,
Mark.
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