Re: [RFC] perf/x86/amd: add support for Large Increment per Cycle Events

[Stephane Eranian]

On Wed, Aug 28, 2019 at 5:47 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Aug 26, 2019 at 02:59:15PM -0500, Kim Phillips wrote:
> > The core AMD PMU has a 4-bit wide per-cycle increment for each
> > performance monitor counter.  That works for most counters, but
> > now with AMD Family 17h and above processors, for some, more than 15
> > events can occur in a cycle.  Those events are called "Large
> > Increment per Cycle" events, and one example is the number of
> > SSE/AVX FLOPs retired (event code 0x003).  In order to count these
> > events, two adjacent h/w PMCs get their count signals merged
> > to form 8 bits per cycle total.
>
> *groan*
>
> > In addition, the PERF_CTR count
> > registers are merged to be able to count up to 64 bits.
>
> That is daft; why can't you extend the existing MSR to 64bit?
>
My understanding is that the problem is not the width of the counter
but its ability to increment by more than 15 per cycle. They need two
counters to swallow 16+ events/cycle.


> > Normally, events like instructions retired, get programmed on a single
> > counter like so:
> >
> > PERF_CTL0 (MSR 0xc0010200) 0x000000000053ff0c # event 0x0c, umask 0xff
> > PERF_CTR0 (MSR 0xc0010201) 0x0000800000000001 # r/w 48-bit count
> >
> > The next counter at MSRs 0xc0010202-3 remains unused, or can be used
> > independently to count something else.
> >
> > When counting Large Increment per Cycle events, such as FLOPs,
> > however, we now have to reserve the next counter and program the
> > PERF_CTL (config) register with the Merge event (0xFFF), like so:
> >
> > PERF_CTL0 (msr 0xc0010200) 0x000000000053ff03 # FLOPs event, umask 0xff
> > PERF_CTR0 (msr 0xc0010201) 0x0000800000000001 # read 64-bit count, wr low 48b
> > PERF_CTL1 (msr 0xc0010202) 0x0000000f004000ff # Merge event, enable bit
> > PERF_CTR1 (msr 0xc0010203) 0x0000000000000000 # write higher 16-bits of count
> >
> > The count is widened from the normal 48-bits to 64 bits by having the
> > second counter carry the higher 16 bits of the count in its lower 16
> > bits of its counter register.  Support for mixed 48- and 64-bit counting
> > is not supported in this version.
>
> This is diguisting.. please talk to your hardware people. I sort of
> understand the pairing, but that upper 16 bit split for writes is just
> woeful crap.
>
> > For more details, search a Family 17h PPR for the "Large Increment per
> > Cycle Events" section, e.g., section 2.1.15.3 on p. 173 in this version:
> >
> > https://www.amd.com/system/files/TechDocs/56176_ppr_Family_17h_Model_71h_B0_pub_Rev_3.06.zip
>
> My mama told me not to open random zip files of the interweb :-)
>
> Also; afaict the only additional information there is that it works in
> odd/even pairs and you have to program the odd one before the even one.
> Surely you could've included that here.
>
> > In order to support reserving the extra counter for a single Large
> > Increment per Cycle event in the perf core, we:
> >
> > 1. Add a f17h get_event_constraints() that returns only an even counter
> > bitmask, since Large Increment events can only be placed on counters 0,
> > 2, and 4 out of the currently available 0-5.
>
> So hereby you promise that all LI events are unconstrained, right?
> Also, what marks the paired counter in the used mask? Aaah, you modify
> __perf_sched_find_counter(). Comments below.
>
> > 2. We add a commit_scheduler hook that adds the Merge event (0xFFF) to
> > any Large Increment event being scheduled.  If the event being scheduled
> > is not a Large Increment event, we check for, and remove any
> > pre-existing Large Increment events on the next counter.
>
> That is weird at best; the scheduling hooks shouldn't be the one doing
> the programming; that should be done in x86_pmu_enable(). Can't you do
> this by changing amd_pmu::{en,dis}able() ?
>
> (also; we really should rename some of those x86_pmu::ops :/)
>
> > 3. In the main x86 scheduler, we reduce the number of available
> > counters by the number of Large Increment per Cycle events being added.
> > This improves the counter scheduler success rate.
> >
> > 4. In perf_assign_events(), if a counter is assigned to a Large
> > Increment event, we increment the current counter variable, so the
> > counter used for the Merge event is skipped.
> >
> > 5. In find_counter(), if a counter has been found for the
> > Large Increment event, we set the next counter as used, to
> > prevent other events from using it.
> >
> > A side-effect of assigning a new get_constraints function for f17h
> > disables calling the old (prior to f15h) amd_get_event_constraints
> > implementation left enabled by commit e40ed1542dd7 ("perf/x86: Add perf
> > support for AMD family-17h processors"), which is no longer
> > necessary since those North Bridge events are obsolete.
>
> > RFC because I'd like input on the approach, including how to add support
> > for mixed-width (48- and 64-bit) counting for a single PMU.
>
> Ideally I'd tell you to wait for sane hardware :/
>
>
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > index 325959d19d9a..4596c141f348 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -787,6 +787,18 @@ static bool __perf_sched_find_counter(struct perf_sched *sched)
> >               if (!__test_and_set_bit(idx, sched->state.used)) {
> >                       if (sched->state.nr_gp++ >= sched->max_gp)
> >                               return false;
> > +                     if (c->flags & PERF_X86_EVENT_LARGE_INC) {
>
> Can we please call that something like:
>
>   PERF_X86_EVENT_PAIR
>
> > +                             /*
> > +                              * merged events need the Merge event
> > +                              * on the next counter
> > +                              */
> > +                             if (__test_and_set_bit(idx + 1,
> > +                                                    sched->state.used))
> > +                                     /* next counter already used */
> > +                                     return false;
>
> Coding Style wants { } there. Also, remove that line-break.
>
> > +
> > +                             set_bit(idx + 1, sched->state.used);
>
>   __set_bit() surely
>
> > +                     }
> >
> >                       goto done;
> >               }
> > @@ -849,14 +861,20 @@ int perf_assign_events(struct event_constraint **constraints, int n,
> >                       int wmin, int wmax, int gpmax, int *assign)
> >  {
> >       struct perf_sched sched;
> > +     struct event_constraint *c;
> > +
> >
> >       perf_sched_init(&sched, constraints, n, wmin, wmax, gpmax);
> >
> >       do {
> >               if (!perf_sched_find_counter(&sched))
> >                       break;  /* failed */
> > -             if (assign)
> > +             if (assign) {
> >                       assign[sched.state.event] = sched.state.counter;
> > +                     c = constraints[sched.state.event];
> > +                     if (c->flags & PERF_X86_EVENT_LARGE_INC)
> > +                             sched.state.counter++;
> > +             }
>
> How about you make __perf_sched_find_count() set the right value? That
> already knows it did this.
>
> >       } while (perf_sched_next_event(&sched));
> >
> >       return sched.state.unassigned;
> > @@ -952,6 +970,18 @@ int x86_schedule_events(struct cpu_hw_events *cpuc, int n, int *assign)
> >                   READ_ONCE(cpuc->excl_cntrs->exclusive_present))
> >                       gpmax /= 2;
> >
> > +             /*
> > +              * reduce the amount of available counters
> > +              * to allow fitting the Merge event along
> > +              * with their large increment event
> > +              */
> > +             if (x86_pmu.flags & PMU_FL_MERGE) {
> > +                     for (i = 0; i < n; i++) {
> > +                             hwc = &cpuc->event_list[i]->hw;
> > +                             if (is_lg_inc_event(hwc) && gpmax > 1)
>
> It should not be possible to hit !gpmax; make that a WARN.
>
> > +                                     gpmax--;
> > +                     }
>
> Alternatively you could have collect_events() cound the number of
> 'lg_inc' (we really have to come up with a better name) events on the
> cpuc. Then you can do a simple subtraction and avoid the loop.
>
> > +             }
> >               unsched = perf_assign_events(cpuc->event_constraint, n, wmin,
> >                                            wmax, gpmax, assign);
> >       }
> > @@ -1210,6 +1240,13 @@ int x86_perf_event_set_period(struct perf_event *event)
> >
> >       wrmsrl(hwc->event_base, (u64)(-left) & x86_pmu.cntval_mask);
> >
> > +     /*
> > +      * Clear the Merge event counter's upper 16 bits since
> > +      * we currently declare a 48-bit counter width
> > +      */
> > +     if (is_lg_inc_event(hwc))
> > +             wrmsrl(x86_pmu_event_addr(idx + 1), 0);
> > +
>
> *yuck*...
>
> >       /*
> >        * Due to erratum on certan cpu we need
> >        * a second write to be sure the register