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SubjectRe: [PATCH] perf_events: fix event scheduling issues introduced by transactional API
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On Mon, 2010-05-24 at 22:40 +0800, Stephane Eranian wrote:
> The transactional API patch between the generic and model-specific
> code introduced several important bugs with event scheduling, at
> least on X86. If you had pinned events, e.g., watchdog, and were
> over-committing the PMU, you would get bogus counts. The bug was
> showing up on Intel CPU because events would move around more
> often that on AMD. But the problem also existed on AMD, though
> harder to expose.
>
> The issues were:
>
> - group_sched_in() was missing a cancel_txn() in the error path
>
> - cpuc->n_added was not properly maintained, leading to missing
> actions in hw_perf_enable(), i.e., n_running being 0. You cannot
> update n_added until you know the transaction has succeeded. In
> case of failed transaction n_added was not adjusted back.
>
> - in case of failed transactions, event_sched_out() was called
> and eventually invoked x86_disable_event() to touch the HW reg.
> But with transactions, on X86, event_sched_in() does not touch
> HW registers, it simply collects events into a list. Thus, you

event_sched_in always does not touch HW registers, both with and without
transactions.

> could end up calling x86_disable_event() on a counter which
> did not correspond to the current event when idx != -1.
>
> The patch modifies the generic and X86 code to avoid all those
> problems.
>
> First, n_added is now ONLY updated once we know the transaction has
> succeeded.
>
> Second, we encapsulate the event_sched_in() and event_sched_out() in
> group_sched_in() inside the transaction. That way, in the X6 code, we
> can detect that we are being asked to stop an event which was not yet
> started by checking cpuc->group_flag.
>
> With this patch, you can now overcommit the PMU even with pinned
> system-wide events present and still get valid counts.
>
> Signed-off-by: Stephane Eranian <eranian@google.com>
>
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index fd4db0d..25dfd30 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -105,6 +105,7 @@ struct cpu_hw_events {
> int enabled;
>
> int n_events;
> + int n_events_trans;
> int n_added;
> int assign[X86_PMC_IDX_MAX]; /* event to counter assignment */
> u64 tags[X86_PMC_IDX_MAX];
> @@ -591,7 +592,7 @@ void hw_perf_disable(void)
> if (!cpuc->enabled)
> return;
>
> - cpuc->n_added = 0;
> + cpuc->n_added = cpuc->n_events_trans = 0;
> cpuc->enabled = 0;
> barrier();
>
> @@ -820,7 +821,7 @@ void hw_perf_enable(void)
> * apply assignment obtained either from
> * hw_perf_group_sched_in() or x86_pmu_enable()
> *
> - * step1: save events moving to new counters
> + * step1: stop-save old events moving to new counters
> * step2: reprogram moved events into new counters
> */
> for (i = 0; i < n_running; i++) {
> @@ -982,7 +983,7 @@ static int x86_pmu_enable(struct perf_event *event)
>
> out:
> cpuc->n_events = n;
> - cpuc->n_added += n - n0;
> + cpuc->n_events_trans += n - n0;
>
> return 0;
> }
> @@ -1070,7 +1071,7 @@ static void x86_pmu_stop(struct perf_event *event)
> struct hw_perf_event *hwc = &event->hw;
> int idx = hwc->idx;
>
> - if (!__test_and_clear_bit(idx, cpuc->active_mask))
> + if (idx == -1 || !__test_and_clear_bit(idx, cpuc->active_mask))
> return;
>
> x86_pmu.disable(event);
> @@ -1087,14 +1088,30 @@ static void x86_pmu_stop(struct perf_event *event)
> static void x86_pmu_disable(struct perf_event *event)
> {
> struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
> + bool no_trans;
> int i;
>
> - x86_pmu_stop(event);
> + /*
> + * when coming from a transaction, then the event has not
> + * actually been scheduled in yet. It has only been collected
> + * (collect_events), thus we cannot apply a full disable:
> + * - put_constraints() assumes went thru schedule_events()
> + * - x86_pmu_stop() assumes went thru x86_pmu_start() because
> + * maybe idx != -1 and thus we may overwrite another the wrong
> + * counter (e.g, pinned event).
> + *
> + * When called from a transaction, we need to un-collect the
> + * event, i..e, remove the event from event_list[]
> + */
> + no_trans = !(cpuc->group_flag & PERF_EVENT_TXN_STARTED);
> +
> + if (no_trans)
> + x86_pmu_stop(event);
>
> for (i = 0; i < cpuc->n_events; i++) {
> if (event == cpuc->event_list[i]) {
>
> - if (x86_pmu.put_event_constraints)
> + if (no_trans && x86_pmu.put_event_constraints)
> x86_pmu.put_event_constraints(cpuc, event);
>
> while (++i < cpuc->n_events)
> @@ -1104,7 +1121,8 @@ static void x86_pmu_disable(struct perf_event *event)
> break;
> }
> }
> - perf_event_update_userpage(event);
> + if (no_trans)
> + perf_event_update_userpage(event);
> }
>
> static int x86_pmu_handle_irq(struct pt_regs *regs)
> @@ -1418,7 +1436,8 @@ static int x86_pmu_commit_txn(const struct pmu *pmu)
> * will be used by hw_perf_enable()
> */
> memcpy(cpuc->assign, assign, n*sizeof(int));
> -
> + cpuc->n_added += cpuc->n_events_trans;
> + cpuc->n_events_trans = 0;
> return 0;
> }
>
> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> index 7e3bcf1..b0ccf4b 100644
> --- a/kernel/perf_event.c
> +++ b/kernel/perf_event.c
> @@ -652,8 +652,11 @@ group_sched_in(struct perf_event *group_event,
> if (txn)
> pmu->start_txn(pmu);
>
> - if (event_sched_in(group_event, cpuctx, ctx))
> + if (event_sched_in(group_event, cpuctx, ctx)) {
> + if (txn)
> + pmu->cancel_txn(pmu);
> return -EAGAIN;
> + }

Can't imagine I forgot to call cancel_txn here.
Thanks very much.

>
> /*
> * Schedule in siblings as one group (if any):
> @@ -675,8 +678,6 @@ group_sched_in(struct perf_event *group_event,
> }
>
> group_error:
> - if (txn)
> - pmu->cancel_txn(pmu);
>
> /*
> * Groups can be scheduled in as one unit only, so undo any
> @@ -689,6 +690,9 @@ group_error:
> }
> event_sched_out(group_event, cpuctx, ctx);
>
> + if (txn)
> + pmu->cancel_txn(pmu);
> +
> return -EAGAIN;
> }
>

Looks good to me.

Thanks,
Lin Ming



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