Re: [RFC] perf_events: support for uncore a.k.a. nest units

[stephane eranian <eranian@googlemail.com>]

Seems to me that struct pmu is a shared resource across all CPUs.
I don't understand why scheduling on one CPU would have to impact
all the other CPUs, unless I am missing something here.


On Wed, Apr 21, 2010 at 10:08 AM, Lin Ming <ming.m.lin@intel.com> wrote:
> On Tue, 2010-04-20 at 20:03 +0800, Peter Zijlstra wrote:
>> On Tue, 2010-04-20 at 19:55 +0800, Lin Ming wrote:
>>
>> > > One thing not on that list, which should happen first I guess, is to
>> > > remove hw_perf_group_sched_in(). The idea is to add some sort of
>> > > transactional API to the struct pmu, so that we can delay the
>> > > schedulability check until commit time (and roll back when it fails).
>> > >
>> > > Something as simple as:
>> > >
>> > >   struct pmu {
>> > >     void start_txn(struct pmu *);
>> > >     void commit_txn(struct pmu *);
>> > >
>> > >     ,,,
>> > >   };
>> >
>> > Could you please explain a bit more?
>> >
>> > Does it mean that "start_txn" perform the schedule events stuff
>> > and "commit_txn" perform the assign events stuff?
>> >
>> > Does "commit time" mean the actual activation in hw_perf_enable?
>>
>> No, the idea behind hw_perf_group_sched_in() is to not perform
>> schedulability tests on each event in the group, but to add the group as
>> a whole and then perform one test.
>>
>> Of course, when that test fails, you'll have to roll-back the whole
>> group again.
>>
>> So start_txn (or a better name) would simply toggle a flag in the pmu
>> implementation that will make pmu::enable() not perform the
>> schedulablilty test.
>>
>> Then commit_txn() will perform the schedulability test (so note the
>> method has to have a !void return value, my mistake in the earlier
>> email).
>>
>> This will allow us to use the regular
>> kernel/perf_event.c::group_sched_in() and all the rollback code.
>> Currently each hw_perf_group_sched_in() implementation duplicates all
>> the rolllback code (with various bugs).
>>
>>
>>
>> We must get rid of all weak hw_perf_*() functions before we can properly
>> consider multiple struct pmu implementations.
>>
>
> Thanks for the clear explanation.
>
> Does below patch show what you mean?
>
> I only touch the x86 arch code now.
>
> ---
>  arch/x86/kernel/cpu/perf_event.c |  161 +++++++++++--------------------------
>  include/linux/perf_event.h       |   10 ++-
>  kernel/perf_event.c              |   28 +++----
>  3 files changed, 67 insertions(+), 132 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 626154a..62aa9a1 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -944,6 +944,9 @@ static int x86_pmu_enable(struct perf_event *event)
>        if (n < 0)
>                return n;
>
> +       if (!(event->pmu->flag & PERF_EVENT_TRAN_STARTED))
> +               goto out;
> +
>        ret = x86_pmu.schedule_events(cpuc, n, assign);
>        if (ret)
>                return ret;
> @@ -953,6 +956,7 @@ static int x86_pmu_enable(struct perf_event *event)
>         */
>        memcpy(cpuc->assign, assign, n*sizeof(int));
>
> +out:
>        cpuc->n_events = n;
>        cpuc->n_added += n - n0;
>
> @@ -1210,119 +1214,6 @@ x86_get_event_constraints(struct cpu_hw_events *cpuc, struct perf_event *event)
>        return &unconstrained;
>  }
>
> -static int x86_event_sched_in(struct perf_event *event,
> -                         struct perf_cpu_context *cpuctx)
> -{
> -       int ret = 0;
> -
> -       event->state = PERF_EVENT_STATE_ACTIVE;
> -       event->oncpu = smp_processor_id();
> -       event->tstamp_running += event->ctx->time - event->tstamp_stopped;
> -
> -       if (!is_x86_event(event))
> -               ret = event->pmu->enable(event);
> -
> -       if (!ret && !is_software_event(event))
> -               cpuctx->active_oncpu++;
> -
> -       if (!ret && event->attr.exclusive)
> -               cpuctx->exclusive = 1;
> -
> -       return ret;
> -}
> -
> -static void x86_event_sched_out(struct perf_event *event,
> -                           struct perf_cpu_context *cpuctx)
> -{
> -       event->state = PERF_EVENT_STATE_INACTIVE;
> -       event->oncpu = -1;
> -
> -       if (!is_x86_event(event))
> -               event->pmu->disable(event);
> -
> -       event->tstamp_running -= event->ctx->time - event->tstamp_stopped;
> -
> -       if (!is_software_event(event))
> -               cpuctx->active_oncpu--;
> -
> -       if (event->attr.exclusive || !cpuctx->active_oncpu)
> -               cpuctx->exclusive = 0;
> -}
> -
> -/*
> - * Called to enable a whole group of events.
> - * Returns 1 if the group was enabled, or -EAGAIN if it could not be.
> - * Assumes the caller has disabled interrupts and has
> - * frozen the PMU with hw_perf_save_disable.
> - *
> - * called with PMU disabled. If successful and return value 1,
> - * then guaranteed to call perf_enable() and hw_perf_enable()
> - */
> -int hw_perf_group_sched_in(struct perf_event *leader,
> -              struct perf_cpu_context *cpuctx,
> -              struct perf_event_context *ctx)
> -{
> -       struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
> -       struct perf_event *sub;
> -       int assign[X86_PMC_IDX_MAX];
> -       int n0, n1, ret;
> -
> -       if (!x86_pmu_initialized())
> -               return 0;
> -
> -       /* n0 = total number of events */
> -       n0 = collect_events(cpuc, leader, true);
> -       if (n0 < 0)
> -               return n0;
> -
> -       ret = x86_pmu.schedule_events(cpuc, n0, assign);
> -       if (ret)
> -               return ret;
> -
> -       ret = x86_event_sched_in(leader, cpuctx);
> -       if (ret)
> -               return ret;
> -
> -       n1 = 1;
> -       list_for_each_entry(sub, &leader->sibling_list, group_entry) {
> -               if (sub->state > PERF_EVENT_STATE_OFF) {
> -                       ret = x86_event_sched_in(sub, cpuctx);
> -                       if (ret)
> -                               goto undo;
> -                       ++n1;
> -               }
> -       }
> -       /*
> -        * copy new assignment, now we know it is possible
> -        * will be used by hw_perf_enable()
> -        */
> -       memcpy(cpuc->assign, assign, n0*sizeof(int));
> -
> -       cpuc->n_events  = n0;
> -       cpuc->n_added  += n1;
> -       ctx->nr_active += n1;
> -
> -       /*
> -        * 1 means successful and events are active
> -        * This is not quite true because we defer
> -        * actual activation until hw_perf_enable() but
> -        * this way we* ensure caller won't try to enable
> -        * individual events
> -        */
> -       return 1;
> -undo:
> -       x86_event_sched_out(leader, cpuctx);
> -       n0  = 1;
> -       list_for_each_entry(sub, &leader->sibling_list, group_entry) {
> -               if (sub->state == PERF_EVENT_STATE_ACTIVE) {
> -                       x86_event_sched_out(sub, cpuctx);
> -                       if (++n0 == n1)
> -                               break;
> -               }
> -       }
> -       return ret;
> -}
> -
>  #include "perf_event_amd.c"
>  #include "perf_event_p6.c"
>  #include "perf_event_p4.c"
> @@ -1454,6 +1345,47 @@ static inline void x86_pmu_read(struct perf_event *event)
>        x86_perf_event_update(event);
>  }
>
> +/*
> + * Set the flag to make pmu::enable() not perform the
> + * schedulablilty test.
> + */
> +static void x86_pmu_start_txn(struct pmu *pmu)
> +{
> +       pmu->flag |= PERF_EVENT_TRAN_STARTED;
> +}
> +
> +static void x86_pmu_stop_txn(struct pmu *pmu)
> +{
> +       pmu->flag &= ~PERF_EVENT_TRAN_STARTED;
> +}
> +
> +/*
> + * Return 0 if commit transaction success
> + */
> +static int x86_pmu_commit_txn(struct pmu *pmu)
> +{
> +       struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
> +       int assign[X86_PMC_IDX_MAX];
> +       int n, ret;
> +
> +       n = cpuc->n_events;
> +
> +       if (!x86_pmu_initialized())
> +               return -EAGAIN;
> +
> +       ret = x86_pmu.schedule_events(cpuc, n, assign);
> +       if (ret)
> +               return ret;
> +
> +       /*
> +        * copy new assignment, now we know it is possible
> +        * will be used by hw_perf_enable()
> +        */
> +       memcpy(cpuc->assign, assign, n*sizeof(int));
> +
> +       return 0;
> +}
> +
>  static const struct pmu pmu = {
>        .enable         = x86_pmu_enable,
>        .disable        = x86_pmu_disable,
> @@ -1461,6 +1393,9 @@ static const struct pmu pmu = {
>        .stop           = x86_pmu_stop,
>        .read           = x86_pmu_read,
>        .unthrottle     = x86_pmu_unthrottle,
> +       .start_txn      = x86_pmu_start_txn,
> +       .stop_txn       = x86_pmu_stop_txn,
> +       .commit_txn     = x86_pmu_commit_txn,
>  };
>
>  /*
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index bf896d0..93aa8d8 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -524,6 +524,8 @@ struct hw_perf_event {
>
>  struct perf_event;
>
> +#define PERF_EVENT_TRAN_STARTED 1
> +
>  /**
>  * struct pmu - generic performance monitoring unit
>  */
> @@ -534,6 +536,11 @@ struct pmu {
>        void (*stop)                    (struct perf_event *event);
>        void (*read)                    (struct perf_event *event);
>        void (*unthrottle)              (struct perf_event *event);
> +       void (*start_txn)               (struct pmu *pmu);
> +       void (*stop_txn)                (struct pmu *pmu);
> +       int (*commit_txn)               (struct pmu *pmu);
> +
> +       u8 flag;
>  };
>
>  /**
> @@ -799,9 +806,6 @@ extern void perf_disable(void);
>  extern void perf_enable(void);
>  extern int perf_event_task_disable(void);
>  extern int perf_event_task_enable(void);
> -extern int hw_perf_group_sched_in(struct perf_event *group_leader,
> -              struct perf_cpu_context *cpuctx,
> -              struct perf_event_context *ctx);
>  extern void perf_event_update_userpage(struct perf_event *event);
>  extern int perf_event_release_kernel(struct perf_event *event);
>  extern struct perf_event *
> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> index 07b7a43..4537676 100644
> --- a/kernel/perf_event.c
> +++ b/kernel/perf_event.c
> @@ -83,14 +83,6 @@ extern __weak const struct pmu *hw_perf_event_init(struct perf_event *event)
>  void __weak hw_perf_disable(void)              { barrier(); }
>  void __weak hw_perf_enable(void)               { barrier(); }
>
> -int __weak
> -hw_perf_group_sched_in(struct perf_event *group_leader,
> -              struct perf_cpu_context *cpuctx,
> -              struct perf_event_context *ctx)
> -{
> -       return 0;
> -}
> -
>  void __weak perf_event_print_debug(void)       { }
>
>  static DEFINE_PER_CPU(int, perf_disable_count);
> @@ -642,14 +634,13 @@ group_sched_in(struct perf_event *group_event,
>               struct perf_event_context *ctx)
>  {
>        struct perf_event *event, *partial_group;
> +       struct pmu *pmu = (struct pmu *)group_event->pmu;
>        int ret;
>
>        if (group_event->state == PERF_EVENT_STATE_OFF)
>                return 0;
>
> -       ret = hw_perf_group_sched_in(group_event, cpuctx, ctx);
> -       if (ret)
> -               return ret < 0 ? ret : 0;
> +       pmu->start_txn(pmu);
>
>        if (event_sched_in(group_event, cpuctx, ctx))
>                return -EAGAIN;
> @@ -664,16 +655,21 @@ group_sched_in(struct perf_event *group_event,
>                }
>        }
>
> -       return 0;
> +       ret = pmu->commit_txn(pmu);
> +       if (!ret) {
> +               pmu->stop_txn(pmu);
> +               return 0;
> +       }
>
>  group_error:
> +       pmu->stop_txn(pmu);
> +
>        /*
> -        * Groups can be scheduled in as one unit only, so undo any
> -        * partial group before returning:
> +        * Commit transaction fails, rollback
> +        * Groups can be scheduled in as one unit only, so undo
> +        * whole group before returning:
>         */
>        list_for_each_entry(event, &group_event->sibling_list, group_entry) {
> -               if (event == partial_group)
> -                       break;
>                event_sched_out(event, cpuctx, ctx);
>        }
>        event_sched_out(group_event, cpuctx, ctx);
>
>
>