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Date:	Wed, 5 Feb 2014 17:01:42 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 02/14] perf, core: introduce pmu context switch callback

On Fri, Jan 3, 2014 at 6:47 AM, Yan, Zheng <zheng.z.yan@...el.com> wrote:
> The callback is invoked when process is scheduled in or out. It
> provides mechanism for later patches to save/store the LBR stack.
> It can also replace the flush branch stack callback.
>
I think you need to say this callback may be invoked on context switches
with per-thread events attached. As far I understand, the callback cannot
be invoked for system-wide events.

> To avoid unnecessary overhead, the callback is enabled dynamically
>
> Signed-off-by: Yan, Zheng <zheng.z.yan@...el.com>
> ---
>  arch/x86/kernel/cpu/perf_event.c |  7 +++++
>  arch/x86/kernel/cpu/perf_event.h |  4 +++
>  include/linux/perf_event.h       |  8 ++++++
>  kernel/events/core.c             | 60 +++++++++++++++++++++++++++++++++++++++-
>  4 files changed, 78 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 8e13293..6703d17 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -1846,6 +1846,12 @@ static const struct attribute_group *x86_pmu_attr_groups[] = {
>         NULL,
>  };
>
> +static void x86_pmu_sched_task(struct perf_event_context *ctx, bool sched_in)
> +{
> +       if (x86_pmu.sched_task)
> +               x86_pmu.sched_task(ctx, sched_in);
> +}
> +
>  static void x86_pmu_flush_branch_stack(void)
>  {
>         if (x86_pmu.flush_branch_stack)
> @@ -1879,6 +1885,7 @@ static struct pmu pmu = {
>
>         .event_idx              = x86_pmu_event_idx,
>         .flush_branch_stack     = x86_pmu_flush_branch_stack,
> +       .sched_task             = x86_pmu_sched_task,
>  };
>
>  void arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)
> diff --git a/arch/x86/kernel/cpu/perf_event.h b/arch/x86/kernel/cpu/perf_event.h
> index 745f6fb..3fdb751 100644
> --- a/arch/x86/kernel/cpu/perf_event.h
> +++ b/arch/x86/kernel/cpu/perf_event.h
> @@ -417,6 +417,8 @@ struct x86_pmu {
>
>         void            (*check_microcode)(void);
>         void            (*flush_branch_stack)(void);
> +       void            (*sched_task)(struct perf_event_context *ctx,
> +                                     bool sched_in);
>
>         /*
>          * Intel Arch Perfmon v2+
> @@ -675,6 +677,8 @@ void intel_pmu_pebs_disable_all(void);
>
>  void intel_ds_init(void);
>
> +void intel_pmu_lbr_sched_task(struct perf_event_context *ctx, bool sched_in);
> +
There is no mention of this function anywhere else. Should not be here.

>  void intel_pmu_lbr_reset(void);
>
>  void intel_pmu_lbr_enable(struct perf_event *event);
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 8f4a70f..6a3e603 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -251,6 +251,12 @@ struct pmu {
>          * flush branch stack on context-switches (needed in cpu-wide mode)
>          */
>         void (*flush_branch_stack)      (void);
> +
> +       /*
> +        * PMU callback for context-switches. optional
> +        */
> +       void (*sched_task)              (struct perf_event_context *ctx,
> +                                        bool sched_in);
>  };
>
>  /**
> @@ -546,6 +552,8 @@ extern void perf_event_delayed_put(struct task_struct *task);
>  extern void perf_event_print_debug(void);
>  extern void perf_pmu_disable(struct pmu *pmu);
>  extern void perf_pmu_enable(struct pmu *pmu);
> +extern void perf_sched_cb_disable(struct pmu *pmu);
> +extern void perf_sched_cb_enable(struct pmu *pmu);
>  extern int perf_event_task_disable(void);
>  extern int perf_event_task_enable(void);
>  extern int perf_event_refresh(struct perf_event *event, int refresh);
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 89d34f9..d110a23 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -141,6 +141,7 @@ enum event_type_t {
>  struct static_key_deferred perf_sched_events __read_mostly;
>  static DEFINE_PER_CPU(atomic_t, perf_cgroup_events);
>  static DEFINE_PER_CPU(atomic_t, perf_branch_stack_events);
> +static DEFINE_PER_CPU(int, perf_sched_cb_usages);
>
>  static atomic_t nr_mmap_events __read_mostly;
>  static atomic_t nr_comm_events __read_mostly;
> @@ -150,6 +151,7 @@ static atomic_t nr_freq_events __read_mostly;
>  static LIST_HEAD(pmus);
>  static DEFINE_MUTEX(pmus_lock);
>  static struct srcu_struct pmus_srcu;
> +static struct idr pmu_idr;
>
>  /*
>   * perf event paranoia level:
> @@ -2327,6 +2329,57 @@ unlock:
>         }
>  }
>
> +void perf_sched_cb_disable(struct pmu *pmu)
> +{
> +       __get_cpu_var(perf_sched_cb_usages)--;
> +}
> +
> +void perf_sched_cb_enable(struct pmu *pmu)
> +{
> +       __get_cpu_var(perf_sched_cb_usages)++;
> +}
> +
I think you want to use jump_labels instead of this to make
the callback optional. This is already used all over the place
in the generic code.

> +/*
> + * This function provides the context switch callback to the lower code
> + * layer. It is invoked ONLY when the context switch callback is enabled.
> + */
> +static void perf_pmu_sched_task(struct task_struct *prev,
> +                               struct task_struct *next,
> +                               bool sched_in)
> +{
> +       struct perf_cpu_context *cpuctx;
> +       struct pmu *pmu;
> +       unsigned long flags;
> +
> +       if (prev == next)
> +               return;
> +
> +       local_irq_save(flags);
> +
> +       rcu_read_lock();
> +
> +       pmu = idr_find(&pmu_idr, PERF_TYPE_RAW);
> +
> +       if (pmu && pmu->sched_task) {
> +               cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
> +               pmu = cpuctx->ctx.pmu;
> +
> +               perf_ctx_lock(cpuctx, cpuctx->task_ctx);
> +
> +               perf_pmu_disable(pmu);
> +
> +               pmu->sched_task(cpuctx->task_ctx, sched_in);
> +
> +               perf_pmu_enable(pmu);
> +
> +               perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
> +       }
> +
> +       rcu_read_unlock();
> +
> +       local_irq_restore(flags);
> +}
> +
>  #define for_each_task_context_nr(ctxn)                                 \
>         for ((ctxn) = 0; (ctxn) < perf_nr_task_contexts; (ctxn)++)
>
> @@ -2346,6 +2399,9 @@ void __perf_event_task_sched_out(struct task_struct *task,
>  {
>         int ctxn;
>
> +       if (__get_cpu_var(perf_sched_cb_usages))
> +               perf_pmu_sched_task(task, next, false);
> +
>         for_each_task_context_nr(ctxn)
>                 perf_event_context_sched_out(task, ctxn, next);
>
> @@ -2605,6 +2661,9 @@ void __perf_event_task_sched_in(struct task_struct *prev,
>         /* check for system-wide branch_stack events */
>         if (atomic_read(&__get_cpu_var(perf_branch_stack_events)))
>                 perf_branch_stack_sched_in(prev, task);
> +
> +       if (__get_cpu_var(perf_sched_cb_usages))
> +               perf_pmu_sched_task(prev, task, true);
>  }
>
>  static u64 perf_calculate_period(struct perf_event *event, u64 nsec, u64 count)
> @@ -6326,7 +6385,6 @@ static void free_pmu_context(struct pmu *pmu)
>  out:
>         mutex_unlock(&pmus_lock);
>  }
> -static struct idr pmu_idr;
>
>  static ssize_t
>  type_show(struct device *dev, struct device_attribute *attr, char *page)
> --
> 1.8.4.2
>
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