linux-kernel - Re: [PATCH 09/14] perf, x86: Save/resotre LBR stack during context switch

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Date:	Wed, 5 Feb 2014 18:45:15 +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 09/14] perf, x86: Save/resotre LBR stack during context switch

On Fri, Jan 3, 2014 at 6:48 AM, Yan, Zheng <zheng.z.yan@...el.com> wrote:
> When the LBR call stack is enabled, it is necessary to save/restore
> the LBR stack on context switch. The solution is saving/restoring
> the LBR stack to/from task's perf event context.
>
> The LBR stack is saved/restored only when there are events that use
> the LBR call stack. If no event uses LBR call stack, the LBR stack
> is reset when task is scheduled in.
>
> Signed-off-by: Yan, Zheng <zheng.z.yan@...el.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel_lbr.c | 80 ++++++++++++++++++++++++------
>  1 file changed, 66 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> index 2137a9f..51e1842 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> @@ -187,18 +187,82 @@ void intel_pmu_lbr_reset(void)
>                 intel_pmu_lbr_reset_64();
>  }
>
> +/*
> + * TOS = most recently recorded branch
> + */
> +static inline u64 intel_pmu_lbr_tos(void)
> +{
> +       u64 tos;
> +       rdmsrl(x86_pmu.lbr_tos, tos);
> +       return tos;
> +}
> +
> +enum {
> +       LBR_UNINIT,
> +       LBR_NONE,
> +       LBR_VALID,
> +};
> +
I don't see where the x86_perf_task_context struct gets initialized with
your task_ctx_data/task_ctx_size mechanism. You are relying on 0
as a valid default value. But if later more fields are needed and they need
non-zero init values, it will be easy to forget.....

So I think you need to provide a callback from alloc_perf_context().
Should have mentioned that in Patch 05/14.

> +static void __intel_pmu_lbr_restore(struct x86_perf_task_context *task_ctx)
> +{
> +       int i;
> +       unsigned lbr_idx, mask = x86_pmu.lbr_nr - 1;
> +       u64 tos = intel_pmu_lbr_tos();
> +
> +       for (i = 0; i < x86_pmu.lbr_nr; i++) {
> +               lbr_idx = (tos - i) & mask;
> +               wrmsrl(x86_pmu.lbr_from + lbr_idx, task_ctx->lbr_from[i]);
> +               wrmsrl(x86_pmu.lbr_to + lbr_idx, task_ctx->lbr_to[i]);
> +       }
> +       task_ctx->lbr_stack_state = LBR_NONE;
> +}
> +
> +static void __intel_pmu_lbr_save(struct x86_perf_task_context *task_ctx)
> +{
> +       int i;
> +       unsigned lbr_idx, mask = x86_pmu.lbr_nr - 1;
> +       u64 tos = intel_pmu_lbr_tos();
> +
> +       for (i = 0; i < x86_pmu.lbr_nr; i++) {
> +               lbr_idx = (tos - i) & mask;
> +               rdmsrl(x86_pmu.lbr_from + lbr_idx, task_ctx->lbr_from[i]);
> +               rdmsrl(x86_pmu.lbr_to + lbr_idx, task_ctx->lbr_to[i]);
> +       }
> +       task_ctx->lbr_stack_state = LBR_VALID;
> +}
> +
> +
>  void intel_pmu_lbr_sched_task(struct perf_event_context *ctx, bool sched_in)
>  {
> +       struct cpu_hw_events *cpuc;
> +       struct x86_perf_task_context *task_ctx;
> +
>         if (!x86_pmu.lbr_nr)
>                 return;
>
> +       cpuc = &__get_cpu_var(cpu_hw_events);
> +       task_ctx = ctx ? ctx->task_ctx_data : NULL;
> +
> +
>         /*
>          * It is necessary to flush the stack on context switch. This happens
>          * when the branch stack does not tag its entries with the pid of the
>          * current task.
>          */
> -       if (sched_in)
> -               intel_pmu_lbr_reset();
> +       if (sched_in) {
> +               if (!task_ctx ||
> +                   !task_ctx->lbr_callstack_users ||
> +                   task_ctx->lbr_stack_state != LBR_VALID)
> +                       intel_pmu_lbr_reset();
> +               else
> +                       __intel_pmu_lbr_restore(task_ctx);
> +       } else if (task_ctx) {
> +               if (task_ctx->lbr_callstack_users &&
> +                   task_ctx->lbr_stack_state != LBR_UNINIT)
> +                       __intel_pmu_lbr_save(task_ctx);
> +               else
> +                       task_ctx->lbr_stack_state = LBR_NONE;
> +       }
>  }
>
There ought to be a better way of structuring this if/else. It is
ugly.

>  static inline bool branch_user_callstack(unsigned br_sel)
> @@ -267,18 +331,6 @@ void intel_pmu_lbr_disable_all(void)
>                 __intel_pmu_lbr_disable();
>  }
>
> -/*
> - * TOS = most recently recorded branch
> - */
> -static inline u64 intel_pmu_lbr_tos(void)
> -{
> -       u64 tos;
> -
> -       rdmsrl(x86_pmu.lbr_tos, tos);
> -
> -       return tos;
> -}
> -
>  static void intel_pmu_lbr_read_32(struct cpu_hw_events *cpuc)
>  {
>         unsigned long mask = x86_pmu.lbr_nr - 1;
> --
> 1.8.4.2
>
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