linux-kernel - Re: [PATCH 1/1] perf/x86: filter branches for PEBS event

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Date:	Thu, 26 Mar 2015 12:40:14 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] perf/x86: filter branches for PEBS event

On Thu, Mar 26, 2015 at 11:13 AM,  <kan.liang@...el.com> wrote:
> From: Kan Liang <kan.liang@...el.com>
>
> For supporting Intel LBR branches filtering, Intel LBR sharing logic
> mechanism is introduced from commit b36817e88630 ("perf/x86: Add Intel
> LBR sharing logic"). It modifies __intel_shared_reg_get_constraints to
> config lbr_sel, which is finally used to set LBR_SELECT.
> However, the intel_shared_regs_constraints is called after
> intel_pebs_constraints. The PEBS event will return immediately after
> intel_pebs_constraints. So it's impossible to filter branches for PEBS
> event.
>
> This patch move intel_shared_regs_constraints for branch_reg ahead of
> intel_pebs_constraints.
> intel_shared_regs_constraints for branch_reg doesn't modify event->hw,
> so it's safe to be called before intel_pebs_constraints.
> intel_shared_regs_constraints for branch_reg also special case when it
> returns &emptyconstraint. It put constraints for extra_reg. This patch
> remove it. Because it will never get constraints for extra_reg if return
> is &emptyconstraint.
>
> Signed-off-by: Kan Liang <kan.liang@...el.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel.c | 34 ++++++++++++----------------------
>  1 file changed, 12 insertions(+), 22 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
> index 9f1dd18..247780a 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> @@ -1587,25 +1587,14 @@ __intel_shared_reg_put_constraints(struct cpu_hw_events *cpuc,
>
>  static struct event_constraint *
>  intel_shared_regs_constraints(struct cpu_hw_events *cpuc,
> -                             struct perf_event *event)
> +                             struct perf_event *event,
> +                             struct hw_perf_event_extra *reg)
>  {
> -       struct event_constraint *c = NULL, *d;
> -       struct hw_perf_event_extra *xreg, *breg;
> +       struct event_constraint *c = NULL;
> +
> +       if (reg->idx != EXTRA_REG_NONE)
> +               c = __intel_shared_reg_get_constraints(cpuc, event, reg);
>
> -       xreg = &event->hw.extra_reg;
> -       if (xreg->idx != EXTRA_REG_NONE) {
> -               c = __intel_shared_reg_get_constraints(cpuc, event, xreg);
> -               if (c == &emptyconstraint)
> -                       return c;
> -       }
> -       breg = &event->hw.branch_reg;
> -       if (breg->idx != EXTRA_REG_NONE) {
> -               d = __intel_shared_reg_get_constraints(cpuc, event, breg);
> -               if (d == &emptyconstraint) {
> -                       __intel_shared_reg_put_constraints(cpuc, xreg);
> -                       c = d;
> -               }
> -       }
>         return c;
>  }
>
> @@ -1629,17 +1618,18 @@ x86_get_event_constraints(struct cpu_hw_events *cpuc, struct perf_event *event)
>  static struct event_constraint *
>  intel_get_event_constraints(struct cpu_hw_events *cpuc, struct perf_event *event)
>  {
> -       struct event_constraint *c;
> +       struct event_constraint *c, *d;
>
>         c = intel_bts_constraints(event);
>         if (c)
>                 return c;
>
> -       c = intel_pebs_constraints(event);
> -       if (c)
> -               return c;
> +       c = intel_shared_regs_constraints(cpuc, event, &event->hw.branch_reg);
> +       d = intel_pebs_constraints(event);
> +       if (d || c)
> +               return (d) ? (d) : (c);
>
> -       c = intel_shared_regs_constraints(cpuc, event);
> +       c = intel_shared_regs_constraints(cpuc, event, &event->hw.extra_reg);
>         if (c)
>                 return c;
>
You are addressing one of the problems of this routine. But I think
there is a more serious
issue which is not addressed here. The intel_shared_regs_constraints()
assumes that
the associated event is necessarily unconstrained:

__intel_shared_reg_get_constraints()
{
    struct event_constraint *c = &emptyconstraint;
   ...
}

This is true for offcore_response, but for LBR this may not always be the case.
I may want to use LBR on the L1D_PEND_MISS event and it would need to
be on counter 2. But I believe that the current code could place it on counter 0
simply because you return if shared_reg_get_constraint() is successful, but
it looks only at the LBR constraint not the event constraint. I think
in the presence
of LBR, you always need to call share_get_reg() and x86_get_event_constraint().
This is to ensure that both the shared constraint AND the event constraint are
satisfied (and of course, in case one fails, the other needs to be released).


> --
> 1.8.3.2
>
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