linux-kernel - Re: [PATCH v2 2/4] perf/x86: Fix data source decoding for Skylake

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Date:   Thu, 8 Jun 2017 22:03:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jiri Olsa <jolsa@...nel.org>, Andi Kleen <ak@...ux.intel.com>,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 2/4] perf/x86: Fix data source decoding for Skylake

On Thu, Jun 08, 2017 at 12:40:59PM -0700, Stephane Eranian wrote:
> Hi,
> 
> On Thu, Jun 8, 2017 at 1:15 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Wed, Jun 07, 2017 at 04:22:24PM -0700, Andi Kleen wrote:
> >
> > > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> > > index b1c0b187acfe..95daade294d7 100644
> > > --- a/include/uapi/linux/perf_event.h
> > > +++ b/include/uapi/linux/perf_event.h
> > > @@ -931,14 +931,18 @@ union perf_mem_data_src {
> > >                       mem_snoop:5,    /* snoop mode */
> > >                       mem_lock:2,     /* lock instr */
> > >                       mem_dtlb:7,     /* tlb access */
> > > -                     mem_rsvd:31;
> > > +                     mem_lvlx:8,     /* memory hierarchy level, ext */
> > > +                     mem_snoopx:2,   /* snoop mode, ext */
> > > +                     mem_rsvd:21;
> > >       };
> > >  };
> > >  #elif defined(__BIG_ENDIAN_BITFIELD)
> > >  union perf_mem_data_src {
> > >       __u64 val;
> > >       struct {
> > > -             __u64   mem_rsvd:31,
> > > +             __u64   mem_rsvd:21,
> > > +                     mem_snoopx:2,   /* snoop mode, ext */
> > > +                     mem_lvlx:8,     /* memory hierarchy level, ext */
> > >                       mem_dtlb:7,     /* tlb access */
> > >                       mem_lock:2,     /* lock instr */
> > >                       mem_snoop:5,    /* snoop mode */
> >
> > So one thing we could do is add a mem_hops field and always set that,
> > even for the old stuff. The old stuff will not know about that field and
> > ignore the bits, but new stuff will then not need as many LVL bits.
> >
> That would be better than lvlx I think. I am guessing you're suggesting
> an integer count here and not a bitmask. Right? 

Yah, 0 hops = local, etc..

> Then I wonder why it
> would need 8 bits or 255 possible levels!

I thing we still need lvlx, simply because the current lvl doesn't have
room to encode L4.

But having a mem_hops field avoids having to have local/remote/remote2
variants of everything.

That said, I'm afraid SGI can actually fill a mem_hops:8 or something like
that ;-)