Re: [PATCH 02/26] perf: Introduce mmap3 version of mmap event

[Jiri Olsa <jolsa@redhat.com>]

On Sun, Sep 13, 2020 at 11:41:00PM -0700, Stephane Eranian wrote:
> On Sun, Sep 13, 2020 at 2:03 PM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Add new version of mmap event. The MMAP3 record is an
> > augmented version of MMAP2, it adds build id value to
> > identify the exact binary object behind memory map:
> >
> >   struct {
> >     struct perf_event_header header;
> >
> >     u32                      pid, tid;
> >     u64                      addr;
> >     u64                      len;
> >     u64                      pgoff;
> >     u32                      maj;
> >     u32                      min;
> >     u64                      ino;
> >     u64                      ino_generation;
> >     u32                      prot, flags;
> >     u32                      reserved;
> >     u8                       buildid[20];
> >     char                     filename[];
> >     struct sample_id         sample_id;
> >   };
> >
> > Adding 4 bytes reserved field to align buildid data to 8 bytes,
> > so sample_id data is properly aligned.
> >
> > The mmap3 event is enabled by new mmap3 bit in perf_event_attr
> > struct.  When set for an event, it enables the build id retrieval
> > and will use mmap3 format for the event.
> >
> > Keeping track of mmap3 events and calling build_id_parse
> > in perf_event_mmap_event only if we have any defined.
> >
> > Having build id attached directly to the mmap event will help
> > tool like perf to skip final search through perf data for
> > binaries that are needed in the report time. Also it prevents
> > possible race when the binary could be removed or replaced
> > during profiling.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  include/uapi/linux/perf_event.h | 27 ++++++++++++++++++++++-
> >  kernel/events/core.c            | 38 +++++++++++++++++++++++++++------
> >  2 files changed, 57 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> > index 077e7ee69e3d..facfc3c673ed 100644
> > --- a/include/uapi/linux/perf_event.h
> > +++ b/include/uapi/linux/perf_event.h
> > @@ -384,7 +384,8 @@ struct perf_event_attr {
> >                                 aux_output     :  1, /* generate AUX records instead of events */
> >                                 cgroup         :  1, /* include cgroup events */
> >                                 text_poke      :  1, /* include text poke events */
> > -                               __reserved_1   : 30;
> > +                               mmap3          :  1, /* include bpf events */
> > +                               __reserved_1   : 29;
> >
> what happens if I set mmap3 and mmap2?

hum bad things probably ;-) I think mmap3 would overload mmap2

> 
> I think using mmap3 for every mmap may be overkill as you add useless
> 20 bytes to an mmap record.
> I am not sure if your code handles the case where mmap3 is not needed
> because there is no buildid, e.g, anonymous memory.
> It seems to me you've written the patch in such a way that if the user
> tool supports mmap3, then it supersedes mmap2, and thus
> you need all the fields of mmap2. But if could be more interesting to
> return either MMAP2 or MMAP3 depending on tool support
> and type of mmap, that would certainly save 20 bytes on any anon mmap.
> But maybe that logic is already in your patch and I missed it.

I like peter's idea of ditching mmap3 and use that maj/min..
area in mmap2 for buildid based on misc bits

jirka