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

Jiri Olsa
2020-09-14T19:51:07Z

On Mon, Sep 14, 2020 at 12:31:34PM -0300, Arnaldo Carvalho de Melo wrote:

SNIP

> > > ---
> > > 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?
> >
> > I think using mmap3 for every mmap may be overkill as you add useless
> > 20 bytes to an mmap record.
>
> So use just PERF_RECORD_MMAP2.
>
> I think if the user says: I need buildids, then, in kernels with support
> for getting the buildid in MMAP records, use it as its more accurate,
> otherwise fall back to traversing all records at the end to go over lots
> of files haversting those build-ids.

ok, so special record option to enable this

>
> If the user says I don't want build-ids, nothing changes, no collection
> at the end, perf continues using PERF_RECORD_MMAP2.

and that's -B option in 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.
>
> Right, it should take into account if the user asked for build-ids or
> not in addition to checking if the kernel supports MMAP3.

right, thanks,

jirka