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SubjectRe: [PATCH] perf: Allow suppressing AUX records

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Jan 15, 2018 at 05:00:20PM +0200, Alexander Shishkin wrote:
> > It has been pointed out to me many times that it is useful to be able
> > to switch off AUX records to save the bandwidth for records that actually
> > matter, for example, in AUX overwrite mode.
> >
> > The usefulness of PERF_RECORD_AUX is in some of its flags, like the
> > TRUNCATED flag that tells the decoder where exactly gaps in the trace are.
> > The OVERWRITE flag, on the other hand will be set on every single record
> > in overwrite mode. However, a PERF_RECORD_AUX[flags=OVERWRITE] is
> > generated on every target task's sched_out, which over time adds up to
> > a lot of useless information.
> >
> > In case the existing userspace depends on AUX records in the overwrite
> > mode, we preserve the original behavior and add an opt-in for the new
> > behavior, wherein the 'useless' records get suppressed.
> >
> > This patch adds an attribute bit to the described effect.
> >
> > Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > Cc: Markus Metzger <markus.t.metzger@intel.com>
> > Cc: Adrian Hunter <adrian.hunter@intel.com>
> > ---
> > include/uapi/linux/perf_event.h | 3 ++-
> > kernel/events/core.c | 5 +++++
> > kernel/events/ring_buffer.c | 13 +++++++++++--
> > 3 files changed, 18 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> > index c77c9a2ebbbb..d7a981130561 100644
> > --- a/include/uapi/linux/perf_event.h
> > +++ b/include/uapi/linux/perf_event.h
> > @@ -370,7 +370,8 @@ struct perf_event_attr {
> > context_switch : 1, /* context switch data */
> > write_backward : 1, /* Write ring buffer from end to beginning */
> > namespaces : 1, /* include namespaces data */
> > - __reserved_1 : 35;
> > + suppress_aux : 1, /* don't generate PERF_RECORD_AUX */
> > + __reserved_1 : 34;
> >
> > union {
> > __u32 wakeup_events; /* wakeup every n events */
>
> So I'm basically fine with this patch, however I wonder if we really
> need this suppress flag and can't just unconditionally drop these
> events.
>
> Ash said that as far as he knows no Intel-PT user actually relies on it;
> Will is there anything ARM that is known to rely on them?
>
> In anycase, tentative ACK on this, unless we wants to be brave and forgo
> this flag.
>
> Ingo, any opinions?

Yeah, I'd suggest we just supress those record, and wait for complaints - let's
not complicate the ABI if not necessary?

Thanks,

Ingo

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