Re: [PATCH RFC] hist lookups

Jiri Olsa
2018-11-07T19:43:49Z

On Wed, Nov 07, 2018 at 09:32:17AM +0100, Jiri Olsa wrote:
> On Tue, Nov 06, 2018 at 10:13:49PM -0800, David Miller wrote:
> > From: Jiri Olsa <jolsa@redhat.com>
> > Date: Tue, 6 Nov 2018 21:42:55 +0100
> >
> > > I pushed that fix in perf/fixes branch, but I'm still occasionaly
> > > hitting the namespace crash.. working on it ;-)
> >
> > Jiri, how can this new scheme work without setting copy_on_queue
> > for the queued_events we use here?
>
> aahh.. it won't, setting it up ;-)
>
> >
> > I don't see copy_on_queue being set and that means the queued event
> > structures reference the event memory directly in the mmaps, after the
> > mmap thread has released them back to the queue.
> >
> > That means new events can come in to the mmap ring and overwrite what
> > was there previously, maybe even while deliver_event() is in the
> > middle of parsing the event.
> >
> > Setting copy_on_queue for data[0] and data[1] makes all of the crashes
> > go away for me.
> >
> > I get a lot of "[unknown]" shared objects shortly after perf top
> > starts up during a full workload. I've been wondering about one
> > side effect of how the mmap queues are processed, consider the
> > following:
> >
> > cpu 0 cpu 1
> >
> > exec
> > create new mmap2 events
> > scheduled to cpu 0 for whatever reason
> > sample 1
> > sample 2
> >
> > And let's say that perf top is backlogged processing the mmap ring of
> > events generated for cpu 0, and sees sample 1 and sample 2 before
> > getting to any of cpu 1's events.
> >
> > This means the thread and map and symbol objects won't exist and
> > we'll get those '[Unknown]' histogram entries, and they won't go
> > away.
> >
> > When it finally stops looping over the mmap ring for cpu 0's events
> > it gets to cpu 1's mmap ring and sees the exec and mmap2 events
> > but at that point it's far too late.
> >
> > I surmise from what I see with perf top right now that this happens
> > a lot.
>
> right, there's no reason why top should have different standards than
> record/report.. above can definitely happen, I'll enable time sample
> type and use ordered events for the queue

I pushed new version in my perf/fixes branch

jirka