linux-kernel - Re: [BUG] segfault in perf-top -- thread refcnt

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Date:	Tue, 31 Mar 2015 09:15:15 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc:	Jiri Olsa <jolsa@...hat.com>, David Ahern <dsahern@...il.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] segfault in perf-top -- thread refcnt

Hi Arnaldo,

On Mon, Mar 30, 2015 at 11:02:39AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Mar 30, 2015 at 10:06:35PM +0900, Namhyung Kim escreveu:
> > On Mon, Mar 30, 2015 at 9:56 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> > > On Mon, Mar 30, 2015 at 09:48:52PM +0900, Namhyung Kim wrote:
> > >> > @@ -380,10 +381,13 @@ static struct thread *__machine__findnew_thread(struct machine *machine,
> > >> >     if (!create)
> > >> >             return NULL;
> > >> >
> > >> > -   th = thread__new(pid, tid);
> > >> > +   th = thread__new(machine, pid, tid);
> > >> >     if (th != NULL) {
> > >> > +
> > >> > +           pthread_mutex_lock(&machine->threads_lock);
> > >> >             rb_link_node(&th->rb_node, parent, p);
> > >> >             rb_insert_color(&th->rb_node, &machine->threads);
> > >> > +           pthread_mutex_unlock(&machine->threads_lock);
> > >>
> > >> I think you also need to protect the rb tree traversal above.
> > >
> > > yep, I already have another version.. but it blows on another place ;-)
> > >
> > >>
> > >> But this makes every sample processing grabs and releases the lock so
> > >> might cause high overhead.  It can be a problem if such processing is
> > >> done parallelly like my multi-thread work. :-/
> > >
> > > yep.. perhaps instead of more locking we need to find a way where
> > > only single thread do the update on hists/threads
> > 
> > Agreed.
> > 
> > AFAIK the reason we do ref-counting is to cleanup dead/exited thread
> > for live session like perf top.  In that case we can somehow mark
> > to-be-deleted thread and kill it in a safe time/place..
> 
> Humm, you mean have another list node in struct threads and add threads
> to another dead_threads like list, i.e. one that is _really_ dead as no
> more refcounts point to it, and then amortize the costs of removing it
> from the rb_tree by removing multiple threads instead of just one?

Yes, I really want to avoid any overhead on the fastpath.  Instead of
refcnt, how about marking and deleting dead threads by the perf top's
display thread unless it's not a selected one?

Thanks,
Namhyung
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