On Mon, Sep 08, 2014 at 01:39:58PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 08, 2014 at 12:01:22PM +0200, Peter Zijlstra wrote:
> > On Mon, Sep 08, 2014 at 11:48:55AM +0200, Peter Zijlstra wrote:
> >
> > > > The thing is; I don't understand those reasons. That commit log doesn't
> > > > explain.
> > >
> > > Ah wait, I finally see. I think we want to fix that exit path, not
> > > disallow the cloning.
> > >
> > > The thing is, by not allowing this optimization simple things like eg.
> > > pipe-test say very expensive.
> >
> > So its 179033b3e064 ("perf: Add PERF_EVENT_STATE_EXIT state for events
> > with exited task") that introduces the problem. Before that things would
> > work correctly afaict.
> >
> > The exit would remove from the context but leave the event in existence.
> > Both the fd and the inherited events would have references to it, only
> > once those are gone do we destroy the actual event.
>
> I have another 'problem' with 179033b3e064. What if you 'want' to
> continue monitoring after the initial task died? Eg. if you want to
> monitor crap that unconditionally daemonizes.
right.. did not think of that.. need to check more, but
seems like just the check for children should be enough
jirka
---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index bf482ccbdbe1..341d0b47ca14 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3568,6 +3568,19 @@ static int perf_event_read_one(struct perf_event *event,
return n * sizeof(u64);
}
+static bool is_event_hup(struct perf_event *event)
+{
+ bool no_children;
+
+ if (event->state != PERF_EVENT_STATE_EXIT)
+ return false;
+
+ mutex_lock(&event->child_mutex);
+ no_children = list_empty(&event->child_list);
+ mutex_unlock(&event->child_mutex);
+ return no_children;
+}
+
/*
* Read the performance event - simple non blocking version for now
*/
@@ -3582,8 +3595,7 @@ perf_read_hw(struct perf_event *event, char __user *buf,
size_t count)
* error state (i.e. because it was pinned but it couldn't be
* scheduled on to the CPU at some point).
*/
- if ((event->state == PERF_EVENT_STATE_ERROR) ||
- (event->state == PERF_EVENT_STATE_EXIT))
+ if ((event->state == PERF_EVENT_STATE_ERROR) || (is_event_hup(event)))
return 0;
if (count < event->read_size)
@@ -3614,7 +3626,7 @@ static unsigned int perf_poll(struct file *file,
poll_table *wait)
poll_wait(file, &event->waitq, wait);
- if (event->state == PERF_EVENT_STATE_EXIT)
+ if (is_event_hup(event))
return events;
/*
--
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