Re: [PATCH v3 1/n] perf/core: addressing 4x slowdown during per-process profiling of STREAM benchmark on Intel Xeon Phi

[Alexey Budankov]

On 16.06.2017 12:09, Mark Rutland wrote:
> On Fri, Jun 16, 2017 at 01:10:10AM +0300, Alexey Budankov wrote:
> > On 15.06.2017 22:56, Mark Rutland wrote:
> > > On Thu, Jun 15, 2017 at 08:41:42PM +0300, Alexey Budankov wrote:
> > > > This series of patches continues v2 and addresses captured comments.
>
> > > > Specifically this patch replaces pinned_groups and flexible_groups
> > > > lists of perf_event_context by red-black cpu indexed trees avoiding
> > > > data structures duplication and introducing possibility to iterate
> > > > event groups for a specific CPU only.
> > >
> > > If you use --per-thread, I take it the overhead is significantly
> > > lowered?
> >
> > Please ask more.
>
> IIUC, you're seeing the slowdown when using perf record, correct?

Correct. Specifically in per-process mode - without -a option.

> There's a --per-thread option to ask perf record to not duplicate the
> event per-cpu.
>
> If you use that, what amount of slowdown do you see?
>
> It might be preferable to not open task-bound per-cpu events on systems
> with large cpu counts, and it would be good to know what the trade-off
> looks like for this case.
>
> > > > +static void
> > > > +perf_cpu_tree_insert(struct rb_root *tree, struct perf_event *event)
> > > > +{
> > > > +	struct rb_node **node;
> > > > +	struct rb_node *parent;
> > > > +
> > > > +	WARN_ON_ONCE(!tree || !event);
> > > > +
> > > > +	node = &tree->rb_node;
> > > > +	parent = *node;
> > >
> > > The first iteration of the loop handles this, so it can go.
> >
> > If tree is empty parent will be uninitialized what is harmful.
>
> Sorry; my bad.
>
> Thanks,
> Mark.