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

[Alexey Budankov]

On 20.06.2017 16:44, Mark Rutland wrote:
> On Fri, Jun 16, 2017 at 02:03:58AM +0300, Alexey Budankov wrote:
> > perf/core: use rb trees for pinned/flexible groups
> >
> > By default, the userspace perf tool opens per-cpu task-bound events
> > when sampling, so for N logical events requested by the user, the tool
> > will open N * NR_CPUS events.
> >
> > In the kernel, we mux events with a hrtimer, periodically rotating the
> > flexible group list and trying to schedule each group in turn. We
> > skip groups whose cpu filter doesn't match. So when we get unlucky,
> > we can walk N * (NR_CPUS - 1) groups pointlessly for each hrtimer
> > invocation.
> >
> > This has been observed to result in significant overhead when running
> > the STREAM benchmark on 272 core Xeon Phi systems.
> >
> > One way to avoid this is to place our events into an rb tree sorted by
> > CPU filter, so that our hrtimer can skip to the current CPU's
> > list and ignore everything else.
> >
> > As a step towards that, this patch replaces event group lists with rb
> > trees.
> >
> > Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
> > ---
> >   include/linux/perf_event.h |  18 ++-
> >   kernel/events/core.c       | 393
> > +++++++++++++++++++++++++++++++++------------
> >   2 files changed, 307 insertions(+), 104 deletions(-)
> >
> > Addressed Mark Rutland's comments from the previous patch version.
>
> ... then this should be v4, no?
>
> Which comments? Could you pelase write a changelog in future?

Changes are:

1. changed type of pinned_groups/flexible_groups to rb_tree;
2. removed group_list_entry and reused group_entry for that purposes;
3. added add_to_groups()/del_from_groups() helper functions;

> In future, please send patches as a series, with a known upper-bound
> rather than N. It's really painful to find them when they're sent
> separately, without a known upper bound.

Accepted.

> [...]
>
> > +/*
> > + * Delete a group from a tree. If the group is directly attached to
> > the tree
> > + * it also detaches all groups on the group's group_list list.
> > + */
> > +static void
> > +perf_cpu_tree_delete(struct rb_root *tree, struct perf_event *event)
> > +{
> > +	WARN_ON_ONCE(!tree || !event);
> > +
> > +	if (RB_EMPTY_NODE(&event->group_node)) {
> > +		list_del_init(&event->group_entry);
> > +	} else {
> > +		struct perf_event *list_event, *list_next;
> > +
> > +		rb_erase(&event->group_node, tree);
> > +		list_for_each_entry_safe(list_event, list_next,
> > +				&event->group_list, group_entry)
> > +			list_del_init(&list_event->group_entry);
> > +	}
> > +}
>
> As I commented on the last version, this means that all groups which
> were (incidentally) hanging off of this one are removed, and can
> no longer be reached via the tree.
>
> Surely one of the remaining groups should be added to the tree?

Aww, I see. That needs to implemented. Thanks.

> I don't beleive that is correct.
>
> I beleive it would be simpler to reason about a threaded rb-tree here,
> since that special case wouldn't exist.
>
> Thanks,
> Mark.