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SubjectRe: [PATCH v3 08/10] perf: cache perf_event_groups_first for cgroups
On Thu, Nov 14, 2019 at 2:25 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Nov 13, 2019 at 04:30:40PM -0800, Ian Rogers wrote:
> > Add a per-CPU cache of the pinned and flexible perf_event_groups_first
> > value for a cgroup avoiding an O(log(#perf events)) searches during
> > sched_in.
> >
> > Based-on-work-by: Kan Liang <kan.liang@linux.intel.com>
> > Signed-off-by: Ian Rogers <irogers@google.com>
> > ---
> > include/linux/perf_event.h | 6 +++
> > kernel/events/core.c | 79 +++++++++++++++++++++++++++-----------
> > 2 files changed, 62 insertions(+), 23 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index b3580afbf358..cfd0b320418c 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -877,6 +877,12 @@ struct perf_cgroup_info {
> > struct perf_cgroup {
> > struct cgroup_subsys_state css;
> > struct perf_cgroup_info __percpu *info;
> > + /* A cache of the first event with the perf_cpu_context's
> > + * perf_event_context for the first event in pinned_groups or
> > + * flexible_groups. Avoids an rbtree search during sched_in.
> > + */
>
> Broken comment style.

Done.

> > + struct perf_event * __percpu *pinned_event;
> > + struct perf_event * __percpu *flexible_event;
>
> Where is the actual storage allocated? There is a conspicuous lack of
> alloc_percpu() in this patch, see for example perf_cgroup_css_alloc()
> which fills out the above @info field.

Apologies, missed from Kan's original patch but was in v2. Added again.

> > };
> >
> > /*
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 11594d8bbb2e..9f0febf51d97 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -1638,6 +1638,25 @@ perf_event_groups_insert(struct perf_event_groups *groups,
> >
> > rb_link_node(&event->group_node, parent, node);
> > rb_insert_color(&event->group_node, &groups->tree);
> > +#ifdef CONFIG_CGROUP_PERF
> > + if (is_cgroup_event(event)) {
> > + struct perf_event **cgrp_event;
> > +
> > + if (event->attr.pinned)
> > + cgrp_event = per_cpu_ptr(event->cgrp->pinned_event,
> > + event->cpu);
> > + else
> > + cgrp_event = per_cpu_ptr(event->cgrp->flexible_event,
> > + event->cpu);
>
> Codingstyle requires { } here (or just bust the line length a little).

Done.

> > + /*
> > + * Cgroup events for the same cgroup on the same CPU will
> > + * always be inserted at the right because of bigger
> > + * @groups->index. Only need to set *cgrp_event when it's NULL.
> > + */
> > + if (!*cgrp_event)
> > + *cgrp_event = event;
>
> I would feel much better if you had some actual leftmost logic in the
> insertion iteration.

Done. Also altered the comment to address the possibility of overflow.

> > + }
> > +#endif
> > }
> >
> > /*
> > @@ -1652,6 +1671,9 @@ add_event_to_groups(struct perf_event *event, struct perf_event_context *ctx)
> > perf_event_groups_insert(groups, event);
> > }
> >
> > +static struct perf_event *
> > +perf_event_groups_next(struct perf_event *event);
> > +
> > /*
> > * Delete a group from a tree.
> > */
> > @@ -1662,6 +1684,22 @@ perf_event_groups_delete(struct perf_event_groups *groups,
> > WARN_ON_ONCE(RB_EMPTY_NODE(&event->group_node) ||
> > RB_EMPTY_ROOT(&groups->tree));
> >
> > +#ifdef CONFIG_CGROUP_PERF
> > + if (is_cgroup_event(event)) {
> > + struct perf_event **cgrp_event;
> > +
> > + if (event->attr.pinned)
> > + cgrp_event = per_cpu_ptr(event->cgrp->pinned_event,
> > + event->cpu);
> > + else
> > + cgrp_event = per_cpu_ptr(event->cgrp->flexible_event,
> > + event->cpu);
>
> Codingstyle again.

Done.

> > +
> > + if (*cgrp_event == event)
> > + *cgrp_event = perf_event_groups_next(event);
> > + }
> > +#endif
> > +
> > rb_erase(&event->group_node, &groups->tree);
> > init_event_group(event);
> > }
> > @@ -1679,20 +1717,14 @@ del_event_from_groups(struct perf_event *event, struct perf_event_context *ctx)
> > }
> >
> > /*
> > - * Get the leftmost event in the cpu/cgroup subtree.
> > + * Get the leftmost event in the cpu subtree without a cgroup (ie task or
> > + * system-wide).
> > */
> > static struct perf_event *
> > -perf_event_groups_first(struct perf_event_groups *groups, int cpu,
> > - struct cgroup *cgrp)
> > +perf_event_groups_first_no_cgroup(struct perf_event_groups *groups, int cpu)
>
> I'm going to impose a function name length limit soon :/ That's insane
> (again).

Done, with the argument added back in.

> > {
> > struct perf_event *node_event = NULL, *match = NULL;
> > struct rb_node *node = groups->tree.rb_node;
> > -#ifdef CONFIG_CGROUP_PERF
> > - int node_cgrp_id, cgrp_id = 0;
> > -
> > - if (cgrp)
> > - cgrp_id = cgrp->id;
> > -#endif
> >
> > while (node) {
> > node_event = container_of(node, struct perf_event, group_node);
> > @@ -1706,18 +1738,10 @@ perf_event_groups_first(struct perf_event_groups *groups, int cpu,
> > continue;
> > }
> > #ifdef CONFIG_CGROUP_PERF
> > - node_cgrp_id = 0;
> > - if (node_event->cgrp && node_event->cgrp->css.cgroup)
> > - node_cgrp_id = node_event->cgrp->css.cgroup->id;
> > -
> > - if (cgrp_id < node_cgrp_id) {
> > + if (node_event->cgrp) {
> > node = node->rb_left;
> > continue;
> > }
> > - if (cgrp_id > node_cgrp_id) {
> > - node = node->rb_right;
> > - continue;
> > - }
> > #endif
> > match = node_event;
> > node = node->rb_left;
>
> Also, just leave that in and let callers have: .cgrp = NULL. Then you
> can forgo that monstrous name.

Done. It is a shame that there is some extra logic for the task/no-cgroup case.

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