linux-kernel - Re: [PATCH v3 1/2] perf/core: Share an event with multiple cgroups

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Date:   Tue, 20 Apr 2021 12:28:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Song Liu <songliubraving@...com>, Tejun Heo <tj@...nel.org>,
        kernel test robot <lkp@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/2] perf/core: Share an event with multiple cgroups

On Fri, Apr 16, 2021 at 06:49:09PM +0900, Namhyung Kim wrote:
> On Thu, Apr 15, 2021 at 11:51 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > > +static void perf_update_cgroup_node(struct perf_event *event, struct cgroup *cgrp)
> > > +{
> > > +     u64 delta_count, delta_time_enabled, delta_time_running;
> > > +     int i;
> > > +
> > > +     if (event->cgrp_node_count == 0)
> > > +             goto out;
> > > +
> > > +     delta_count = local64_read(&event->count) - event->cgrp_node_count;

>From here...

> > > +     delta_time_enabled = event->total_time_enabled - event->cgrp_node_time_enabled;
> > > +     delta_time_running = event->total_time_running - event->cgrp_node_time_running;
> > > +
> > > +     /* account delta to all ancestor cgroups */
> > > +     for (i = 0; i <= cgrp->level; i++) {
> > > +             struct perf_cgroup_node *node;
> > > +
> > > +             node = find_cgroup_node(event, cgrp->ancestor_ids[i]);
> > > +             if (node) {
> > > +                     node->count += delta_count;
> > > +                     node->time_enabled += delta_time_enabled;
> > > +                     node->time_running += delta_time_running;
> > > +             }
> > > +     }

... till here, NMI could hit and increment event->count, which then
means that:

> > > +
> > > +out:
> > > +     event->cgrp_node_count = local64_read(&event->count);

This load doesn't match the delta_count load and events will go missing.

Obviously correct solution is:

	event->cgrp_node_count += delta_count;


> > > +     event->cgrp_node_time_enabled = event->total_time_enabled;
> > > +     event->cgrp_node_time_running = event->total_time_running;

And while total_time doesn't have that problem, consistency would then
have you do:

	event->cgrp_node_time_foo += delta_time_foo;

> >
> > This is wrong; there's no guarantee these are the same values you read
> > at the begin, IOW you could be loosing events.
> 
> Could you please elaborate?

You forgot NMI.