Re: [PATCH v3 11/14] perf/hw_breakpoint: Reduce contention with large number of tasks

[Marco Elver <elver@google.com>]

On Wed, 17 Aug 2022 at 15:03, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Jul 04, 2022 at 05:05:11PM +0200, Marco Elver wrote:
> > +static bool bp_constraints_is_locked(struct perf_event *bp)
> > +{
> > +     struct mutex *tsk_mtx = get_task_bps_mutex(bp);
> > +
> > +     return percpu_is_write_locked(&bp_cpuinfo_sem) ||
> > +            (tsk_mtx ? mutex_is_locked(tsk_mtx) :
> > +                       percpu_is_read_locked(&bp_cpuinfo_sem));
> > +}
>
> > @@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
> >   */
> >  int dbg_reserve_bp_slot(struct perf_event *bp)
> >  {
> > -     if (mutex_is_locked(&nr_bp_mutex))
> > +     int ret;
> > +
> > +     if (bp_constraints_is_locked(bp))
> >               return -1;
> >
> > -     return __reserve_bp_slot(bp, bp->attr.bp_type);
> > +     /* Locks aren't held; disable lockdep assert checking. */
> > +     lockdep_off();
> > +     ret = __reserve_bp_slot(bp, bp->attr.bp_type);
> > +     lockdep_on();
> > +
> > +     return ret;
> >  }
> >
> >  int dbg_release_bp_slot(struct perf_event *bp)
> >  {
> > -     if (mutex_is_locked(&nr_bp_mutex))
> > +     if (bp_constraints_is_locked(bp))
> >               return -1;
> >
> > +     /* Locks aren't held; disable lockdep assert checking. */
> > +     lockdep_off();
> >       __release_bp_slot(bp, bp->attr.bp_type);
> > +     lockdep_on();
> >
> >       return 0;
> >  }
>
> Urggghhhh... this is horrible crap. That is, the current code is that
> and this makes it worse :/

Heh, yes and when I looked at it I really wanted to see if it can
change. But from what I can tell, when the kernel debugger is being
attached, the kernel does stop everything it does and we need the
horrible thing above to not deadlock. And these dbg_ functions are not
normally used, so I decided to leave it as-is. Suggestions?