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

On Mon, 29 Aug 2022 at 10:38, Peter Zijlstra [off-list ref] wrote:

On Wed, Aug 17, 2022 at 03:14:54PM +0200, Marco Elver wrote:

quoted

On Wed, 17 Aug 2022 at 15:03, Peter Zijlstra [off-list ref] wrote:

quoted

On Mon, Jul 04, 2022 at 05:05:11PM +0200, Marco Elver wrote:

quoted

+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));
+}

quoted

@@ -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?

What context is this ran in? NMI should already have lockdep disabled.

kgdb can enter via kgdb_nmicall*() but also via
kgdb_handle_exception(), which isn't for NMI.