On Wed, Mar 13, 2019 at 6:52 AM Peter Zijlstra [off-list ref] wrote:

On Fri, Feb 08, 2019 at 10:05:40AM +0000, Patrick Bellasi wrote:

quoted

+/*
+ * When a task is enqueued on a rq, the clamp bucket currently defined by the
+ * task's uclamp::bucket_id is reference counted on that rq. This also
+ * immediately updates the rq's clamp value if required.
+ *
+ * Since tasks know their specific value requested from user-space, we track
+ * within each bucket the maximum value for tasks refcounted in that bucket.
+ * This provide a further aggregation (local clamping) which allows to track
+ * within each bucket the exact "requested" clamp value whenever all tasks
+ * RUNNABLE in that bucket require the same clamp.
+ */
+static inline void uclamp_rq_inc_id(struct task_struct *p, struct rq *rq,
+                                 unsigned int clamp_id)
+{
+     unsigned int bucket_id = p->uclamp[clamp_id].bucket_id;
+     unsigned int rq_clamp, bkt_clamp, tsk_clamp;
+
+     rq->uclamp[clamp_id].bucket[bucket_id].tasks++;
+
+     /*
+      * Local clamping: rq's buckets always track the max "requested"
+      * clamp value from all RUNNABLE tasks in that bucket.
+      */
+     tsk_clamp = p->uclamp[clamp_id].value;
+     bkt_clamp = rq->uclamp[clamp_id].bucket[bucket_id].value;
+     rq->uclamp[clamp_id].bucket[bucket_id].value = max(bkt_clamp, tsk_clamp);

So, if I read this correct:

 - here we track a max value in a bucket,

quoted

+     rq_clamp = READ_ONCE(rq->uclamp[clamp_id].value);
+     WRITE_ONCE(rq->uclamp[clamp_id].value, max(rq_clamp, tsk_clamp));
+}
+
+/*
+ * When a task is dequeued from a rq, the clamp bucket reference counted by
+ * the task is released. If this is the last task reference counting the rq's
+ * max active clamp value, then the rq's clamp value is updated.
+ * Both the tasks reference counter and the rq's cached clamp values are
+ * expected to be always valid, if we detect they are not we skip the updates,
+ * enforce a consistent state and warn.
+ */
+static inline void uclamp_rq_dec_id(struct task_struct *p, struct rq *rq,
+                                 unsigned int clamp_id)
+{
+     unsigned int bucket_id = p->uclamp[clamp_id].bucket_id;
+     unsigned int rq_clamp, bkt_clamp;
+
+     SCHED_WARN_ON(!rq->uclamp[clamp_id].bucket[bucket_id].tasks);
+     if (likely(rq->uclamp[clamp_id].bucket[bucket_id].tasks))
+             rq->uclamp[clamp_id].bucket[bucket_id].tasks--;
+
+     /*
+      * Keep "local clamping" simple and accept to (possibly) overboost
+      * still RUNNABLE tasks in the same bucket.
+      */
+     if (likely(rq->uclamp[clamp_id].bucket[bucket_id].tasks))
+             return;

(Oh man, I hope that generates semi sane code; long live CSE passes I
suppose)

But we never decrement that bkt_clamp value on dequeue.

quoted

+     bkt_clamp = rq->uclamp[clamp_id].bucket[bucket_id].value;
+
+     /* The rq's clamp value is expected to always track the max */
+     rq_clamp = READ_ONCE(rq->uclamp[clamp_id].value);
+     SCHED_WARN_ON(bkt_clamp > rq_clamp);
+     if (bkt_clamp >= rq_clamp) {

head hurts, this reads ==, how can this ever not be so?

quoted

+             /*
+              * Reset rq's clamp bucket value to its nominal value whenever
+              * there are anymore RUNNABLE tasks refcounting it.

-ENOPARSE

quoted

+              */
+             rq->uclamp[clamp_id].bucket[bucket_id].value =
+                     uclamp_bucket_value(rq_clamp);

But basically you decrement the bucket value to the nominal value.

quoted

+             uclamp_rq_update(rq, clamp_id);
+     }
+}

Given all that, what is to stop the bucket value to climbing to
uclamp_bucket_value(+1)-1 and staying there (provided there's someone
runnable)?

Why are we doing this... ?

I agree with Peter, this part of the patch was the hardest to read.
SCHED_WARN_ON line makes sense to me. The condition that follows and
the following comment are a little baffling. Condition seems to
indicate that the code that follows should be executed only if we are
in the top-most occupied bucket (the bucket which has tasks and has
the highest uclamp value). So this bucket just lost its last task and
we should update rq->uclamp[clamp_id].value. However that's not
exactly what the code does... It also resets
rq->uclamp[clamp_id].bucket[bucket_id].value. So if I understand
correctly, unless the bucket that just lost its last task is the
top-most one its value will not be reset to nominal value. That looks
like a bug to me. Am I missing something?

Side note: some more explanation would be very helpful.