[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH V2 1/1] Improved RTDS scheduler

On 1/26/2016 12:28 PM, Meng Xu wrote:
Hi Dario and Tianyang,

On Tue, Jan 26, 2016 at 9:06 AM, Dario Faggioli
<dario.faggioli@xxxxxxxxxx> wrote:
On Mon, 2016-01-25 at 18:00 -0500, Meng Xu wrote:
On Mon, Jan 25, 2016 at 5:04 PM, Tianyang Chen <tiche@xxxxxxxxxxxxxx>
I have removed some of the Ccs so they won't get bothered as we

On 1/25/2016 4:00 AM, Dario Faggioli wrote:

On Thu, 2015-12-31 at 05:20 -0500, Tianyang Chen wrote:

@@ -147,6 +148,16 @@ static unsigned int nr_rt_ops;
    * Global lock is referenced by schedule_data.schedule_lock
from all
    * physical cpus. It can be grabbed via
+/* dedicated timer for replenishment */
+static struct timer repl_timer;

So, there's always only one timer... Even if we have multiple
with RTDS as their scheduler, they share the replenishment timer?
think it makes more sense to make this per-scheduler.

Yeah, I totally ignored the case for cpu-pools. It looks like when
cpu-pool is created, it copies the scheduler struct and calls
where a private field is initialized. So I assume the timer should
be put
inside the scheduler private struct? Now that I think about it, the
timer is
hard-coded to run on cpu0. If there're lots of cpu-pools but the
replenishment can only be done on the same pcpu, would that be a
Should we keep track of all instances of schedulers (nr_rt_ops
counts how
many) and just put times on different pcpus?

+/* controls when to first start the timer*/
+static int timer_started;

I don't like this, and I don't think we need it. In fact, you
it yourself from v3, AFAICT.

@@ -635,6 +652,13 @@ rt_vcpu_insert(const struct scheduler
struct vcpu *vc)

       /* add rt_vcpu svc to scheduler-specific vcpu list of the
dom */
       list_add_tail(&svc->sdom_elem, &svc->sdom->vcpu);
+    if(!timer_started)
+    {
+        /* the first vcpu starts the timer for the first
+        timer_started = 1;
+        set_timer(&repl_timer,svc->cur_deadline);
+    }

This also seems to be gone in v3, which is good. In fact, it uses
timer_started, which I already said I didn't like.

About the actual startup of the timer (no matter whether for
first time
or not). Here, you were doing it in _vcpu_insert() and not in
_vcpu_wake(); in v3 you're doing it in _vcpu_wake() and not in
_runq_insert()... Which one is the proper way?

Correct me if I'm wrong, at the beginning of the boot process, all
vcpus are
put to sleep/not_runnable after insertions. Therefore, the timer
start when the first vcpu wakes up. I think the wake() in v3 should

@@ -792,44 +816,6 @@ __runq_pick(const struct scheduler *ops,
cpumask_t *mask)

- * Update vcpu's budget and
- * sort runq by insert the modifed vcpu back to runq
- * lock is grabbed before calling this function
- */
-static void
-__repl_update(const struct scheduler *ops, s_time_t now)

Please, allow me to say that seeing this function going away,
fills my
heart with pure joy!! :-D

@@ -889,7 +874,7 @@ rt_schedule(const struct scheduler *ops,
now, bool_t tasklet_work_sched

-    ret.time = MIN(snext->budget, MAX_SCHEDULE); /* sched
quantum */
+    ret.time = snext->budget; /* invoke the scheduler next
time */
       ret.task = snext->vcpu;

This is ok as it is done in v3 (i.e., snext->budget if !idle, -1

@@ -1074,14 +1055,7 @@ rt_vcpu_wake(const struct scheduler
struct vcpu *vc)
       /* insert svc to runq/depletedq because svc is not in
queue now
       __runq_insert(ops, svc);

-    __repl_update(ops, now);
-    ASSERT(!list_empty(&prv->sdom));
-    sdom = list_entry(prv->sdom.next, struct rt_dom,
-    online = cpupool_scheduler_cpumask(sdom->dom->cpupool);
-    snext = __runq_pick(ops, online); /* pick snext from ALL
cpus */
-    runq_tickle(ops, snext);
+    runq_tickle(ops, svc);

And this is another thing I especially like of this patch: it
makes the
wakeup path a lot simpler and a lot more similar to how it looks
in the other schedulers.

Good job with this. :-)

@@ -1108,15 +1078,8 @@ rt_context_saved(const struct scheduler
struct vcpu *vc)
       if ( test_and_clear_bit(__RTDS_delayed_runq_add, &svc-
flags) &&
            likely(vcpu_runnable(vc)) )
+        /* only insert the pre-empted vcpu back*/
           __runq_insert(ops, svc);
-        __repl_update(ops, NOW());
-        ASSERT(!list_empty(&prv->sdom));
-        sdom = list_entry(prv->sdom.next, struct rt_dom,
-        online = cpupool_scheduler_cpumask(sdom->dom-
-        snext = __runq_pick(ops, online); /* pick snext from
cpus */
-        runq_tickle(ops, snext);

Mmm... I'll think about this more and let you know... But out of
top of my head, I think the tickling has to stay? You preempted a
from the pcpu where it was running, maybe some other pcpu is
idle or running a vcpu with a later deadline, and should come and
this one up?

If that's the case, why should we preempt this VCPU? We should use
top VCPu in the runq to preempt the lowest priority VCPU, right?

Yeah, in theory, and as far as things goes by "just" looking at
runq_tickle() in sched_rt.c, you're right.

Tickling is (like everything in life :-P) not perfect, though. At least
it's not "instantaneously" that a tickled pcpu come and pick up work...
Something may happen that perturbates the scenario one depicted in his
head when thinking at what will happen after tickling (e.g., the pcpu
being tickled may be doing something which can't be interrupted for a

The scheduler should be robust, and don't explode or don't exhibit
wrong behavior in case, and I think the current code is ok in that

My idea was that adding one more "tickling point" would make it more
robust, exactly in that regard, i.e., in tolerating anomalies due to
tickling resulting in the pcpu that picked up the work was a different
one from what we expected.

But this indeed introduces more overhead... So, I agree, let's not do
that and see if we encounter problems. We'll come back here if we do.

I see the point. OK. We can do some test to see when the extra
tickling point should be used by adding some temporary warning print
in the code and see if it's called and if we can avoid it. If it's not
called too frequently, it may be ok, (which I'm not so sure), that we
use extra tickle for it. (Maybe have some fast path in the tickle for
the common cases.)

gEDF allows this but there is overhead and may not be worth it. I
have no
stats to support this but there are some papers on restricting what
can migrate. We can discuss more if we need extra logic here.

As to gEDF, the scheduling policy does not restrict what tasks can
migrate, except for the VCPU's hard affinity set by users.  So
migrating is an option. but we should avoid the unnecessary


+        if( min_repl> svc->cur_deadline )
+        {
+            min_repl = svc->cur_deadline;
+        }
+        /* reinsert the vcpu if its deadline is updated */
+        __q_remove(svc);
+        __runq_insert(ops, svc);

One more proof of what I was trying to say. Is it really this
job to --basically-- re-sort the runqueue? I don't think so.

What is the specific situation that you are trying to handle like

Right, if we want to count deadline misses, it could be done when a
vcpu is
picked. However, when selecting the most imminent "inter-release
time" of
all runnable vcpu, the head of the runq could be missing its
deadline and
the cur-deadline could be in the past. How do we handle this
situation? We
still need to scan the runq right?

I think Dario's point is that:
All VCPUs on runq should still have remaining budget, so they should
not have come into the situation of replenishing their budget. So in
the replenishment handler, runq should not be called.


I agree the runq
should not be called to replenish the budget. But the top VCPU in the
runq may be the next earliest one that should get its budget

With "the top VCPU in the runq" you mean the one that is running on a
pCPU? No, I don't think you refer to that one since, as you said,
running vCPUs are not in the runqueues.

And then why it is only the first vCPU in the runqueue  you seem to
care about. I appreciate it has the shortest deadline, but I can't tell
if that's enough to assume we only need to care about it. I probably do
not recall with 100% accuracy the details of the DS algorithm that you
want to implement... In particular, whether or not a replenishment need
to be programmed when the task/vcpu becomes running, so do feel free to
summarize it here for me. :-)

Ah, you are right. I forgot that a VCPU with a large deadlien may have
an earliest replenish time in the future. My fault. :-( The replenish
time of the vcpu should be decided once the VCPU got running or once
the VCPU is waked up or once the VCPU got its current budget in the
current period. So the top VCPU in the runq actually should have its
replenish time decided when/before it's added into the runq, since it
has  to have some budget to stay in the runq.

I think you are correct and I like the design you describe later that
uses a separate "queue" to record the replenish time.

In any case, whatever the algorithm says, what I'm proposing is
something completely different and general enough that would,
hopefully, make it easier to reason on things.

Can we have, together with the timer, a list of replenishment events?

Sure! this is a good idea and it will be easier for us to include the
RM scheduling later, if needed. Reusing the runq and depeletedq will
make the RM scheduling policy cause many changes to the RTDS
scheduling framework.

I'm talking about an actual list of entries, where each entry contains
the following information:
  - time at which the replenishment should happen
  - amount of replenishment
  - vcpu to be replenished

The list will be kept in order of replenishment time, and the timer
will always be programmed to fire at the most imminent replenishment
time instant (which will correspond to the first entry in the list).

When the timer fires, it picks up the first entry, takes it out of the
list, does the replenishment and takes care of the effects of the
replenishment itself (deadline update, preemption, runq re-insertion,
moving from depletedq to runq), depending on what the status of the
vcpu being replenished was.

After doing all this, the timer reprograms itself to fire at the
replenishment time of the next (which just became the new first) entry
in the list.

At least, this is the idea. Now, for the implementation:

  1. instead of only checking the first entry, it's wise that the timer
     handler start to walk the list, and, considering the current time
     (what NOW() gives you) takes care of all entries whose
     replenishment time has passed. This is to deal with cases where
     the handler may fire a little bit later than expected, and more
     than just one replenishment event should have occurred already.
     Since the list is in order, it is ok to stop as soon as the first
     entry with a replenishment time which is actually in the future is

  2. embedded lists give us the opportunity to place one data structure
     in more than one list/queue. So, instead of actually dynamically
     allocating and using a dedicated data structure for each
     replenishment event, I think we can:
      - just add another embedded list element in rt_vcpu (just like
      - use that to queue the rt_vcpu-sin the replenishment events list
      - add the fields necessary to handle replenishment directly in
        rt_vcpu (assuming we need any... If next replenishment time is
        the next absolute deadline and amount is the budget, everything
        we need should already be there)

So.. What do you think?

It's nice. Thank you so much for drafting this. :-)
What do you think, Tianyang?

Dario and Meng,

This indeed makes it very clear and it looks like the timer handler doesn't even need to know where a vcpu is at all. Just plain replenishment when it needs it. It also gets rid off extra code in context_saved().

I'm assuming a vcpu should be added to the replq when it wakes up and removed when it's not runnable (checks in timer handler).

This discussion was before I figured out things about idle_vcpu[]
tasklet. A vcpu could be preempted and put back to either runq or
if a tasklet is scheduled. It could also end up in a depletedq in
situations. I guess Meng's point is this vcpu should be running
without being taken off if there is no tasklet, in an effort to
follow EDF.
Hi Tianyang,

I think Dario made a good point. In order to avoid vcpu being taken
off from the core, we can handle it in the rt_schedule(), the budget
enforcement function. This could probably make the code cleaner.

Exactly. Budget enforcement is perfectly fine being done in

+    /* if timer was triggered but none of the vcpus
+     * need to be refilled, set the timer to be the
+     * default period + now
+     */
+    if(min_repl == LONG_MAX)
+    {
+        set_timer(&repl_timer, now + RTDS_DEFAULT_PERIOD);

I agree with Meng's point in this thread: this should not be
If it is, it's most likely because of a bug or to something else.

Let's figure out what it is, and fix it properly. (I see that in
this logic is gone, so hopefully you found and fixed the issue

Yeah. Like I said the timer is originally programmed to fire when
the first
vcpu is inserted but all vcpus are not runnable at the beginning of
process. If the timer is triggered before any vcpu wakes up, there
nothing on queue at all. This should be fixed with wake() in V3.

Great! I'm wondering if we should look at the v3 version, which
have fixed many issues? We can decide if the runningq is needed in

The fact that v3 added runningq is the reason why I'm commenting on
this version: I don't think it's necessary at all, and it was easier to
focus on other issues with that out of the way.

What I've seen in v3 seems ok to me. I can take another look, but I
guess the best thing to do would be to put together a v4 with the fixes
to the issue v2 had taken from v3, the runningq taken away, and the
replenished queue implemented as I described (if you agree with it).

I see the point. Tianyang, what do you think? Maybe we can just put
together a v4 quickly and let Dario comment on v4?

Dario, What do you think? Right now, I'm kind of lost how we should
proceed next step? Should we modify based on this version or the
latest posted version v3?

As you wish, but I'd base the new version on this version, and
"backport" good stuff from v3 to here (again, as it doesn't have the
runningq in the way)

I see. Tianyang, let's do what Dario suggests. Actually, the code
changed from v2 to v3 will be a useful knowledge when we are working
on the v4. What do you think? or we can talk in person today. :-)

Thank you very much for your time on this, Dario!
Yeah definitely. I am still trying to figure out why there is an assertion failure at free_pdata() when I remove a pcpu from a pool... It that takes too long I will just send v4 out for discussion first.

Thanks for your time Dario.


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.