xen: credit1: on vCPU wakeup, kick away current only if makes sense

In fact, when waking up a vCPU, __runq_tickle() is called to allow the new vCPU to run on a pCPU (which one, depends on the relationship between the priority of the new vCPU, and the ones of the vCPUs that are already running).

If there is no idle processor on which the new vCPU can run (e.g., because of pinning/affinity), we try to migrate away the vCPU that is currently running on the new vCPU's processor (i.e., the processor on which the vCPU is waking up).

Now, trying to migrate a vCPU has the effect of pushing it through a

running --> offline --> runnable

transition, which, in turn, has the following negative effects:

1) Credit1 counts that as a wakeup, and it BOOSTs the vCPU, even if it is a CPU-bound one, which wouldn't normally have deserved boosting. This can prevent legit IO-bound vCPUs to get ahold of the processor until such spurious boosting expires, hurting the performance!

2) since the vCPU is fails the vcpu_runnable() test (within the call to csched_schedule() that follows the wakeup, as a consequence of tickling) the scheduling rate-limiting mechanism is also fooled, i.e., the context switch happens even if less than the minimum execution amount of time passed.

In particular, 1) has been reported to cause the following issue:

* VM-IO: 1-vCPU pinned to a pCPU, running netperf
* VM-CPU: 1-vCPU pinned the the same pCPU, running a busy CPU loop
==> Only VM-I/O: throughput is 806.64 Mbps
==> VM-I/O + VM-CPU: throughput is 166.50 Mbps

This patch solves (for the above scenario) the problem by checking whether or not it makes sense to try to migrate away the vCPU currently running on the processor. In fact, if there aren't idle processors where such a vCPU can execute. attempting the migration is just futile (harmful, actually!).

With this patch, in the above configuration, results are:

==> Only VM-I/O: throughput is 807.18 Mbps
==> VM-I/O + VM-CPU: throughput is 731.66 Mbps

---
Cc: George Dunlap <[email protected]>
Cc: suokun <[email protected]>
---
Note that, about 1) above, it is IMO _wrong_ that Credit1 treats wakeups resulting from migration of a vCPU to another pCPU as "regular wakeups", hence granting BOOST priority to the vCPUs experiencing that. However:
- fixing that is non-trivial, and requires being done in its own patch;
- that is orthogonal to the fix being introduced here. That is to say, even when Credit1 will be fixed not to boost migrating vCPUs, this patch will be still corect and necessary.

Regards,
Dario

---
 xen/common/sched_credit.c |   22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

index b8f28fe..1b30e67 100644
@@ -426,9 +426,10 @@ __runq_tickle(unsigned int cpu, struct csched_vcpu *new)
 
             /*
              * If there are no suitable idlers for new, and it's higher
-             * priority than cur, ask the scheduler to migrate cur away.
-             * We have to act like this (instead of just waking some of
-             * the idlers suitable for cur) because cur is running.
+             * priority than cur, check whether we can migrate cur away.
+             * (We have to do it indirectly, via _VPF_migrating, instead
+             * of just tickling any idler suitable for cur) because cur
+             * is running.)
              *
              * If there are suitable idlers for new, no matter priorities,
              * leave cur alone (as it is running and is, likely, cache-hot)

@@ -437,11 +438,18 @@ __runq_tickle(unsigned int cpu, struct csched_vcpu *new)
              */
             if ( new_idlers_empty && new->pri > cur->pri )
             {
+                csched_balance_cpumask(cur->vcpu, balance_step,
+                                       csched_balance_mask(cpu));
+                if ( cpumask_intersects(csched_balance_mask(cpu),
+                                        &idle_mask) )
+                {
+                    SCHED_VCPU_STAT_CRANK(cur, kicked_away);
+                    SCHED_VCPU_STAT_CRANK(cur, migrate_r);
+                    SCHED_STAT_CRANK(migrate_kicked_away);
+                    set_bit(_VPF_migrating, &cur->vcpu->pause_flags);
+                }
+                /* Tickle cpu anyway, to let new preempt cur. */
                 SCHED_STAT_CRANK(tickle_idlers_none);
-                SCHED_VCPU_STAT_CRANK(cur, kicked_away);
-                SCHED_VCPU_STAT_CRANK(cur, migrate_r);
-                SCHED_STAT_CRANK(migrate_kicked_away);
-                set_bit(_VPF_migrating, &cur->vcpu->pause_flags);
                 __cpumask_set_cpu(cpu, &mask);
             }
             else if ( !new_idlers_empty )

I'm getting a bit worried about how long the path is to actually wake up a vcpu; if this only affected the "pin" case, then I might say it wasn't worth it. But it looks to me like this could be a consistent pattern on any system where there was consistently no idlers available; so at this point it's probably better to have than not:

Same here. That's why I started looking at a solution that was more general that the pinned scenario, for which it wasn't worth adding complexity in the wakeup path.

I've got a patch for that almost ready (avoiding boosting in case of wakeups induced by a migration).

However, while working on that, I realized...

...exactly this. I.e., it's not only the pinned case. Even 'free' vCPUs, or vCPUs with arbitrary large affinities, if the system is busy, can incur into this sort of spurious migration attempts, with takes time (migrate-->pick-->wake-->tickle-->rescheduling), and yet leaves the situation unchanged (with the fix I'm preparing; without, it causes the reported anomaly).

At that point, this, and the boosting after migration, became two orthogonal issues, both needing fixing. :-/

Thanks and Regards,
Dario

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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