[Xen-devel] [PATCH v2 3/4] xen: credit2: improve distribution of budget (for domains with caps)

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

Instead of letting the vCPU that for first tries to get
some budget take it all (although temporarily), allow each
vCPU to only get a specific quota of the total budget.

This improves fairness, allows for more parallelism, and
prevents vCPUs from not being able to get any budget (e.g.,
because some other vCPU always comes before and gets it all)
for one or more period, and hence starve (and cause troubles
in guest kernels, such as livelocks, triggering of whatchdogs,
etc.).

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
---
Cc: Anshul Makkar <anshulmakkar@xxxxxxxxx>
---
Changes from v1:
- typos;
- spurious hunk moved to previous patch.
---
 xen/common/sched_credit2.c |   56 ++++++++++++++++++++++++++++++++------------
 1 file changed, 41 insertions(+), 15 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index ce70224..211e2d6 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -522,6 +522,8 @@ struct csched2_vcpu {
     unsigned flags;                    /* Status flags (16 bits would be ok,  
*/
     s_time_t budget;                   /* Current budget (if domains has cap) 
*/
                                        /* but clear_bit() does not like that) 
*/
+    s_time_t budget_quota;             /* Budget to which vCPU is entitled    
*/
+
     s_time_t start_time;               /* Time we were scheduled (for credit) 
*/
 
     /* Individual contribution to load                                        
*/
@@ -1791,17 +1793,16 @@ static bool vcpu_grab_budget(struct csched2_vcpu *svc)
 
     if ( sdom->budget > 0 )
     {
-        /*
-         * NB: we give the whole remaining budget a domain has, to the first
-         * vCPU that comes here and asks for it. This means that, in a domain
-         * with a cap, only 1 vCPU is able to run, at any given time.
-         * /THIS IS GOING TO CHANGE/ in subsequent patches, toward something
-         * that allows much better fairness and parallelism. Proceeding in
-         * two steps, is for making things easy to understand, when looking
-         * at the signle commits.
-         */
-        svc->budget = sdom->budget;
-        sdom->budget = 0;
+        s_time_t budget;
+
+        /* Get our quota, if there's at least as much budget */
+        if ( likely(sdom->budget >= svc->budget_quota) )
+            budget = svc->budget_quota;
+        else
+            budget = sdom->budget;
+
+        svc->budget = budget;
+        sdom->budget -= budget;
     }
     else
     {
@@ -2036,6 +2037,7 @@ csched2_alloc_vdata(const struct scheduler *ops, struct 
vcpu *vc, void *dd)
     svc->tickled_cpu = -1;
 
     svc->budget = STIME_MAX;
+    svc->budget_quota = 0;
     INIT_LIST_HEAD(&svc->parked_elem);
 
     SCHED_STAT_CRANK(vcpu_alloc);
@@ -2822,6 +2824,9 @@ csched2_dom_cntl(
         /* Cap */
         if ( op->u.credit2.cap != 0 )
         {
+            struct csched2_vcpu *svc;
+            spinlock_t *lock;
+
             /* Cap is only valid if it's below 100 * nr_of_vCPUS */
             if ( op->u.credit2.cap > 100 * sdom->nr_vcpus )
             {
@@ -2834,6 +2839,26 @@ csched2_dom_cntl(
             sdom->tot_budget /= 100;
             spin_unlock(&sdom->budget_lock);
 
+            /*
+             * When trying to get some budget and run, each vCPU will grab
+             * from the pool 1/N (with N = nr of vCPUs of the domain) of
+             * the total budget. Roughly speaking, this means each vCPU will
+             * have at least one chance to run during every period.
+             */
+            for_each_vcpu ( d, v )
+            {
+                svc = csched2_vcpu(v);
+                lock = vcpu_schedule_lock(svc->vcpu);
+                /*
+                 * Too small quotas would in theory cause a lot of overhead,
+                 * which then won't happen because, in csched2_runtime(),
+                 * CSCHED2_MIN_TIMER is what would be used anyway.
+                 */
+                svc->budget_quota = max(sdom->tot_budget / sdom->nr_vcpus,
+                                        CSCHED2_MIN_TIMER);
+                vcpu_schedule_unlock(lock, svc->vcpu);
+            }
+
             if ( sdom->cap == 0 )
             {
                 /*
@@ -2865,9 +2890,8 @@ csched2_dom_cntl(
                  */
                 for_each_vcpu ( d, v )
                 {
-                    struct csched2_vcpu *svc = csched2_vcpu(v);
-                    spinlock_t *lock = vcpu_schedule_lock(svc->vcpu);
-
+                    svc = csched2_vcpu(v);
+                    lock = vcpu_schedule_lock(svc->vcpu);
                     if ( v->is_running )
                     {
                         unsigned int cpu = v->processor;
@@ -2917,6 +2941,7 @@ csched2_dom_cntl(
                 spinlock_t *lock = vcpu_schedule_lock(svc->vcpu);
 
                 svc->budget = STIME_MAX;
+                svc->budget_quota = 0;
 
                 vcpu_schedule_unlock(lock, svc->vcpu);
             }
@@ -3601,7 +3626,8 @@ csched2_dump_vcpu(struct csched2_private *prv, struct 
csched2_vcpu *svc)
     printk(" credit=%" PRIi32" [w=%u]", svc->credit, svc->weight);
 
     if ( has_cap(svc) )
-        printk(" budget=%"PRI_stime, svc->budget);
+        printk(" budget=%"PRI_stime"(%"PRI_stime")",
+               svc->budget, svc->budget_quota);
 
     printk(" load=%"PRI_stime" (~%"PRI_stime"%%)", svc->avgload,
            (svc->avgload * 100) >> prv->load_precision_shift);


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