[Xen-changelog] [xen stable-4.8] xen: credit2: don't miss accounting while doing a credit reset.

[patchbot@xxxxxxx]

commit 866f3636f832ecae0260b04e90b8de432aaa3129
Author:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
AuthorDate: Tue Mar 14 12:42:19 2017 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Mar 14 12:42:19 2017 +0100

    xen: credit2: don't miss accounting while doing a credit reset.
    
    A credit reset basically means going through all the
    vCPUs of a runqueue and altering their credits, as a
    consequence of a 'scheduling epoch' having come to an
    end.
    
    Blocked or runnable vCPUs are fine, all the credits
    they've spent running so far have been accounted to
    them when they were scheduled out.
    
    But if a vCPU is running on a pCPU, when a reset event
    occurs (on another pCPU), that does not get properly
    accounted. Let's therefore begin to do so, for better
    accuracy and fairness.
    
    In fact, after this patch, we see this in a trace:
    
     csched2:schedule cpu 10, rq# 1, busy, not tickled
     csched2:burn_credits d1v5, credit = 9998353, delta = 202996
     runstate_continue d1v5 running->running
     ...
     csched2:schedule cpu 12, rq# 1, busy, not tickled
     csched2:burn_credits d1v6, credit = -1327, delta = 9999544
     csched2:reset_credits d0v13, credit_start = 10500000, credit_end = 
10500000, mult = 1
     csched2:reset_credits d0v14, credit_start = 10500000, credit_end = 
10500000, mult = 1
     csched2:reset_credits d0v7, credit_start = 10500000, credit_end = 
10500000, mult = 1
     csched2:burn_credits d1v5, credit = 201805, delta = 9796548
     csched2:reset_credits d1v5, credit_start = 201805, credit_end = 10201805, 
mult = 1
     csched2:burn_credits d1v6, credit = -1327, delta = 0
     csched2:reset_credits d1v6, credit_start = -1327, credit_end = 9998673, 
mult = 1
    
    Which shows how d1v5 actually executed for ~9.796 ms,
    on pCPU 10, when reset_credit() is executed, on pCPU
    12, because of d1v6's credits going below 0.
    
    Without this patch, this 9.796ms are not accounted
    to anyone. With this patch, d1v5 is charged for that,
    and its credits drop down from 9796548 to 201805.
    
    And this is important, as it means that it will
    begin the new epoch with 10201805 credits, instead
    of 10500000 (which he would have, before this patch).
    
    Basically, we were forgetting one round of accounting
    in epoch x, for the vCPUs that are running at the time
    the epoch ends. And this meant favouring a little bit
    these same vCPUs, in epoch x+1, providing them with
    the chance of execute longer than their fair share.
    
    Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    master commit: 4fa4f8a3cd5afd4980ad9517755d002dc316abdc
    master date: 2017-03-01 16:56:34 +0000
---
 xen/common/sched_credit2.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 25ea4d8..6f7860a 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -1119,18 +1119,28 @@ static void reset_credit(const struct scheduler *ops, 
int cpu, s_time_t now,
 
     list_for_each( iter, &rqd->svc )
     {
+        unsigned int svc_cpu;
         struct csched2_vcpu * svc;
         int start_credit;
 
         svc = list_entry(iter, struct csched2_vcpu, rqd_elem);
+        svc_cpu = svc->vcpu->processor;
 
         ASSERT(!is_idle_vcpu(svc->vcpu));
         ASSERT(svc->rqd == rqd);
 
+        /*
+         * If svc is running, it is our responsibility to make sure, here,
+         * that the credit it has spent so far get accounted.
+         */
+        if ( svc->vcpu == curr_on_cpu(svc_cpu) )
+            burn_credits(rqd, svc, now);
+
         start_credit = svc->credit;
 
-        /* And add INIT * m, avoiding integer multiplication in the
-         * common case. */
+        /*
+         * Add INIT * m, avoiding integer multiplication in the common case.
+         */
         if ( likely(m==1) )
             svc->credit += CSCHED2_CREDIT_INIT;
         else
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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