[Xen-changelog] [xen master] xen: sched_credit: when picking, make sure we get an idle one, if any

[patchbot@xxxxxxx]

commit ebb064aa22715ae473f72468d093e7a05e3c3c58
Author:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
AuthorDate: Wed Apr 17 10:57:30 2013 +0000
Commit:     Ian Campbell <ian.campbell@xxxxxxxxxx>
CommitDate: Wed Apr 17 12:11:14 2013 +0100

    xen: sched_credit: when picking, make sure we get an idle one, if any
    
    The pcpu picking algorithm treats two threads of a SMT core the same.
    More specifically, if one is idle and the other one is busy, they both
    will be assigned a weight of 1. Therefore, when picking begins, if the
    first target pcpu is the busy thread (and if there are no other idle
    pcpu than its sibling), that will never change.
    
    This change fixes this by ensuring that, before entering the core of
    the picking algorithm, the target pcpu is an idle one (if there is an
    idle pcpu at all, of course).
    
    Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Acked-by: Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>
    Acked-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
    Acked-by: Keir Fraser <keir@xxxxxxx>
---
 xen/common/sched_credit.c |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 9e9ef33..64e76c5 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -532,6 +532,21 @@ _csched_cpu_pick(const struct scheduler *ops, struct vcpu 
*vc, bool_t commit)
     if ( vc->processor == cpu && IS_RUNQ_IDLE(cpu) )
         cpumask_set_cpu(cpu, &idlers);
     cpumask_and(&cpus, &cpus, &idlers);
+
+    /*
+     * It is important that cpu points to an idle processor, if a suitable
+     * one exists (and we can use cpus to check and, possibly, choose a new
+     * CPU, as we just &&-ed it with idlers). In fact, if we are on SMT, and
+     * cpu points to a busy thread with an idle sibling, both the threads
+     * will be considered the same, from the "idleness" calculation point
+     * of view", preventing vcpu from being moved to the thread that is
+     * actually idle.
+     *
+     * Notice that cpumask_test_cpu() is quicker than cpumask_empty(), so
+     * we check for it first.
+     */
+    if ( !cpumask_test_cpu(cpu, &cpus) && !cpumask_empty(&cpus) )
+        cpu = cpumask_cycle(cpu, &cpus);
     cpumask_clear_cpu(cpu, &cpus);
 
     while ( !cpumask_empty(&cpus) )
--
generated by git-patchbot for /home/xen/git/xen.git#master

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