[Xen-changelog] [xen master] xen: credit1: small optimization in Credit1's tickling logic.

[patchbot@xxxxxxx]

commit b19da0ee4f751ff628662a11b7f5d05ff4038977
Author:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
AuthorDate: Thu Sep 15 12:35:03 2016 +0100
Commit:     George Dunlap <george.dunlap@xxxxxxxxxx>
CommitDate: Thu Sep 15 12:39:32 2016 +0100

    xen: credit1: small optimization in Credit1's tickling logic.
    
    If, when vcpu x wakes up, there are no idle pcpus in x's
    soft-affinity, we just go ahead and look at its hard
    affinity. This basically means that, if, in __runq_tickle(),
    new_idlers_empty is true, balance_step is equal to
    CSCHED_BALANCE_HARD_AFFINITY, and that calling
    csched_balance_cpumask() for whatever vcpu, would just
    return the vcpu's cpu_hard_affinity.
    
    Therefore, don't bother calling it (it's just pure
    overhead) and use cpu_hard_affinity directly.
    
    For this very reason, this patch should only be
    a (slight) optimization, and entail no functional
    change.
    
    As a side note, it would make sense to do what the
    patch does, even if we could be inside the
    [[ new_idlers_empty && new->pri > cur->pri ]] if
    with balance_step equal to CSCHED_BALANCE_SOFT_AFFINITY.
    In fact, what is actually happening is:
     - vcpu x is waking up, and (since there aren't suitable
       idlers, and it's entitled for it) it is preempting
       vcpu y;
     - vcpu y's hard-affinity is a superset of its
       soft-affinity mask.
    
    Therefore, it makes sense to use wider possible mask,
    as by doing that, we maximize the probability of
    finding an idle pcpu in there, to which we can send
    vcpu y, which then will be able to run.
    
    While there, also fix the comment, which included
    an awkward parenthesis nesting.
    
    Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Acked-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
 xen/common/sched_credit.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index c2b4b24..472fb28 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -433,9 +433,9 @@ static inline void __runq_tickle(struct csched_vcpu *new)
             /*
              * If there are no suitable idlers for new, and it's higher
              * priority than cur, check whether we can migrate cur away.
-             * (We have to do it indirectly, via _VPF_migrating, instead
+             * We have to do it indirectly, via _VPF_migrating (instead
              * of just tickling any idler suitable for cur) because cur
-             * is running.)
+             * is running.
              *
              * If there are suitable idlers for new, no matter priorities,
              * leave cur alone (as it is running and is, likely, cache-hot)
@@ -444,9 +444,7 @@ static inline void __runq_tickle(struct csched_vcpu *new)
              */
             if ( new_idlers_empty && new->pri > cur->pri )
             {
-                csched_balance_cpumask(cur->vcpu, balance_step,
-                                       cpumask_scratch_cpu(cpu));
-                if ( cpumask_intersects(cpumask_scratch_cpu(cpu),
+                if ( cpumask_intersects(cur->vcpu->cpu_hard_affinity,
                                         &idle_mask) )
                 {
                     SCHED_VCPU_STAT_CRANK(cur, kicked_away);
--
generated by git-patchbot for /home/xen/git/xen.git#master

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