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[Xen-changelog] [xen stable-4.2] xen: sched_credit: improve picking up the idle CPU for a VCPU

commit 6261d98d6fc5934cd8e5c2188b39fc9590b9095c
Author:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
AuthorDate: Tue Feb 12 13:36:07 2013 +0100
Commit:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
CommitDate: Tue Feb 12 13:36:07 2013 +0100

    xen: sched_credit: improve picking up the idle CPU for a VCPU
    In _csched_cpu_pick() we try to select the best possible CPU for
    running a VCPU, considering the characteristics of the underlying
    hardware (i.e., how many threads, core, sockets, and how busy they
    are). What we want is "the idle execution vehicle with the most
    idling neighbours in its grouping".
    In order to achieve it, we select a CPU from the VCPU's affinity,
    giving preference to its current processor if possible, as the basis
    for the comparison with all the other CPUs. Problem is, to discount
    the VCPU itself when computing this "idleness" (in an attempt to be
    fair wrt its current processor), we arbitrarily and unconditionally
    consider that selected CPU as idle, even when it is not the case,
    for instance:
     1. If the CPU is not the one where the VCPU is running (perhaps due
        to the affinity being changed);
     2. The CPU is where the VCPU is running, but it has other VCPUs in
        its runq, so it won't go idle even if the VCPU in question goes.
    This is exemplified in the trace below:
    ]  3.466115364 x|------|------| d10v1   22005(2:2:5) 3 [ a 1 8 ]
       ... ... ...
       3.466122856 x|------|------| d10v1 runstate_change d10v1
       3.466123046 x|------|------| d?v? runstate_change d32767v0
       ... ... ...
    ]  3.466126887 x|------|------| d32767v0   28004(2:8:4) 3 [ a 1 8 ]
    22005(...) line (the first line) means _csched_cpu_pick() was called
    on VCPU 1 of domain 10, while it is running on CPU 0, and it choose
    CPU 8, which is busy ('|'), even if there are plenty of idle
    CPUs. That is because, as a consequence of changing the VCPU affinity,
    CPU 8 was chosen as the basis for the comparison, and therefore
    considered idle (its bit gets unconditionally set in the bitmask
    representing the idle CPUs). 28004(...) line means the VCPU is woken
    up and queued on CPU 8's runq, where it waits for a context switch or
    a migration, in order to be able to execute.
    This change fixes things by only considering the "guessed" CPU idle if
    the VCPU in question is both running there and is its only runnable
    Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Acked-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    xen-unstable changeset: 26287:127c2c47d440
    xen-unstable date: Tue Dec 18 18:10:18 UTC 2012
 xen/common/sched_credit.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index fa138c5..7df2699 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -72,6 +72,9 @@
 #define CSCHED_VCPU(_vcpu)  ((struct csched_vcpu *) (_vcpu)->sched_priv)
 #define CSCHED_DOM(_dom)    ((struct csched_dom *) (_dom)->sched_priv)
 #define RUNQ(_cpu)          (&(CSCHED_PCPU(_cpu)->runq))
+/* Is the first element of _cpu's runq its idle vcpu? */
+#define IS_RUNQ_IDLE(_cpu)  (list_empty(RUNQ(_cpu)) || \
+                             is_idle_vcpu(__runq_elem(RUNQ(_cpu)->next)->vcpu))
@@ -487,9 +490,14 @@ _csched_cpu_pick(const struct scheduler *ops, struct vcpu 
*vc, bool_t commit)
      * distinct cores first and guarantees we don't do something stupid
      * like run two VCPUs on co-hyperthreads while there are idle cores
      * or sockets.
+     *
+     * Notice that, when computing the "idleness" of cpu, we may want to
+     * discount vc. That is, iff vc is the currently running and the only
+     * runnable vcpu on cpu, we add cpu to the idlers.
     cpumask_and(&idlers, &cpu_online_map, CSCHED_PRIV(ops)->idlers);
-    cpumask_set_cpu(cpu, &idlers);
+    if ( vc->processor == cpu && IS_RUNQ_IDLE(cpu) )
+        cpumask_set_cpu(cpu, &idlers);
     cpumask_and(&cpus, &cpus, &idlers);
     cpumask_clear_cpu(cpu, &cpus);
generated by git-patchbot for /home/xen/git/xen.git#stable-4.2

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