[Xen-changelog] [xen master] xen: credit: consider tickled pCPUs as busy.

[patchbot@xxxxxxx]

commit 47d529299575ef01df59050253dbdc0d24375f0a
Author:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
AuthorDate: Fri Apr 7 18:56:45 2017 +0200
Commit:     George Dunlap <george.dunlap@xxxxxxxxxx>
CommitDate: Fri Apr 7 18:17:39 2017 +0100

    xen: credit: consider tickled pCPUs as busy.
    
    Currently, it can happen that __runq_tickle(),
    running on pCPU 2 because vCPU x woke up, decides
    to tickle pCPU 3, because it's idle. Just after
    that, but before pCPU 3 manages to schedule and
    pick up x, either __runq_tickel() or
    __csched_cpu_pick(), running on pCPU 6, sees that
    idle pCPUs are 0, 1 and also 3, and for whatever
    reason it also chooses 3 for waking up (or
    migrating) vCPU y.
    
    When pCPU 3 goes through the scheduler, it will
    pick up, say, vCPU x, and y will sit in its
    runqueue, even if there are idle pCPUs.
    
    Alleviate this by marking a pCPU to be idle
    right away when tickling it (like, e.g., it happens
    in Credit2).
    
    Note that this does not eliminate the race. That
    is not possible without introducing proper locking
    for the cpumasks the scheduler uses. It significantly
    reduces the window during which it can happen, though.
    
    Introduce proper locking for the cpumask can, in
    theory, be done, and may be investigated in future.
    It is a significant amount of work to do it properly
    (e.g., avoiding deadlock), and it is likely to adversely
    affect scalability, and so it may be a path it is just
    not worth following.
    
    Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
 xen/common/sched_credit.c | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 59ed4ca..7b4ea02 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -489,7 +489,17 @@ static inline void __runq_tickle(struct csched_vcpu *new)
                 __trace_var(TRC_CSCHED_TICKLE, 1, sizeof(cpu), &cpu);
         }
 
-        /* Send scheduler interrupts to designated CPUs */
+        /*
+         * Mark the designated CPUs as busy and send them all the scheduler
+         * interrupt. We need the for_each_cpu for dealing with the
+         * !opt_tickle_one_idle case. We must use cpumask_clear_cpu() and
+         * can't use cpumask_andnot(), because prv->idlers needs atomic access.
+         *
+         * In the default (and most common) case, when opt_rickle_one_idle is
+         * true, the loop does only one step, and only one bit is cleared.
+         */
+        for_each_cpu(cpu, &mask)
+            cpumask_clear_cpu(cpu, prv->idlers);
         cpumask_raise_softirq(&mask, SCHEDULE_SOFTIRQ);
     }
     else
@@ -990,6 +1000,13 @@ csched_vcpu_acct(struct csched_private *prv, unsigned int 
cpu)
             SCHED_VCPU_STAT_CRANK(svc, migrate_r);
             SCHED_STAT_CRANK(migrate_running);
             set_bit(_VPF_migrating, &current->pause_flags);
+            /*
+             * As we are about to tickle cpu, we should clear its bit in
+             * idlers. But, if we are here, it means there is someone running
+             * on it, and hence the bit must be zero already.
+             */
+            ASSERT(!cpumask_test_cpu(cpu,
+                                     CSCHED_PRIV(per_cpu(scheduler, 
cpu))->idlers));
             cpu_raise_softirq(cpu, SCHEDULE_SOFTIRQ);
         }
     }
@@ -1082,13 +1099,22 @@ static void
 csched_vcpu_sleep(const struct scheduler *ops, struct vcpu *vc)
 {
     struct csched_vcpu * const svc = CSCHED_VCPU(vc);
+    unsigned int cpu = vc->processor;
 
     SCHED_STAT_CRANK(vcpu_sleep);
 
     BUG_ON( is_idle_vcpu(vc) );
 
-    if ( curr_on_cpu(vc->processor) == vc )
-        cpu_raise_softirq(vc->processor, SCHEDULE_SOFTIRQ);
+    if ( curr_on_cpu(cpu) == vc )
+    {
+        /*
+         * We are about to tickle cpu, so we should clear its bit in idlers.
+         * But, we are here because vc is going to sleep while running on cpu,
+         * so the bit must be zero already.
+         */
+        ASSERT(!cpumask_test_cpu(cpu, CSCHED_PRIV(per_cpu(scheduler, 
cpu))->idlers));
+        cpu_raise_softirq(cpu, SCHEDULE_SOFTIRQ);
+    }
     else if ( __vcpu_on_runq(svc) )
         __runq_remove(svc);
 }
--
generated by git-patchbot for /home/xen/git/xen.git#master

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