[Xen-devel] [PATCH] xen/sched: fix cpu offlining with core scheduling

[Juergen Gross <jgross@xxxxxxxx>]

Offlining a cpu with core scheduling active can result in a hanging
system. Reason is the scheduling resource and unit of the to be removed
cpus needs to be split in order to remove the cpu from its cpupool and
move it to the idle scheduler. In case one of the involved cpus happens
to have received a sched slave event due to a vcpu former having been
running on that cpu being woken up again, it can happen that this cpu
will enter sched_wait_rendezvous_in() while its scheduling resource is
just about to be split. It might wait for ever for the other sibling
to join, which will never happen due to the resources already being
modified.

This can easily be avoided by:
- resetting the rendezvous counters of the idle unit which is kept
- checking for a new scheduling resource in sched_wait_rendezvous_in()
  after reacquiring the scheduling lock and resetting the counters in
  that case without scheduling another vcpu
- moving schedule resource modifications (in schedule_cpu_rm()) and
  retrieving (schedule(), sched_slave() is fine already, others are not
  critical) into locked regions

Reported-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 xen/common/sched/core.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
index 7e8e7d2c39..723283ed00 100644
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -2415,7 +2415,8 @@ static struct sched_unit *sched_wait_rendezvous_in(struct 
sched_unit *prev,
 {
     struct sched_unit *next;
     struct vcpu *v;
-    unsigned int gran = get_sched_res(cpu)->granularity;
+    struct sched_resource *sr = get_sched_res(cpu);
+    unsigned int gran = sr->granularity;
 
     if ( !--prev->rendezvous_in_cnt )
     {
@@ -2482,6 +2483,19 @@ static struct sched_unit 
*sched_wait_rendezvous_in(struct sched_unit *prev,
             atomic_set(&prev->next_task->rendezvous_out_cnt, 0);
             prev->rendezvous_in_cnt = 0;
         }
+
+        /*
+         * Check for scheduling resourced switched. This happens when we are
+         * moved away from our cpupool and cpus are subject of the idle
+         * scheduler now.
+         */
+        if ( unlikely(sr != get_sched_res(cpu)) )
+        {
+            ASSERT(is_idle_unit(prev));
+            atomic_set(&prev->next_task->rendezvous_out_cnt, 0);
+            prev->rendezvous_in_cnt = 0;
+            return NULL;
+        }
     }
 
     return prev->next_task;
@@ -2538,7 +2552,10 @@ static void sched_slave(void)
 
     next = sched_wait_rendezvous_in(prev, &lock, cpu, now);
     if ( !next )
+    {
+        rcu_read_unlock(&sched_res_rculock);
         return;
+    }
 
     pcpu_schedule_unlock_irq(lock, cpu);
 
@@ -2567,11 +2584,11 @@ static void schedule(void)
 
     rcu_read_lock(&sched_res_rculock);
 
+    lock = pcpu_schedule_lock_irq(cpu);
+
     sr = get_sched_res(cpu);
     gran = sr->granularity;
 
-    lock = pcpu_schedule_lock_irq(cpu);
-
     if ( prev->rendezvous_in_cnt )
     {
         /*
@@ -2599,7 +2616,10 @@ static void schedule(void)
         cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ);
         next = sched_wait_rendezvous_in(prev, &lock, cpu, now);
         if ( !next )
+        {
+            rcu_read_unlock(&sched_res_rculock);
             return;
+        }
     }
     else
     {
@@ -3151,7 +3171,10 @@ int schedule_cpu_rm(unsigned int cpu)
         per_cpu(sched_res_idx, cpu_iter) = 0;
         if ( cpu_iter == cpu )
         {
-            idle_vcpu[cpu_iter]->sched_unit->priv = NULL;
+            unit = idle_vcpu[cpu_iter]->sched_unit;
+            unit->priv = NULL;
+            atomic_set(&unit->next_task->rendezvous_out_cnt, 0);
+            unit->rendezvous_in_cnt = 0;
         }
         else
         {
@@ -3182,6 +3205,8 @@ int schedule_cpu_rm(unsigned int cpu)
     }
     sr->scheduler = &sched_idle_ops;
     sr->sched_priv = NULL;
+    sr->granularity = 1;
+    sr->cpupool = NULL;
 
     smp_mb();
     sr->schedule_lock = &sched_free_cpu_lock;
@@ -3194,9 +3219,6 @@ int schedule_cpu_rm(unsigned int cpu)
     sched_free_udata(old_ops, vpriv_old);
     sched_free_pdata(old_ops, ppriv_old, cpu);
 
-    sr->granularity = 1;
-    sr->cpupool = NULL;
-
 out:
     rcu_read_unlock(&sched_res_rculock);
     xfree(sr_new);
-- 
2.16.4


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