[Xen-devel] [PATCH 4/5] xen: sched: impove use of cpumask scratch space in Credit1.

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

It is ok to use just cpumask_scratch in csched_runq_steal().
In fact, the cpu parameter comes from the cpu local variable
in csched_load_balance(), which in turn comes from cpu in
csched_schedule(), which is smp_processor_id().

While there, also:
 - move the comment about cpumask_scratch in the header
   where the scratch space is declared;
 - spell more clearly (in that same comment) what are the
   serialization rules.

No functional change intended.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/common/sched_credit.c  |    5 ++---
 xen/common/schedule.c      |    7 +------
 xen/include/xen/sched-if.h |    7 +++++++
 3 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index dfe8545..ad20819 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -1636,9 +1636,8 @@ csched_runq_steal(int peer_cpu, int cpu, int pri, int 
balance_step)
                  && !__vcpu_has_soft_affinity(vc, vc->cpu_hard_affinity) )
                 continue;
 
-            csched_balance_cpumask(vc, balance_step, cpumask_scratch_cpu(cpu));
-            if ( __csched_vcpu_is_migrateable(vc, cpu,
-                                              cpumask_scratch_cpu(cpu)) )
+            csched_balance_cpumask(vc, balance_step, cpumask_scratch);
+            if ( __csched_vcpu_is_migrateable(vc, cpu, cpumask_scratch) )
             {
                 /* We got a candidate. Grab it! */
                 TRACE_3D(TRC_CSCHED_STOLEN_VCPU, peer_cpu,
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index 36ff2e9..bee5d1f 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -65,12 +65,7 @@ static void poll_timer_fn(void *data);
 DEFINE_PER_CPU(struct schedule_data, schedule_data);
 DEFINE_PER_CPU(struct scheduler *, scheduler);
 
-/*
- * Scratch space, for avoiding having too many cpumask_var_t on the stack.
- * Properly serializing access, if necessary, is responsibility of each
- * scheduler (typically, one can expect this to be protected by the per pCPU
- * or per runqueue lock).
- */
+/* Scratch space for cpumasks. */
 DEFINE_PER_CPU(cpumask_t, cpumask_scratch);
 
 extern const struct scheduler *__start_schedulers_array[], 
*__end_schedulers_array[];
diff --git a/xen/include/xen/sched-if.h b/xen/include/xen/sched-if.h
index bc0e794..c25cda6 100644
--- a/xen/include/xen/sched-if.h
+++ b/xen/include/xen/sched-if.h
@@ -47,6 +47,13 @@ DECLARE_PER_CPU(struct schedule_data, schedule_data);
 DECLARE_PER_CPU(struct scheduler *, scheduler);
 DECLARE_PER_CPU(struct cpupool *, cpupool);
 
+/*
+ * Scratch space, for avoiding having too many cpumask_var_t on the stack.
+ * Within each scheduler, when using the scratch mask of one pCPU:
+ * - the pCPU must belong to the scheduler,
+ * - the caller must own the per-pCPU scheduler lock (a.k.a. runqueue
+ *   lock).
+ */
 DECLARE_PER_CPU(cpumask_t, cpumask_scratch);
 #define cpumask_scratch        (&this_cpu(cpumask_scratch))
 #define cpumask_scratch_cpu(c) (&per_cpu(cpumask_scratch, c))


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