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[Xen-devel] [PATCH 4/4] xen: sched: simplify (and speedup) checking soft-affinity

The fact of whether or not a vCPU has a soft-affinity
which is effective, i.e., with the power of actually
affecting the scheduling of the vCPU itself rarely
changes. Very, very rarely, as compared to how often
we need to check for the same thing (basically, at
every scheduling decision!).

That can be improved by storing in a (per-vCPU) flag
(it's actually a boolean field in struct vcpu) whether
or not, considering how hard-affinity and soft-affinity
look like, soft-affinity should or not be taken into
account during scheduling decisions.

This saves some cpumask manipulations, which is nice,
considering how frequently they were being done. Note
that we can't get rid of 100% of the cpumask operations
involved in the check, because soft-affinity being
effective or not, not only depends on the relationship
between the hard and soft-affinity masks of a vCPU, but
also of the online pCPUs and/or of what pCPUs are part
of the cpupool where the vCPU lives, and that's rather
impractical to store in a per-vCPU flag. Still the
overhead is reduced to "just" one cpumask_subset() (and
only if the newly introduced flag is 'true')!

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Anshul Makkar <anshulmakkar@xxxxxxxxx>
 xen/common/schedule.c      |    5 +++++
 xen/include/xen/sched-if.h |    7 +++----
 xen/include/xen/sched.h    |    2 ++
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index a8b82fd..284df66 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -854,6 +854,11 @@ void sched_set_affinity(
         cpumask_copy(v->cpu_hard_affinity, hard);
     if ( soft )
         cpumask_copy(v->cpu_hard_affinity, soft);
+    v->soft_aff_effective = !cpumask_subset(v->cpu_hard_affinity,
+                                            v->cpu_soft_affinity) &&
+                            cpumask_intersects(v->cpu_soft_affinity,
+                                               v->cpu_hard_affinity);
 static int vcpu_set_affinity(
diff --git a/xen/include/xen/sched-if.h b/xen/include/xen/sched-if.h
index 417789a..1f4ff1b 100644
--- a/xen/include/xen/sched-if.h
+++ b/xen/include/xen/sched-if.h
@@ -247,10 +247,9 @@ static inline cpumask_t* cpupool_domain_cpumask(struct 
domain *d)
 static inline int has_soft_affinity(const struct vcpu *v)
-    return !cpumask_subset(cpupool_domain_cpumask(v->domain),
-                           v->cpu_soft_affinity) &&
-           !cpumask_subset(v->cpu_hard_affinity, v->cpu_soft_affinity) &&
-           cpumask_intersects(v->cpu_soft_affinity, v->cpu_hard_affinity);
+    return v->soft_aff_effective &&
+           !cpumask_subset(cpupool_domain_cpumask(v->domain),
+                           v->cpu_soft_affinity);
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 4f386f1..68c66f3 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -210,6 +210,8 @@ struct vcpu
     bool             hcall_compat;
+    /* Does soft affinity actually play a role (given hard affinity)? */
+    bool             soft_aff_effective;
      * > 0: a single port is being polled;

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