File credit2-avoid-picking-a-spurious-idle-unit-when-caps-are-used.patch of Package xen.19912

From 4d879b7afbc61dde3a641a50a09e5ef439ad510c Mon Sep 17 00:00:00 2001
From: Dario Faggioli <dfaggioli@suse.com>
Date: Mon, 21 Jun 2021 18:25:34 +0200
Subject: [PATCH 2/2] Xen: credit2: avoid picking a spurious idle unit when
 caps are used

Commit 07b0eb5d0ef0 ("credit2: make sure we pick a runnable unit from the
runq if there is one") did not fix completely the problem of potentially
selecting a scheduling unit that will then not be able to run.

In fact, in case caps are used and the unit we are currently looking
at, during the runqueue scan, does not have enough budget for being run,
we should continue looking instead than giving up and picking the idle
unit.

Suggested-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
 xen/common/sched/credit2.c | 32 +++++++++++++++++++++++++-------
 1 file changed, 25 insertions(+), 7 deletions(-)

diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c
index f5c1e5b944..92789dddaa 100644
--- a/xen/common/sched/credit2.c
+++ b/xen/common/sched/credit2.c
@@ -3463,6 +3463,15 @@ runq_candidate(struct csched2_runqueue_data *rqd,
                         (unsigned char *)&d);
         }
 
+        /*
+         * If the unit in the runqueue has more credits than current (or than
+         * idle, if current is not runnable) or if current is yielding, we may
+         * want to pick it up. Otherwise, there's no need to keep scanning the
+         * runqueue any further.
+         */
+        if ( !yield && svc->credit <= snext->credit )
+            break;
+
         /* Skip non runnable units that we (temporarily) have in the runq */
         if ( unlikely(!unit_runnable_state(svc->unit)) )
             continue;
@@ -3494,16 +3503,25 @@ runq_candidate(struct csched2_runqueue_data *rqd,
         }
 
         /*
-         * If the one in the runqueue has more credit than current (or idle,
-         * if current is not runnable), or if current is yielding, and also
-         * if the one in runqueue either is not capped, or is capped but has
-         * some budget, then choose it.
+         * If we are here, we are almost sure we want to pick the unit in
+         * the runqueue. Last thing we need to check is that it either is
+         * not capped or, if it is, it has some budget.
+         *
+         * Note that budget availability must be the very last check that we
+         * do, in this loop, due to the side effects that unit_grab_budget().
+         * causes.
+         *
+         * In fact, if there is budget available in the unit's domain's
+         * budget pool, the function will pick some for running this unit.
+         * And we clearly want to do that only if we're otherwise sure that
+         * the unit will actually run, consume it, and return the leftover
+         * (if any) in the usual way.
          */
-        if ( (yield || svc->credit > snext->credit) &&
-             (!has_cap(svc) || unit_grab_budget(svc)) )
-            snext = svc;
+        if ( has_cap(svc) && !unit_grab_budget(svc) )
+            continue;
 
         /* In any case, if we got this far, break. */
+        snext = svc;
         break;
     }
 
-- 
2.32.0