sched/fair: Add lsub_positive() and use it consistently

The following pattern:

   var -= min_t(typeof(var), var, val);

is used multiple times in fair.c.

The existing sub_positive() already captures that pattern, but it also
adds an explicit load-store to properly support lockless observations.
In other cases the pattern above is used to update local, and/or not
concurrently accessed, variables.

Let's add a simpler version of sub_positive(), targeted at local variables
updates, which gives the same readability benefits at calling sites,
without enforcing {READ,WRITE}_ONCE() barriers.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Perret <quentin.perret@arm.com>
Cc: Steve Muckle <smuckle@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Todd Kjos <tkjos@google.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/lkml/20181031184527.GA3178@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 28ee60cabba197ea292a4719d095a29e8e00764c..2cac9a469df417cf531d3c67d2a21a2998a4d82c 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2734,6 +2734,17 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
        WRITE_ONCE(*ptr, res);                                  \
 } while (0)
 
+/*
+ * Remove and clamp on negative, from a local variable.
+ *
+ * A variant of sub_positive(), which does not use explicit load-store
+ * and is thus optimized for local variable updates.
+ */
+#define lsub_positive(_ptr, _val) do {                         \
+       typeof(_ptr) ptr = (_ptr);                              \
+       *ptr -= min_t(typeof(*ptr), *ptr, _val);                \
+} while (0)
+
 #ifdef CONFIG_SMP
 static inline void
 enqueue_runnable_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
@@ -4639,7 +4650,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
                cfs_b->distribute_running = 0;
                throttled = !list_empty(&cfs_b->throttled_cfs_rq);
 
-               cfs_b->runtime -= min(runtime, cfs_b->runtime);
+               lsub_positive(&cfs_b->runtime, runtime);
        }
 
        /*
@@ -4773,7 +4784,7 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
 
        raw_spin_lock(&cfs_b->lock);
        if (expires == cfs_b->runtime_expires)
-               cfs_b->runtime -= min(runtime, cfs_b->runtime);
+               lsub_positive(&cfs_b->runtime, runtime);
        cfs_b->distribute_running = 0;
        raw_spin_unlock(&cfs_b->lock);
 }
@@ -6240,7 +6251,7 @@ static unsigned long cpu_util_without(int cpu, struct task_struct *p)
        util = READ_ONCE(cfs_rq->avg.util_avg);
 
        /* Discount task's util from CPU's util */
-       util -= min_t(unsigned int, util, task_util(p));
+       lsub_positive(&util, task_util(p));
 
        /*
         * Covered cases:
@@ -6289,10 +6300,9 @@ static unsigned long cpu_util_without(int cpu, struct task_struct *p)
                 * properly fix the execl regression and it helps in further
                 * reducing the chances for the above race.
                 */
-               if (unlikely(task_on_rq_queued(p) || current == p)) {
-                       estimated -= min_t(unsigned int, estimated,
-                                          _task_util_est(p));
-               }
+               if (unlikely(task_on_rq_queued(p) || current == p))
+                       lsub_positive(&estimated, _task_util_est(p));
+
                util = max(util, estimated);
        }