sched/fair: Add NOHZ balancer flag for nohz.next_balance updates

A following patch will trigger NOHZ idle balances as a means to update
nohz.next_balance. Vincent noted that blocked load updates can have
non-negligible overhead, which should be avoided if the intent is to only
update nohz.next_balance.

Add a new NOHZ balance kick flag, NOHZ_NEXT_KICK. Gate NOHZ blocked load
update by the presence of NOHZ_STATS_KICK - currently all NOHZ balance
kicks will have the NOHZ_STATS_KICK flag set, so no change in behaviour is
expected.

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210823111700.2842997-2-valentin.schneider@arm.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f6a05d9..f4de7f5 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10375,7 +10375,7 @@ static void nohz_balancer_kick(struct rq *rq)
                goto out;
 
        if (rq->nr_running >= 2) {
-               flags = NOHZ_KICK_MASK;
+               flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
                goto out;
        }
 
@@ -10389,7 +10389,7 @@ static void nohz_balancer_kick(struct rq *rq)
                 * on.
                 */
                if (rq->cfs.h_nr_running >= 1 && check_cpu_capacity(rq, sd)) {
-                       flags = NOHZ_KICK_MASK;
+                       flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
                        goto unlock;
                }
        }
@@ -10403,7 +10403,7 @@ static void nohz_balancer_kick(struct rq *rq)
                 */
                for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
                        if (sched_asym_prefer(i, cpu)) {
-                               flags = NOHZ_KICK_MASK;
+                               flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
                                goto unlock;
                        }
                }
@@ -10416,7 +10416,7 @@ static void nohz_balancer_kick(struct rq *rq)
                 * to run the misfit task on.
                 */
                if (check_misfit_status(rq, sd)) {
-                       flags = NOHZ_KICK_MASK;
+                       flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
                        goto unlock;
                }
 
@@ -10443,7 +10443,7 @@ static void nohz_balancer_kick(struct rq *rq)
                 */
                nr_busy = atomic_read(&sds->nr_busy_cpus);
                if (nr_busy > 1) {
-                       flags = NOHZ_KICK_MASK;
+                       flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
                        goto unlock;
                }
        }
@@ -10605,7 +10605,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
         * setting the flag, we are sure to not clear the state and not
         * check the load of an idle cpu.
         */
-       WRITE_ONCE(nohz.has_blocked, 0);
+       if (flags & NOHZ_STATS_KICK)
+               WRITE_ONCE(nohz.has_blocked, 0);
 
        /*
         * Ensures that if we miss the CPU, we must see the has_blocked
@@ -10627,13 +10628,15 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
                 * balancing owner will pick it up.
                 */
                if (need_resched()) {
-                       has_blocked_load = true;
+                       if (flags & NOHZ_STATS_KICK)
+                               has_blocked_load = true;
                        goto abort;
                }
 
                rq = cpu_rq(balance_cpu);
 
-               has_blocked_load |= update_nohz_stats(rq);
+               if (flags & NOHZ_STATS_KICK)
+                       has_blocked_load |= update_nohz_stats(rq);
 
                /*
                 * If time for next balance is due,
@@ -10664,8 +10667,9 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
        if (likely(update_next_balance))
                nohz.next_balance = next_balance;
 
-       WRITE_ONCE(nohz.next_blocked,
-               now + msecs_to_jiffies(LOAD_AVG_PERIOD));
+       if (flags & NOHZ_STATS_KICK)
+               WRITE_ONCE(nohz.next_blocked,
+                          now + msecs_to_jiffies(LOAD_AVG_PERIOD));
 
 abort:
        /* There is still blocked load, enable periodic update */

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3d3e579..1fec313 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2709,12 +2709,18 @@ extern void cfs_bandwidth_usage_dec(void);
 #define NOHZ_BALANCE_KICK_BIT  0
 #define NOHZ_STATS_KICK_BIT    1
 #define NOHZ_NEWILB_KICK_BIT   2
+#define NOHZ_NEXT_KICK_BIT     3
 
+/* Run rebalance_domains() */
 #define NOHZ_BALANCE_KICK      BIT(NOHZ_BALANCE_KICK_BIT)
+/* Update blocked load */
 #define NOHZ_STATS_KICK                BIT(NOHZ_STATS_KICK_BIT)
+/* Update blocked load when entering idle */
 #define NOHZ_NEWILB_KICK       BIT(NOHZ_NEWILB_KICK_BIT)
+/* Update nohz.next_balance */
+#define NOHZ_NEXT_KICK         BIT(NOHZ_NEXT_KICK_BIT)
 
-#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK)
+#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK | NOHZ_NEXT_KICK)
 
 #define nohz_flags(cpu)        (&cpu_rq(cpu)->nohz_flags)