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From:   Alexander Graf <graf@amazon.com>
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Subject: [PATCH 2/3] sched: Trigger new hrtick if timer expires too fast
Date:   Thu, 27 Aug 2020 02:42:49 +0200
Message-ID: <20200827004250.4853-3-graf@amazon.com>
In-Reply-To: <20200827004250.4853-1-graf@amazon.com>
References: <20200827004250.4853-1-graf@amazon.com>
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Sender: linux-kernel-owner@vger.kernel.org
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When an hrtick timer event occurs too quickly, we just bail out and don't
attempt to set a new hrtick timeout. That means that the time slice for
that particular task grows until the next HZ tick occurs. That again may
create significant jitter for the respective task, as it will not get
scheduled for as long as it executed before, to bring the overall queue's
vruntime into balance again.

With this patch, even a too early hrtick timer event will just reconfigure
the hrtick to when we expected it to fire, removing overall jitter from
the system.

Signed-off-by: Alexander Graf <graf@amazon.com>
---
 kernel/sched/fair.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0d4ff3ab2572..66e7aae8b15e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -99,6 +99,8 @@ static int __init setup_sched_thermal_decay_shift(char *str)
 }
 __setup("sched_thermal_decay_shift=", setup_sched_thermal_decay_shift);
 
+static void hrtick_update(struct rq *rq);
+
 #ifdef CONFIG_SMP
 /*
  * For asym packing, by default the lower numbered CPU has higher priority.
@@ -4458,8 +4460,10 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 	 * narrow margin doesn't have to wait for a full slice.
 	 * This also mitigates buddy induced latencies under load.
 	 */
-	if (delta_exec < sysctl_sched_min_granularity)
+	if (delta_exec < sysctl_sched_min_granularity) {
+		hrtick_update(rq_of(cfs_rq));
 		return;
+	}
 
 	se = __pick_first_entity(cfs_rq);
 	delta = curr->vruntime - se->vruntime;
-- 
2.26.2


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