[PATCH] sched/fair: limit burst to zero when cfs bandwidth is toggled off

[Zhao Wenhui]

When the quota value in CFS bandwidth is set to -1, that imples the
cfs bandwidth is toggled off. So the burst feature is supposed to
be disable as well.

Currently, when quota is -1, burst will not be check, so that it can be
set to almost arbitery value. Examples:
        mkdir /sys/fs/cgroup/cpu/test
        echo -1 > /sys/fs/cgroup/cpu/test/cpu.cfs_quota_us
        echo 10000000000000000 > /sys/fs/cgroup/cpu/test/cpu.cfs_burst_us

Moreover, after the burst modified by this way, quota can't be set
to any value:
        echo 100000 > cpu.cfs_quota_us
        -bash: echo: write error: Invalid argument

This patch can ensure the burst value being zero and unalterable,
when quota is set to -1.

Fixes: f4183717b370 ("sched/fair: Introduce the burstable CFS controller")
Reported-by: Zhao Gongyi <zhaogongyi@xxxxxxxxxx>
Signed-off-by: Zhao Wenhui <zhaowenhui8@xxxxxxxxxx>
---
 kernel/sched/core.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ee28253c9ac0..4c1fc01d8c68 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10584,6 +10584,12 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota,
 				     burst + quota > max_cfs_runtime))
 		return -EINVAL;
 
+	/*
+	 * Ensure burst equals to zero when quota is -1.
+	 */
+	if (quota == RUNTIME_INF && burst)
+		return -EINVAL;
+
 	/*
 	 * Prevent race between setting of cfs_rq->runtime_enabled and
 	 * unthrottle_offline_cfs_rqs().
@@ -10643,8 +10649,10 @@ static int tg_set_cfs_quota(struct task_group *tg, long cfs_quota_us)
 
 	period = ktime_to_ns(tg->cfs_bandwidth.period);
 	burst = tg->cfs_bandwidth.burst;
-	if (cfs_quota_us < 0)
+	if (cfs_quota_us < 0) {
 		quota = RUNTIME_INF;
+		burst = 0;
+	}
 	else if ((u64)cfs_quota_us <= U64_MAX / NSEC_PER_USEC)
 		quota = (u64)cfs_quota_us * NSEC_PER_USEC;
 	else
-- 
2.17.1