[PATCH v3 1/3] sched/fair: remove quota/burst write-order dependency

[Zhe Liu <liuzhe1@kylinos.cn>]

Burst validation currently depends on the active quota, so the same quota
and burst values may be accepted or rejected depending on the order in
which userspace updates them.

Make burst validation independent of the current quota while preserving,
for finite quotas, the previous maximum configurable burst at
max_bw_runtime_us / 2. Keep the configured burst across quota updates and
cap the effective burst to min(burst, quota) when CFS runtime is refilled.

Fixes: f4183717b370 ("sched/fair: Introduce the burstable CFS controller")
Signed-off-by: Zhe Liu <liuzhe1@kylinos.cn>
Tested-by: Tao Cui <cuitao@kylinos.cn>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
---
 kernel/sched/core.c | 3 +--
 kernel/sched/fair.c | 3 ++-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b998ef6b87af..c399f0a59c25 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10181,8 +10181,7 @@ static int tg_set_bandwidth(struct task_group *tg,
 	if (quota_us != RUNTIME_INF && quota_us > max_bw_runtime_us)
 		return -EINVAL;
 
-	if (quota_us != RUNTIME_INF && (burst_us > quota_us ||
-					burst_us + quota_us > max_bw_runtime_us))
+	if (burst_us > max_bw_runtime_us / 2)
 		return -EINVAL;
 
 #ifdef CONFIG_CFS_BANDWIDTH
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ade1eceb39b8..e6f9d9296fd2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6627,7 +6627,8 @@ void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)
 		cfs_b->nr_burst++;
 	}
 
-	cfs_b->runtime = min(cfs_b->runtime, cfs_b->quota + cfs_b->burst);
+	cfs_b->runtime = min(cfs_b->runtime,
+			     cfs_b->quota + min(cfs_b->burst, cfs_b->quota));
 	cfs_b->runtime_snap = cfs_b->runtime;
 }
 
-- 
2.25.1