Re: [PATCH v2 1/3] sched/fair: Remove the write-order dependency between cpu.max and cpu.max.burst

[Tao Cui]

Hi, Zhe,

在 2026/9/4 14:20, Zhe Liu 写道:
> Keep the configured burst independent of the current quota and cap it
> when CFS refills runtime. This allows quota and burst updates in either
> order.
> 

Ran the selftests on next-20260903 in a VM: all 10 test_cpu cases pass
with the series applied, and test_cpucg_max_burst fails on the unpatched
kernel, so the test does catch the old behavior.

One problem (spotted by sashiko, an automated overflow checker; I
re-did the arithmetic): dropping the burst_us + quota_us <=
max_bw_runtime_us check is not safe. With quota and burst both near
MAX_BW, the period-scaling path in sched_cfs_period_timer() can double
both up to 512x, and the new clamp quota + min(burst, quota) in
__refill_cfs_bandwidth_runtime() then wraps u64 to 0, leaving the group
with zero runtime on every refill.

I believe you can just drop the burst_us > quota_us comparison and keep
the sum check. Both write-order cases still pass that way: burst=80ms
with quota=50ms gives 130ms, well under the limit.

Tested-by: Tao Cui <cuitao@xxxxxxxxxx>

> Fixes: f4183717b370 ("sched/fair: Introduce the burstable CFS controller")
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Zhe Liu <liuzhe1@xxxxxxxxxx>
> ---
>  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 f78275192036..5269b8cfcf7f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10159,8 +10159,7 @@ 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)
>  		return -EINVAL;
>  
>  #ifdef CONFIG_CFS_BANDWIDTH
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6d881e530f89..488de18d477e 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;
>  }
>