linux-kernel - Re: [RESEND] sched/fair: Add min_ratio for cfs bandwidth

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Date:   Thu, 20 Oct 2022 19:08:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Chuyi Zhou <zhouchuyi@...edance.com>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, htejun@...il.com,
        lizefan.x@...edance.com, vschneid@...hat.com, bsegall@...gle.com,
        Abel Wu <wuyun.abel@...edance.com>
Subject: Re: [RESEND] sched/fair: Add min_ratio for cfs bandwidth_control

On Wed, Oct 19, 2022 at 11:21:19AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Wed, Oct 19, 2022 at 11:15:51AM +0800, Chuyi Zhou wrote:
> > Tasks may be throttled when holding locks for a long time by current
> > cfs bandwidth control mechanism once users set a too small quota/period
> > ratio, which can result whole system get stuck[1].
> > 
> > In order to prevent the above situation from happening, this patch adds
> > sysctl_sched_cfs_bandwidth_min_ratio in /proc/sys/kernel, which indicates
> > the minimum percentage of quota/period users can set. The default value is
> > zero and users can set quota and period without triggering this constraint.
> > 
> > Link[1]:https://lore.kernel.org/lkml/5987be34-b527-4ff5-a17d-5f6f0dc94d6d@huawei.com/T/
> > Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
> > Suggested-by: Abel Wu <wuyun.abel@...edance.com>
> 
> This is a bit of a bandaid. I think what we really need to do is only
> throttling when running in userspace. In kernel space, it should just keep
> accumulating used cycles as debt which should be paid back before userspace
> code can run again so that we don't throttle at random places in the kernel.

That's just moving the problem. But yeah; perhaps. Starving random
userspace is less of a problem I suppose.