Re: [RFC PATCH v2 0/7] Defer throttle when task exits to user

Aaron Lu
2025-04-14T11:48:09Z

On Mon, Apr 14, 2025 at 11:05:30AM +0800, Chengming Zhou wrote:
> On 2025/4/9 20:07, Aaron Lu wrote:
> > This is a continuous work based on Valentin Schneider's posting here:
> > Subject: [RFC PATCH v3 00/10] sched/fair: Defer CFS throttle to user entry
> > https://lore.kernel.org/lkml/20240711130004.2157737-1-vschneid@redhat.com/
> >
> > Valentin has described the problem very well in the above link. We also
> > have task hung problem from time to time in our environment due to cfs quota.
> > It is mostly visible with rwsem: when a reader is throttled, writer comes in
> > and has to wait, the writer also makes all subsequent readers wait,
> > causing problems of priority inversion or even whole system hung.
> >
> > To improve this situation, change the throttle model to task based, i.e.
> > when a cfs_rq is throttled, mark its throttled status but do not
> > remove it from cpu's rq. Instead, for tasks that belong to this cfs_rq,
> > when they get picked, add a task work to them so that when they return
> > to user, they can be dequeued. In this way, tasks throttled will not
> > hold any kernel resources. When cfs_rq gets unthrottled, enqueue back
> > those throttled tasks.
> >
> > There are consequences because of this new throttle model, e.g. for a
> > cfs_rq that has 3 tasks attached, when 2 tasks are throttled on their
> > return2user path, one task still running in kernel mode, this cfs_rq is
> > in a partial throttled state:
> > - Should its pelt clock be frozen?
> > - Should this state be accounted into throttled_time?
> >
> > For pelt clock, I chose to keep the current behavior to freeze it on
> > cfs_rq's throttle time. The assumption is that tasks running in kernel
> > mode should not last too long, freezing the cfs_rq's pelt clock can keep
> > its load and its corresponding sched_entity's weight. Hopefully, this can
> > result in a stable situation for the remaining running tasks to quickly
> > finish their jobs in kernel mode.
>
> Seems reasonable to me, although I'm wondering is it possible or desirable
> to implement per-task PELT freeze?

Interesting idea.

One thing I'm thinking, would per-task PELT freeze cause task and its
cfs_rq's pelt clock un-sync? If so, I feel it would create some headaches
but I haven't thought through this yet.

> >
> > For throttle time accounting, I can see several possibilities:
> > - Similar to current behavior: starts accounting when cfs_rq gets
> > throttled(if cfs_rq->nr_queued > 0) and stops accounting when cfs_rq
> > gets unthrottled. This has one drawback, e.g. if this cfs_rq has one
> > task when it gets throttled and eventually, that task doesn't return
> > to user but blocks, then this cfs_rq has no tasks on throttled list
> > but time is accounted as throttled; Patch2 and patch3 implements this
> > accounting(simple, fewer code change).
> > - Starts accounting when the throttled cfs_rq has at least one task on
> > its throttled list; stops accounting when it's unthrottled. This kind
> > of over accounts throttled time because partial throttle state is
> > accounted.
> > - Starts accounting when the throttled cfs_rq has no tasks left and its
> > throttled list is not empty; stops accounting when this cfs_rq is
> > unthrottled; This kind of under accounts throttled time because partial
> > throttle state is not accounted. Patch7 implements this accounting.
> > I do not have a strong feeling which accounting is the best, it's open
> > for discussion.
>
> I personally prefer option 2, which has a more practical throttled time,
> so we can know how long there are some tasks throttled in fact.
>
> Thanks!

Thanks for the input.

Now I think about this more, I feel option 2 is essentially a better
version of option 1 because it doesn't have the drawback of option 1
I mentioned above, so option 1 should probably just be ruled out.

Then there are only 2 options to consider and their difference is
basically whether to treat partial throttle state as throttled or not.

Thanks,
Aaron

> >
> > There is also the concern of increased duration of (un)throttle operations
> > in v1. I've done some tests and with a 2000 cgroups/20K runnable tasks
> > setup on a 2sockets/384cpus AMD server, the longest duration of
> > distribute_cfs_runtime() is in the 2ms-4ms range. For details, please see:
> > https://lore.kernel.org/lkml/20250324085822.GA732629@bytedance/
> > For throttle path, with Chengming's suggestion to move "task work setup"
> > from throttle time to pick time, it's not an issue anymore.
> >
> > Patches:
> > Patch1 is preparation work;
> >
> > Patch2-3 provide the main functionality.
> > Patch2 deals with throttle path: when a cfs_rq is to be throttled, mark
> > throttled status for this cfs_rq and when tasks in throttled hierarchy
> > gets picked, add a task work to them so that when those tasks return to
> > user space, the task work can throttle it by dequeuing the task and
> > remember this by adding the task to its cfs_rq's limbo list;
> > Patch3 deals with unthrottle path: when a cfs_rq is to be unthrottled,
> > enqueue back those tasks in limbo list;
> >
> > Patch4 deals with the dequeue path when task changes group, sched class
> > etc. Task that is throttled is dequeued in fair, but task->on_rq is
> > still set so when it changes task group or sched class or has affinity
> > setting change, core will firstly dequeue it. But since this task is
> > already dequeued in fair class, this patch handle this situation.
> >
> > Patch5-6 are clean ups. Some code are obsolete after switching to task
> > based throttle mechanism.
> >
> > Patch7 implements an alternative accounting mechanism for task based
> > throttle.
> >
> > Changes since v1:
> > - Move "add task work" from throttle time to pick time, suggested by
> > Chengming Zhou;
> > - Use scope_gard() and cond_resched_tasks_rcu_qs() in
> > throttle_cfs_rq_work(), suggested by K Prateek Nayak;
> > - Remove now obsolete throttled_lb_pair(), suggested by K Prateek Nayak;
> > - Fix cfs_rq->runtime_remaining condition check in unthrottle_cfs_rq(),
> > suggested by K Prateek Nayak;
> > - Fix h_nr_runnable accounting for delayed dequeue case when task based
> > throttle is in use;
> > - Implemented an alternative way of throttle time accounting for
> > discussion purpose;
> > - Make !CONFIG_CFS_BANDWIDTH build.
> > I hope I didn't omit any feedbacks I've received, but feel free to let me
> > know if I did.
> >
> > As in v1, all change logs are written by me and if they read bad, it's
> > my fault.
> >
> > Comments are welcome.
> >
> > Base commit: tip/sched/core, commit 6432e163ba1b("sched/isolation: Make
> > use of more than one housekeeping cpu").
> >
> > Aaron Lu (4):
> > sched/fair: Take care of group/affinity/sched_class change for
> > throttled task
> > sched/fair: get rid of throttled_lb_pair()
> > sched/fair: fix h_nr_runnable accounting with per-task throttle
> > sched/fair: alternative way of accounting throttle time
> >
> > Valentin Schneider (3):
> > sched/fair: Add related data structure for task based throttle
> > sched/fair: Handle throttle path for task based throttle
> > sched/fair: Handle unthrottle path for task based throttle
> >
> > include/linux/sched.h | 4 +
> > kernel/sched/core.c | 3 +
> > kernel/sched/fair.c | 449 ++++++++++++++++++++++--------------------
> > kernel/sched/sched.h | 7 +
> > 4 files changed, 248 insertions(+), 215 deletions(-)
> >