From: Tejun Heo <tj@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: torvalds@linux-foundation.org, mingo@redhat.com,
peterz@infradead.org, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
bristot@redhat.com, vschneid@redhat.com, ast@kernel.org,
daniel@iogearbox.net, andrii@kernel.org, martin.lau@kernel.org,
joshdon@google.com, brho@google.com, pjt@google.com,
derkling@google.com, haoluo@google.com, dvernet@meta.com,
dschatzberg@meta.com, dskarlat@cs.cmu.edu, riel@surriel.com,
changwoo@igalia.com, himadrics@inria.fr, memxor@gmail.com,
linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
kernel-team@meta.com, Andrea Righi <andrea.righi@canonical.com>
Subject: Re: [PATCH 12/36] sched_ext: Implement BPF extensible scheduler class
Date: Fri, 26 Apr 2024 11:58:17 -1000 [thread overview]
Message-ID: <ZiwjecOGR3G4ZGbS@slm.duckdns.org> (raw)
In-Reply-To: <CAEXW_YR02g=DetfwM98ZoveWEbGbGGfb1KAikcBeC=Pkvqf4OA@mail.gmail.com>
Hello, Joel.
On Thu, Apr 25, 2024 at 05:28:40PM -0400, Joel Fernandes wrote:
> Got it. I took some time to look at it some more. Now I am wondering
> why check_preempt_curr() has to be separately implemented for a class
> and why the enqueue() handler of each class cannot take care of
> preempting curr via setting resched flags.
>
> The only reason I can see is that, activate_task() is not always
> followed by a check_preempt_curr() and sometimes there is an
> unconditional resched_curr() happening following the call to
> activate_task().
>
> But such issues don't affect sched_ext in its current form I guess.
There's ttwu_runnable() path which just changes the target task's state and
then checks for preemption. The path doesn't involve enqueueing but can
still preempt. Maybe SCX might need to support this in the future too but it
doesn't seem pressing.
> Btw, if sched_ext were to be implemented as a higher priority class
> above CFS [1], then check_preempt_curr() may preempt without even
> calling the class's check_preempt_curr() :
>
> void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
> {
> if (p->sched_class == rq->curr->sched_class)
> rq->curr->sched_class->check_preempt_curr(rq, p, flags);
> else if (sched_class_above(p->sched_class, rq->curr->sched_class))
> resched_curr(rq);
>
> But if I understand, sched_ext is below CFS at the moment, so that
> should not be an issue.
>
> [1] By the way, now that I brought up the higher priority class thing,
> I might as well discuss it here :-D :
>
> One of my use cases is about scheduling high priority latency sensitive threads:
> I think if sched_ext could have 2 classes, one lower than CFS and one
> above CFS, that would be beneficial to those who want a gradual
> transition to use scx, instead of switching all tasks to scx at once.
>
> One reason is EAS (in CFS). It may be beneficial for people to use
> the existing EAS for everything but latency critical tasks (much like
> how people use RT class for those). This is quite involved and
> reimplementing EAS in BPF may be quite a project. Not that it
> shouldn't be implemented that way, but EAS is about a decade old with
> all kinds of energy modeling, math and what not. Having scx higher
> than cfs alongside the lower one is less of an invasive approach than
> switching everything on the system to scx.
I see.
> Do you have any opinions on that? If it makes sense, I can work on
> such an implementation.
>
> Another reason for this is, general purpose systems run very varied
> workloads, and big dramatic changes are likely to be reverted due to
> power and performance regressions. Hence, the request for a higher
> scx, so that we (high priority task scx users) can take baby steps.
Yeah, as a use case, it makes sense to me. Would it suffice to be able to
choose between above or below the fair class tho?
Thanks.
--
tejun
next prev parent reply other threads:[~2024-04-26 21:58 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-11 2:47 [PATCHSET v5] sched: Implement BPF extensible scheduler class Tejun Heo
2023-11-11 2:47 ` [PATCH 01/36] cgroup: Implement cgroup_show_cftypes() Tejun Heo
2023-11-11 2:47 ` [PATCH 02/36] sched: Restructure sched_class order sanity checks in sched_init() Tejun Heo
2023-11-11 2:47 ` [PATCH 03/36] sched: Allow sched_cgroup_fork() to fail and introduce sched_cancel_fork() Tejun Heo
2023-11-11 2:47 ` [PATCH 04/36] sched: Add sched_class->reweight_task() Tejun Heo
2023-11-11 2:47 ` [PATCH 05/36] sched: Add sched_class->switching_to() and expose check_class_changing/changed() Tejun Heo
2023-11-11 2:47 ` [PATCH 06/36] sched: Factor out cgroup weight conversion functions Tejun Heo
2023-11-11 2:47 ` [PATCH 07/36] sched: Expose css_tg() and __setscheduler_prio() Tejun Heo
2023-11-11 2:47 ` [PATCH 08/36] sched: Enumerate CPU cgroup file types Tejun Heo
2023-11-11 2:47 ` [PATCH 09/36] sched: Add @reason to sched_class->rq_{on|off}line() Tejun Heo
2023-11-11 2:47 ` [PATCH 10/36] sched: Add normal_policy() Tejun Heo
2023-11-11 2:47 ` [PATCH 11/36] sched_ext: Add boilerplate for extensible scheduler class Tejun Heo
2023-11-11 2:47 ` [PATCH 12/36] sched_ext: Implement BPF " Tejun Heo
2023-11-13 13:34 ` Changwoo Min
2023-11-14 19:07 ` Tejun Heo
2023-11-13 20:04 ` Andrea Righi
2023-11-14 19:07 ` Tejun Heo
2023-11-23 8:07 ` Andrea Righi
2023-11-25 19:59 ` Tejun Heo
2023-11-26 9:05 ` Andrea Righi
2023-12-07 2:04 ` [PATCH] scx: set p->scx.ops_state using atomic_long_set_release Changwoo Min
2023-12-08 0:16 ` Tejun Heo
2024-03-23 2:37 ` [PATCH 12/36] sched_ext: Implement BPF extensible scheduler class Joel Fernandes
2024-03-23 22:12 ` Tejun Heo
2024-04-25 21:28 ` Joel Fernandes
2024-04-26 16:57 ` Barret Rhoden
2024-04-26 21:58 ` Tejun Heo [this message]
2023-11-11 2:47 ` [PATCH 13/36] sched_ext: Add scx_simple and scx_example_qmap example schedulers Tejun Heo
2023-11-12 4:17 ` kernel test robot
2023-11-12 18:06 ` Tejun Heo
2023-11-11 2:47 ` [PATCH 14/36] sched_ext: Add sysrq-S which disables the BPF scheduler Tejun Heo
2023-11-11 2:47 ` [PATCH 15/36] sched_ext: Implement runnable task stall watchdog Tejun Heo
2023-11-11 2:47 ` [PATCH 16/36] sched_ext: Allow BPF schedulers to disallow specific tasks from joining SCHED_EXT Tejun Heo
2023-11-11 2:47 ` [PATCH 17/36] sched_ext: Allow BPF schedulers to switch all eligible tasks into sched_ext Tejun Heo
2023-11-11 2:47 ` [PATCH 18/36] sched_ext: Print sched_ext info when dumping stack Tejun Heo
2023-11-14 19:23 ` [PATCH v2 " Tejun Heo
2023-11-11 2:47 ` [PATCH 19/36] sched_ext: Implement scx_bpf_kick_cpu() and task preemption support Tejun Heo
2023-11-11 2:47 ` [PATCH 20/36] sched_ext: Add a central scheduler which makes all scheduling decisions on one CPU Tejun Heo
2023-11-11 2:47 ` [PATCH 21/36] sched_ext: Make watchdog handle ops.dispatch() looping stall Tejun Heo
2023-11-11 2:47 ` [PATCH 22/36] sched_ext: Add task state tracking operations Tejun Heo
2023-11-11 2:47 ` [PATCH 23/36] sched_ext: Implement tickless support Tejun Heo
2023-11-11 2:47 ` [PATCH 24/36] sched_ext: Track tasks that are subjects of the in-flight SCX operation Tejun Heo
2023-11-11 2:47 ` [PATCH 25/36] sched_ext: Add cgroup support Tejun Heo
2023-11-11 2:47 ` [PATCH 26/36] sched_ext: Add a cgroup-based core-scheduling scheduler Tejun Heo
2023-11-11 2:47 ` [PATCH 27/36] sched_ext: Add a cgroup scheduler which uses flattened hierarchy Tejun Heo
2023-11-11 2:47 ` [PATCH 28/36] sched_ext: Implement SCX_KICK_WAIT Tejun Heo
2023-11-11 2:47 ` [PATCH 29/36] sched_ext: Implement sched_ext_ops.cpu_acquire/release() Tejun Heo
2023-11-11 2:47 ` [PATCH 30/36] sched_ext: Implement sched_ext_ops.cpu_online/offline() Tejun Heo
2023-11-11 2:47 ` [PATCH 31/36] sched_ext: Implement core-sched support Tejun Heo
2023-11-11 2:47 ` [PATCH 32/36] sched_ext: Add vtime-ordered priority queue to dispatch_q's Tejun Heo
2023-11-11 2:47 ` [PATCH 33/36] sched_ext: Documentation: scheduler: Document extensible scheduler class Tejun Heo
2023-11-11 2:48 ` [PATCH 34/36] sched_ext: Add a basic, userland vruntime scheduler Tejun Heo
2023-11-11 2:48 ` [PATCH 35/36] sched_ext: Add scx_rusty, a rust userspace hybrid scheduler Tejun Heo
2023-11-11 2:48 ` [PATCH 36/36] sched_ext: Add scx_layered, a highly configurable multi-layer scheduler Tejun Heo
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