From: Linus Torvalds <torvalds@linux-foundation.org>
To: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
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,
andrea.righi@canonical.com, joel@joelfernandes.org,
linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
kernel-team@meta.com
Subject: Re: [PATCH sched_ext/for-6.11] sched, sched_ext: Replace scx_next_task_picked() with sched_class->switch_class()
Date: Thu, 20 Jun 2024 15:42:48 -0700 [thread overview]
Message-ID: <CAHk-=wjFPLqo7AXu8maAGEGnOy6reUg-F4zzFhVB0Kyu22h7pw@mail.gmail.com> (raw)
In-Reply-To: <ZnSp5mVp3uhYganb@slm.duckdns.org>
On Thu, 20 Jun 2024 at 15:15, Tejun Heo <tj@kernel.org> wrote:
>
> The changes are straightforward and the code looks better afterwards.
> However, when !CONFIG_SCHED_CLASS_EXT, this just ends up adding an unused
> hook which is unlikely to be useful to other sched_classes. We can #ifdef
> the op with CONFIG_SCHED_CLASS_EXT but then I'm not sure the code
> necessarily looks better afterwards.
So honestly, if people _really_ care about performance here, then I
think that in the long run the right thing to do is
- expose all the DEFINE_SCHED_CLASS() definitions in a header file
- rename for_each_class() to FOR_EACH_CLASS() and make it unroll the
whole damn loop statically
which would turn the indirect branches into actual direct branches,
and would statically just remove any "if (!class->zyz)" conditionals.
Pretty? No. But it probably wouldn't be hugely ugly either, and
honestly, looking at the existing for_each_class() uses (and the one
single "for_class_range()" one), they are so small and the number of
classes is so small that unrolling the loop entirely doesn't sound
bad.
It wouldn't help deal with *this* case (since it's a "call variable
class"), but considering that the current __pick_next_task()
(a) special-cases one class as-is
(b) does a "for_each_class()" and calls an indirect call for each
when that doesn't trigger
I would claim that people don't care enough about this that one test
for a NULL 'switch_class' function would be worth worrying about.
Btw, indirect calls are now expensive enough that when you have only a
handful of choices, instead of a variable
class->some_callback(some_arguments);
you might literally be better off with a macro that does
#define call_sched_fn(class, name, arg...) switch (class) { \
case &fair_name_class: fair_name_class.name(arg); break; \
... unroll them all here..
which then just generates a (very small) tree of if-statements.
Again, this is entirely too ugly to do unless people *really* care.
But for situations where you have a small handful of cases known at
compile-time, it's not out of the question, and it probably does
generate better code.
NOTE NOTE NOTE! This is a comp[letely independent aside, and has
nothing to do with sched_ext except for the very obvious indirect fact
that sched_ext would be one of the classes in this kind of code.
And yes, I suspect it is too ugly to actually do this.
Linus
next prev parent reply other threads:[~2024-06-20 22:43 UTC|newest]
Thread overview: 138+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-01 15:09 [PATCHSET v6] sched: Implement BPF extensible scheduler class Tejun Heo
2024-05-01 15:09 ` [PATCH 01/39] cgroup: Implement cgroup_show_cftypes() Tejun Heo
2024-05-01 15:09 ` [PATCH 02/39] sched: Restructure sched_class order sanity checks in sched_init() Tejun Heo
2024-05-01 15:09 ` [PATCH 03/39] sched: Allow sched_cgroup_fork() to fail and introduce sched_cancel_fork() Tejun Heo
2024-05-01 15:09 ` [PATCH 04/39] sched: Add sched_class->reweight_task() Tejun Heo
2024-06-24 10:23 ` Peter Zijlstra
2024-06-24 10:31 ` Peter Zijlstra
2024-06-24 23:59 ` Tejun Heo
2024-06-25 7:29 ` Peter Zijlstra
2024-06-25 23:57 ` Tejun Heo
2024-06-26 1:29 ` [PATCH sched/urgent] sched/fair: set_load_weight() must also call reweight_task() for SCHED_IDLE tasks Tejun Heo
2024-06-26 2:19 ` [PATCH sched_ext/for-6.11] sched_ext: Account for idle policy when setting p->scx.weight in scx_ops_enable_task() Tejun Heo
2024-05-01 15:09 ` [PATCH 05/39] sched: Add sched_class->switching_to() and expose check_class_changing/changed() Tejun Heo
2024-06-24 11:06 ` Peter Zijlstra
2024-06-24 22:18 ` Tejun Heo
2024-06-25 8:16 ` Peter Zijlstra
2024-05-01 15:09 ` [PATCH 06/39] sched: Factor out cgroup weight conversion functions Tejun Heo
2024-05-01 15:09 ` [PATCH 07/39] sched: Expose css_tg() and __setscheduler_prio() Tejun Heo
2024-06-24 11:19 ` Peter Zijlstra
2024-06-24 18:56 ` Tejun Heo
2024-05-01 15:09 ` [PATCH 08/39] sched: Enumerate CPU cgroup file types Tejun Heo
2024-05-01 15:09 ` [PATCH 09/39] sched: Add @reason to sched_class->rq_{on|off}line() Tejun Heo
2024-06-24 11:32 ` Peter Zijlstra
2024-06-24 21:18 ` Tejun Heo
2024-06-25 8:29 ` Peter Zijlstra
2024-06-25 23:41 ` Tejun Heo
2024-06-26 8:23 ` Peter Zijlstra
2024-06-26 18:01 ` Tejun Heo
2024-06-27 1:27 ` [PATCH sched_ext/for-6.11] sched_ext: Disallow loading BPF scheduler if isolcpus= domain isolation is in effect Tejun Heo
2024-05-01 15:09 ` [PATCH 10/39] sched: Factor out update_other_load_avgs() from __update_blocked_others() Tejun Heo
2024-06-24 12:35 ` Peter Zijlstra
2024-06-24 16:15 ` Vincent Guittot
2024-06-24 19:24 ` Tejun Heo
2024-06-25 9:13 ` Vincent Guittot
2024-06-26 20:49 ` Tejun Heo
2024-05-01 15:09 ` [PATCH 11/39] cpufreq_schedutil: Refactor sugov_cpu_is_busy() Tejun Heo
2024-05-01 15:09 ` [PATCH 12/39] sched: Add normal_policy() Tejun Heo
2024-05-01 15:09 ` [PATCH 13/39] sched_ext: Add boilerplate for extensible scheduler class Tejun Heo
2024-05-01 15:09 ` [PATCH 14/39] sched_ext: Implement BPF " Tejun Heo
2024-05-01 15:09 ` [PATCH 15/39] sched_ext: Add scx_simple and scx_example_qmap example schedulers Tejun Heo
2024-05-01 15:09 ` [PATCH 16/39] sched_ext: Add sysrq-S which disables the BPF scheduler Tejun Heo
2024-05-01 15:09 ` [PATCH 17/39] sched_ext: Implement runnable task stall watchdog Tejun Heo
2024-05-01 15:09 ` [PATCH 18/39] sched_ext: Allow BPF schedulers to disallow specific tasks from joining SCHED_EXT Tejun Heo
2024-06-24 12:40 ` Peter Zijlstra
2024-06-24 19:06 ` Tejun Heo
2024-05-01 15:09 ` [PATCH 19/39] sched_ext: Print sched_ext info when dumping stack Tejun Heo
2024-06-24 12:46 ` Peter Zijlstra
2024-06-24 14:25 ` Linus Torvalds
2024-06-24 18:34 ` Tejun Heo
2024-05-01 15:09 ` [PATCH 20/39] sched_ext: Print debug dump after an error exit Tejun Heo
2024-05-01 15:09 ` [PATCH 21/39] tools/sched_ext: Add scx_show_state.py Tejun Heo
2024-05-01 15:09 ` [PATCH 22/39] sched_ext: Implement scx_bpf_kick_cpu() and task preemption support Tejun Heo
2024-05-01 15:09 ` [PATCH 23/39] sched_ext: Add a central scheduler which makes all scheduling decisions on one CPU Tejun Heo
2024-05-01 15:09 ` [PATCH 24/39] sched_ext: Make watchdog handle ops.dispatch() looping stall Tejun Heo
2024-05-01 15:10 ` [PATCH 25/39] sched_ext: Add task state tracking operations Tejun Heo
2024-05-01 15:10 ` [PATCH 26/39] sched_ext: Implement tickless support Tejun Heo
2024-05-01 15:10 ` [PATCH 27/39] sched_ext: Track tasks that are subjects of the in-flight SCX operation Tejun Heo
2024-05-01 15:10 ` [PATCH 28/39] sched_ext: Add cgroup support Tejun Heo
2024-05-01 15:10 ` [PATCH 29/39] sched_ext: Add a cgroup scheduler which uses flattened hierarchy Tejun Heo
2024-05-01 15:10 ` [PATCH 30/39] sched_ext: Implement SCX_KICK_WAIT Tejun Heo
2024-05-01 15:10 ` [PATCH 31/39] sched_ext: Implement sched_ext_ops.cpu_acquire/release() Tejun Heo
2024-05-01 15:10 ` [PATCH 32/39] sched_ext: Implement sched_ext_ops.cpu_online/offline() Tejun Heo
2024-05-01 15:10 ` [PATCH 33/39] sched_ext: Bypass BPF scheduler while PM events are in progress Tejun Heo
2024-05-01 15:10 ` [PATCH 34/39] sched_ext: Implement core-sched support Tejun Heo
2024-05-01 15:10 ` [PATCH 35/39] sched_ext: Add vtime-ordered priority queue to dispatch_q's Tejun Heo
2024-05-01 15:10 ` [PATCH 36/39] sched_ext: Implement DSQ iterator Tejun Heo
2024-05-01 15:10 ` [PATCH 37/39] sched_ext: Add cpuperf support Tejun Heo
2024-05-01 15:10 ` [PATCH 38/39] sched_ext: Documentation: scheduler: Document extensible scheduler class Tejun Heo
2024-05-02 2:24 ` Bagas Sanjaya
2024-05-01 15:10 ` [PATCH 39/39] sched_ext: Add selftests Tejun Heo
2024-05-02 8:48 ` [PATCHSET v6] sched: Implement BPF extensible scheduler class Peter Zijlstra
2024-05-02 19:20 ` Tejun Heo
2024-05-03 8:52 ` Peter Zijlstra
2024-05-05 23:31 ` Tejun Heo
2024-05-13 8:03 ` Peter Zijlstra
2024-05-13 18:26 ` Steven Rostedt
2024-05-14 0:07 ` Qais Yousef
2024-05-14 21:34 ` David Vernet
2024-05-27 21:25 ` Qais Yousef
2024-05-28 23:46 ` Tejun Heo
2024-05-29 22:09 ` Qais Yousef
2024-05-17 9:58 ` Peter Zijlstra
2024-05-27 20:29 ` Qais Yousef
2024-05-14 20:22 ` Chris Mason
2024-05-14 22:06 ` Josh Don
2024-05-15 20:41 ` Tejun Heo
2024-05-21 0:19 ` Tejun Heo
2024-05-30 16:49 ` Tejun Heo
2024-05-06 18:47 ` Rik van Riel
2024-05-07 19:33 ` Tejun Heo
2024-05-07 19:47 ` Rik van Riel
2024-05-09 7:38 ` Changwoo Min
2024-05-10 18:24 ` Peter Jung
2024-05-13 20:36 ` Andrea Righi
2024-06-11 21:34 ` Linus Torvalds
2024-06-13 23:38 ` Tejun Heo
2024-06-19 20:56 ` Thomas Gleixner
2024-06-19 22:10 ` Linus Torvalds
2024-06-19 22:27 ` Thomas Gleixner
2024-06-19 22:55 ` Linus Torvalds
2024-06-20 2:35 ` Thomas Gleixner
2024-06-20 5:07 ` Linus Torvalds
2024-06-20 17:11 ` Linus Torvalds
2024-06-20 17:41 ` Tejun Heo
2024-06-20 22:15 ` [PATCH sched_ext/for-6.11] sched, sched_ext: Replace scx_next_task_picked() with sched_class->switch_class() Tejun Heo
2024-06-20 22:42 ` Linus Torvalds [this message]
2024-06-21 19:46 ` Tejun Heo
2024-06-24 9:04 ` Peter Zijlstra
2024-06-24 18:41 ` Tejun Heo
2024-06-24 9:02 ` Peter Zijlstra
2024-06-21 19:52 ` Tejun Heo
2024-06-24 8:59 ` Peter Zijlstra
2024-06-24 21:01 ` Tejun Heo
2024-06-25 7:49 ` Peter Zijlstra
2024-06-25 23:30 ` Tejun Heo
2024-06-26 8:28 ` Peter Zijlstra
2024-06-26 17:56 ` Tejun Heo
2024-06-20 18:47 ` [PATCHSET v6] sched: Implement BPF extensible scheduler class Thomas Gleixner
2024-06-20 19:20 ` Linus Torvalds
2024-06-21 9:35 ` Thomas Gleixner
2024-06-21 16:34 ` Linus Torvalds
2024-06-23 2:00 ` Tejun Heo
2024-06-23 10:31 ` Thomas Gleixner
2024-06-23 10:33 ` Thomas Gleixner
2024-06-24 14:23 ` Jason Gunthorpe
2024-06-20 19:58 ` Tejun Heo
2024-06-24 9:34 ` Peter Zijlstra
2024-06-24 20:17 ` Tejun Heo
2024-06-24 20:51 ` [PATCH sched_ext/for-6.11] sched, sched_ext: Simplify dl_prio() case handling in sched_fork() Tejun Heo
2024-06-20 19:35 ` [PATCHSET v6] sched: Implement BPF extensible scheduler class Tejun Heo
2024-06-21 10:46 ` Thomas Gleixner
2024-06-21 21:14 ` Chris Mason
2024-06-23 8:14 ` Thomas Gleixner
2024-06-24 16:42 ` Chris Mason
2024-06-24 18:11 ` Tejun Heo
2024-06-24 22:01 ` Peter Oskolkov
2024-06-24 22:17 ` David Vernet
2024-06-24 21:54 ` Peter Oskolkov
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