Re: [PATCH v2] sched/deadline: only set free_cpus for online runqueues
From: Florian Fainelli
Date: Thu Oct 02 2025 - 15:16:07 EST
On 9/23/2025 11:03 AM, Doug Berger wrote:
On 9/4/2025 9:43 PM, Doug Berger wrote:
On 9/3/2025 12:54 AM, Peter Zijlstra wrote:
On Mon, Aug 18, 2025 at 02:58:19PM +0200, Juri Lelli wrote:Thanks for taking a look.
Hello,
On 14/08/25 18:22, Doug Berger wrote:
Commit 16b269436b72 ("sched/deadline: Modify cpudl::free_cpus
to reflect rd->online") introduced the cpudl_set/clear_freecpu
functions to allow the cpu_dl::free_cpus mask to be manipulated
by the deadline scheduler class rq_on/offline callbacks so the
mask would also reflect this state.
Commit 9659e1eeee28 ("sched/deadline: Remove cpu_active_mask
from cpudl_find()") removed the check of the cpu_active_mask to
save some processing on the premise that the cpudl::free_cpus
mask already reflected the runqueue online state.
Unfortunately, there are cases where it is possible for the
cpudl_clear function to set the free_cpus bit for a CPU when the
deadline runqueue is offline. When this occurs while a CPU is
connected to the default root domain the flag may retain the bad
state after the CPU has been unplugged. Later, a different CPU
that is transitioning through the default root domain may push a
deadline task to the powered down CPU when cpudl_find sees its
free_cpus bit is set. If this happens the task will not have the
opportunity to run.
One example is outlined here:
https://lore.kernel.org/lkml/20250110233010.2339521-1- opendmb@xxxxxxxxx
Another occurs when the last deadline task is migrated from a
CPU that has an offlined runqueue. The dequeue_task member of
the deadline scheduler class will eventually call cpudl_clear
and set the free_cpus bit for the CPU.
This commit modifies the cpudl_clear function to be aware of the
online state of the deadline runqueue so that the free_cpus mask
can be updated appropriately.
It is no longer necessary to manage the mask outside of the
cpudl_set/clear functions so the cpudl_set/clear_freecpu
functions are removed. In addition, since the free_cpus mask is
now only updated under the cpudl lock the code was changed to
use the non-atomic __cpumask functions.
Signed-off-by: Doug Berger <opendmb@xxxxxxxxx>
---
This looks now good to me.
Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
So I just had a look at said patch because Juri here poked me; and I
came away with the feeling that cpudl_clear() is now a misnomen, seeing
how it is called from rq_online_dl().
Would cpudl_update() be a better name?
I don't really have a dog in any fights over naming here, but it seems to me that cpudl_clear and cpudl_set are intended to be complementary functions and the naming reflects that. It would appear that these are primarily intended to maintain the cpudl max-heap entries which is what are being set and cleared.
rq_online_dl() would now call one or the other based on whether any deadline tasks are running on the queue when it is onlined to ensure that the max-heap is valid. This either clears a stale entry that may occur from scenarios like the ones I'm running into or set the entry to the current deadline. In this context the names seem appropriate to me.
Renaming cpudl_clear to cpudl_update may be more confusing since the comment for cpudl_set reads "cpudl_set - update the cpudl max-heap".
I don't feel that the name change is relevant to my patch, but if we want to do it concurrently maybe cpudl_clear_max_heap() and cpudl_set_max_heap() would be more meaningful.
Please let me know how you would like to proceed,
Doug
Is there any way I can help to move this patch forward?
OK, so we are all busy and whatnot but the response time on this patch seems to be completely abysmal, and reminder that it fixes an actual bug that we have been running into on production systems. Can we just get it applied and move on?
--
Florian