On 2025-02-13 08:25, Gabriele Monaco wrote:
On Thu, 2025-02-13 at 14:52 +0800, kernel test robot wrote:
kernel test robot noticed
"WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:
[ 2.640924][ T0] ------------[ cut here ]------------
[ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at kernel/workqueue.c:2495
__queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9))
[ 2.642874][ T0] Modules linked in:
[ 2.643381][ T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted
6.14.0-rc2-00002-g287adf9e9c1f #1
[ 2.644582][ T0] Hardware name: QEMU Standard PC (i440FX +
PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
[ 2.645943][ T0] RIP: 0010:__queue_delayed_work
(kernel/workqueue.c:2495 (discriminator 9))
There seem to be major problems with this configuration, I'm trying to
understand what's wrong but, for the time being, this patchset is not
ready for inclusion.
I'm staring at this now, and I'm thinking we could do a simpler change
that would solve your RT issues without having to introduce a dependency
on workqueue.c.
So if the culprit is that task_mm_cid_work() runs for too long on large
many-cpus systems, why not break it up into smaller iterations ?
So rather than iterating on "for_each_possible_cpu", we could simply
break this down into iteration on at most N cpus, so:
tick #1: iteration on CPUs 0 .. N - 1
tick #2: iteration on CPUs N .. 2*N - 1
...
circling back to 0 when it reaches the number of possible cpus.
This N value could be configurable, e.g. CONFIG_RSEQ_CID_SCAN_BATCH,
with a sane default. An RT system could decide to make that value lower.
Then all we need to do is remember which was that last observed cpu
number in the mm struct, so the next tick picks up from there.
The main downside of this approach compared to scheduling delayed
work in a workqueue is that it depends on having the mm be current when
the scheduler tick happens. But perhaps this is something we could fix
in a different way that does not add a dependency on workqueue. I'm not
sure how though.
Thoughts ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
