From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Gabriele Monaco <gmonaco@redhat.com>, linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, aubrey.li@linux.intel.com,
yu.c.chen@intel.com, Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
"Paul E. McKenney" <paulmck@kernel.org>,
Shuah Khan <shuah@kernel.org>
Subject: Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work
Date: Thu, 13 Feb 2025 12:31:02 -0500 [thread overview]
Message-ID: <0888d6a3-8dea-455b-893f-d8d929e827e2@efficios.com> (raw)
In-Reply-To: <17bda9071b6962414f61668698fa840501819172.camel@redhat.com>
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
next prev parent reply other threads:[~2025-02-13 17:31 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-02-10 15:32 [PATCH v6 0/3] sched: Restructure task_mm_cid_work for predictability Gabriele Monaco
2025-02-10 15:32 ` [PATCH v6 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity Gabriele Monaco
2025-02-13 14:56 ` Mathieu Desnoyers
2025-02-18 7:53 ` Peter Zijlstra
2025-02-20 15:08 ` [tip: sched/urgent] " tip-bot2 for Mathieu Desnoyers
2025-02-10 15:32 ` [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work Gabriele Monaco
2025-02-13 6:52 ` kernel test robot
2025-02-13 13:25 ` Gabriele Monaco
2025-02-13 13:55 ` Mathieu Desnoyers
2025-02-13 14:37 ` Gabriele Monaco
2025-02-13 14:52 ` Mathieu Desnoyers
2025-02-13 14:54 ` Gabriele Monaco
2025-02-13 17:31 ` Mathieu Desnoyers [this message]
2025-02-14 6:44 ` Gabriele Monaco
2025-02-10 15:32 ` [PATCH v6 3/3] rseq/selftests: Add test for mm_cid compaction Gabriele Monaco
2025-02-10 16:09 ` [PATCH v6 0/3] sched: Restructure task_mm_cid_work for predictability Mathieu Desnoyers
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