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Message-ID: <CAJZ5v0j_fFk=EX0Z9_w1twQH-FpntHJvr4d0WSMBM6PevfEqNg@mail.gmail.com>
Date: Sat, 10 May 2025 13:31:15 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>, Linux PM <linux-pm@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>, Lukasz Luba <lukasz.luba@....com>,
Peter Zijlstra <peterz@...radead.org>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
Dietmar Eggemann <dietmar.eggemann@....com>, Morten Rasmussen <morten.rasmussen@....com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
Pierre Gondois <pierre.gondois@....com>, Christian Loehle <christian.loehle@....com>
Subject: Re: [PATCH v2 2/7] cpufreq/sched: Move cpufreq-specific EAS checks to cpufreq
On Sat, May 10, 2025 at 1:49 AM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
>
> On 06.05.2025 22:37, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Doing cpufreq-specific EAS checks that require accessing policy
> > internals directly from sched_is_eas_possible() is a bit unfortunate,
> > so introduce cpufreq_ready_for_eas() in cpufreq, move those checks
> > into that new function and make sched_is_eas_possible() call it.
> >
> > While at it, address a possible race between the EAS governor check
> > and governor change by doing the former under the policy rwsem.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Reviewed-by: Christian Loehle <christian.loehle@....com>
> > Tested-by: Christian Loehle <christian.loehle@....com>
> > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
>
> In my tests I've noticed that this patch, merged as commit 4854649b1fb4
> ("cpufreq/sched: Move cpufreq-specific EAS checks to cpufreq"), causes a
> regression on ARM64 Amlogic Meson SoC based OdroidN2 board. The board
> finally lockups. Reverting $subject on top of next-20250509 fixes this
> issue. Here is the lockdep warning observed before the lockup:
Thanks for the report!
> ======================================================
> WARNING: possible circular locking dependency detected
> 6.15.0-rc5-next-20250509-dirty #10335 Tainted: G C
> cpufreq: cpufreq_policy_online: CPU2: Running at unlisted initial
> frequency: 999999 kHz, changing to: 1000000 kHz
> ------------------------------------------------------
> kworker/3:1/79 is trying to acquire lock:
> ffff00000494b380 (&policy->rwsem){++++}-{4:4}, at:
> cpufreq_ready_for_eas+0x60/0xbc
>
> but task is already holding lock:
> ffff8000832887a0 (sched_domains_mutex){+.+.}-{4:4}, at:
> partition_sched_domains+0x54/0x938
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
>
> -> #2 (sched_domains_mutex){+.+.}-{4:4}:
> __mutex_lock+0xa8/0x598
> mutex_lock_nested+0x24/0x30
> partition_sched_domains+0x54/0x938
> rebuild_sched_domains_locked+0x2d4/0x900
> rebuild_sched_domains+0x2c/0x48
> rebuild_sched_domains_energy+0x3c/0x58
> rebuild_sd_workfn+0x10/0x1c
> process_one_work+0x208/0x604
> worker_thread+0x244/0x388
> kthread+0x150/0x228
> ret_from_fork+0x10/0x20
>
> -> #1 (cpuset_mutex){+.+.}-{4:4}:
> __mutex_lock+0xa8/0x598
> mutex_lock_nested+0x24/0x30
> cpuset_lock+0x1c/0x28
> __sched_setscheduler+0x31c/0x830
> sched_setattr_nocheck+0x18/0x24
> sugov_init+0x1b4/0x388
> cpufreq_init_governor.part.0+0x58/0xd4
> cpufreq_set_policy+0x2c8/0x3ec
> cpufreq_online+0x520/0xb20
> cpufreq_add_dev+0x80/0x98
> subsys_interface_register+0xfc/0x118
> cpufreq_register_driver+0x150/0x238
> dt_cpufreq_probe+0x148/0x488
> platform_probe+0x68/0xdc
> really_probe+0xbc/0x298
> __driver_probe_device+0x78/0x12c
> driver_probe_device+0xdc/0x164
> __device_attach_driver+0xb8/0x138
> bus_for_each_drv+0x80/0xdc
> __device_attach+0xa8/0x1b0
> device_initial_probe+0x14/0x20
> bus_probe_device+0xb0/0xb4
> deferred_probe_work_func+0x8c/0xc8
> process_one_work+0x208/0x604
> worker_thread+0x244/0x388
> kthread+0x150/0x228
> ret_from_fork+0x10/0x20
>
> -> #0 (&policy->rwsem){++++}-{4:4}:
> __lock_acquire+0x1408/0x2254
> lock_acquire+0x1c8/0x354
> down_read+0x60/0x180
> cpufreq_ready_for_eas+0x60/0xbc
> sched_is_eas_possible+0x144/0x170
> partition_sched_domains+0x504/0x938
> rebuild_sched_domains_locked+0x2d4/0x900
> rebuild_sched_domains+0x2c/0x48
> rebuild_sched_domains_energy+0x3c/0x58
> rebuild_sd_workfn+0x10/0x1c
> process_one_work+0x208/0x604
> worker_thread+0x244/0x388
> kthread+0x150/0x228
> ret_from_fork+0x10/0x20
>
> other info that might help us debug this:
>
> Chain exists of:
> &policy->rwsem --> cpuset_mutex --> sched_domains_mutex
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(sched_domains_mutex);
> lock(cpuset_mutex);
> lock(sched_domains_mutex);
> rlock(&policy->rwsem);
>
> *** DEADLOCK ***
Well, it turns out that trying to acquire policy->rwsem under
sched_domains_mutex is a bad idea. It was added to
cpufreq_policy_is_good_for_eas() to address a theoretical race, so it
can be dropped safely. A theoretical race is better than a real
deadlock.
Please test the attached patch.
View attachment "cpufreq-eas-policy-locking.patch" of type "text/x-patch" (264 bytes)
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