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Date: Mon, 29 Nov 2021 20:52:18 +0000
From: Guillaume Tucker <guillaume.tucker@...labora.com>
To: paulmck@...nel.org
Cc: Martin Lau <kafai@...com>, Steven Rostedt <rostedt@...dmis.org>,
Frederic Weisbecker <frederic@...nel.org>,
Robin Murphy <robin.murphy@....com>,
Jonathan Corbet <corbet@....net>,
Lai Jiangshan <jiangshanlai@...il.com>,
Josh Triplett <josh@...htriplett.org>,
Viresh Kumar <viresh.kumar@...aro.org>,
linux-doc@...r.kernel.org, "Maciej W. Rozycki" <macro@...am.me.uk>,
Joerg Roedel <jroedel@...e.de>, rcu@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>, kernelci@...ups.io,
Joel Fernandes <joel@...lfernandes.org>,
linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: next/master bisection: baseline.dmesg.alert on
qemu_arm64-virt-gicv3
On 29/11/2021 19:07, Paul E. McKenney wrote:
> On Mon, Nov 29, 2021 at 06:50:01PM +0000, Guillaume Tucker wrote:
>> Hi Paul,
>>
>> Please see the bisection report below about a new kernel alert
>> message.
>>
>> Reports aren't automatically sent to the public while we're
>> trialing new bisection features on kernelci.org but this one
>> looks valid.
>>
>> Some more details can be found here:
>>
>> https://linux.kernelci.org/test/case/id/61a4f02ad5a9e3a5f218f6f6/
>>
>> The bisection was run on QEMU but it seems to appear on all
>> platforms as it's not an architecture-specific change.
>> Presumably, none of the platforms in KernelCI is setting a value
>> for rcu_task_enqueue_lim at the moment.
>>
>> Alert messages normally highlight potential problems, so ideally
>> they would need to be resolved. Should the message in this case
>> really be a pr_alert(), and if so what would be the recommended
>> way to avoid it?
>
> It is to be unconditionally printed, so I am queueing a fixup that
> downgrades it to pr_info(). Does that work for you?
Yes, that will certainly make the message fall under the radar of
KernelCI. Thanks!
Guillaume
>> GitHub: https://github.com/kernelci/kernelci-project/issues/78
>>
>> On 26/11/2021 21:15, kernelci.org bot wrote:
>>> * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
>>> * This automated bisection report was sent to you on the basis *
>>> * that you may be involved with the breaking commit it has *
>>> * found. No manual investigation has been done to verify it, *
>>> * and the root cause of the problem may be somewhere else. *
>>> * *
>>> * If you do send a fix, please include this trailer: *
>>> * Reported-by: "kernelci.org bot" <bot@...nelci.org> *
>>> * *
>>> * Hope this helps! *
>>> * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
>>>
>>> next/master bisection: baseline.dmesg.alert on qemu_arm64-virt-gicv3
>>>
>>> Summary:
>>> Start: f30a24ed97b40 Add linux-next specific files for 20211126
>>> Plain log: https://storage.kernelci.org/next/master/next-20211126/arm64/defconfig+CONFIG_ARM64_64K_PAGES=y/clang-13/lab-collabora/baseline-qemu_arm64-virt-gicv3.txt
>>> HTML log: https://storage.kernelci.org/next/master/next-20211126/arm64/defconfig+CONFIG_ARM64_64K_PAGES=y/clang-13/lab-collabora/baseline-qemu_arm64-virt-gicv3.html
>>> Result: dc9166f1253df rcu-tasks: Use more callback queues if contention encountered
>>>
>>> Checks:
>>> revert: PASS
>>> verify: PASS
>>>
>>> Parameters:
>>> Tree: next
>>> URL: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
>>> Branch: master
>>> Target: qemu_arm64-virt-gicv3
>>> CPU arch: arm64
>>> Lab: lab-collabora
>>> Compiler: clang-13
>>> Config: defconfig+CONFIG_ARM64_64K_PAGES=y
>>> Test case: baseline.dmesg.alert
>>>
>>> Breaking commit found:
>>>
>>> -------------------------------------------------------------------------------
>>> commit dc9166f1253df20e01b0132ece22efaeb5b0a7e0
>>> Author: Paul E. McKenney <paulmck@...nel.org>
>>> Date: Wed Nov 24 15:12:15 2021 -0800
>>>
>>> rcu-tasks: Use more callback queues if contention encountered
>>>
>>> The rcupdate.rcu_task_enqueue_lim module parameter allows system
>>> administrators to tune the number of callback queues used by the RCU
>>> Tasks flavors. However if callback storms are infrequent, it would
>>> be better to operate with a single queue on a given system unless and
>>> until that system actually needed more queues. Systems not needing
>>> more queues can then avoid the overhead of checking the extra queues
>>> and especially avoid the overhead of fanning workqueue handlers out to
>>> all CPUs to invoke callbacks.
>>>
>>> This commit therefore switches to using all the CPUs' callback queues if
>>> call_rcu_tasks_generic() encounters too much lock contention. The amount
>>> of lock contention to tolerate defaults to 100 contended lock acquisitions
>>> per jiffy, and can be adjusted using the new rcupdate.rcu_task_contend_lim
>>> module parameter.
>>>
>>> Such switching is undertaken only if the rcupdate.rcu_task_enqueue_lim
>>> module parameter is negative, which is its default value (-1).
>>> This allows savvy systems administrators to set the number of queues
>>> to some known good value and to not have to worry about the kernel doing
>>> any second guessing.
>>>
>>> Reported-by: Martin Lau <kafai@...com>
>>> Cc: Neeraj Upadhyay <neeraj.iitr10@...il.com>
>>> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>>>
>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>> index f1f3761fe42c8..fc5e20afaeefa 100644
>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>> @@ -4813,6 +4813,14 @@
>>> period to instead use normal non-expedited
>>> grace-period processing.
>>>
>>> + rcupdate.rcu_task_contend_lim= [KNL]
>>> + Set the minimum number of callback-queuing-time
>>> + lock-contention events per jiffy required to
>>> + cause the RCU Tasks flavors to switch to per-CPU
>>> + callback queuing. This switching only occurs
>>> + when rcupdate.rcu_task_enqueue_lim is set to
>>> + the default value of -1.
>>> +
>>> rcupdate.rcu_task_enqueue_lim= [KNL]
>>> Set the number of callback queues to use for the
>>> RCU Tasks family of RCU flavors. The default
>>> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
>>> index 0cb20e5b3e856..7da3b500fb901 100644
>>> --- a/kernel/rcu/tasks.h
>>> +++ b/kernel/rcu/tasks.h
>>> @@ -142,6 +142,10 @@ module_param(rcu_task_stall_timeout, int, 0644);
>>> static int rcu_task_enqueue_lim __read_mostly = -1;
>>> module_param(rcu_task_enqueue_lim, int, 0444);
>>>
>>> +static bool rcu_task_cb_adjust;
>>> +static int rcu_task_contend_lim __read_mostly = 100;
>>> +module_param(rcu_task_contend_lim, int, 0444);
>>> +
>>> /* RCU tasks grace-period state for debugging. */
>>> #define RTGS_INIT 0
>>> #define RTGS_WAIT_WAIT_CBS 1
>>> @@ -207,8 +211,11 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
>>> int lim;
>>>
>>> raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
>>> - if (rcu_task_enqueue_lim < 0)
>>> - rcu_task_enqueue_lim = nr_cpu_ids;
>>> + if (rcu_task_enqueue_lim < 0) {
>>> + rcu_task_enqueue_lim = 1;
>>> + rcu_task_cb_adjust = true;
>>> + pr_alert("%s: Setting adjustable number of callback queues.\n", __func__);
>>> + }
>>> else if (rcu_task_enqueue_lim == 0)
>>> rcu_task_enqueue_lim = 1;
>>> lim = rcu_task_enqueue_lim;
>>> @@ -251,6 +258,7 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
>>> {
>>> unsigned long flags;
>>> unsigned long j;
>>> + bool needadjust = false;
>>> bool needwake;
>>> struct rcu_tasks_percpu *rtpcp;
>>>
>>> @@ -266,7 +274,9 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
>>> rtpcp->rtp_jiffies = j;
>>> rtpcp->rtp_n_lock_retries = 0;
>>> }
>>> - rtpcp->rtp_n_lock_retries++;
>>> + if (rcu_task_cb_adjust && ++rtpcp->rtp_n_lock_retries > rcu_task_contend_lim &&
>>> + READ_ONCE(rtp->percpu_enqueue_lim) != nr_cpu_ids)
>>> + needadjust = true; // Defer adjustment to avoid deadlock.
>>> }
>>> if (!rcu_segcblist_is_enabled(&rtpcp->cblist)) {
>>> raw_spin_unlock_rcu_node(rtpcp); // irqs remain disabled.
>>> @@ -276,6 +286,15 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
>>> needwake = rcu_segcblist_empty(&rtpcp->cblist);
>>> rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
>>> raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
>>> + if (unlikely(needadjust)) {
>>> + raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
>>> + if (rtp->percpu_enqueue_lim != nr_cpu_ids) {
>>> + WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids));
>>> + smp_store_release(&rtp->percpu_enqueue_lim, nr_cpu_ids);
>>> + pr_alert("Switching %s to per-CPU callback queuing.\n", rtp->name);
>>> + }
>>> + raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
>>> + }
>>> /* We can't create the thread unless interrupts are enabled. */
>>> if (needwake && READ_ONCE(rtp->kthread_ptr)) {
>>> irq_work_queue(&rtpcp->rtp_irq_work);
>>> -------------------------------------------------------------------------------
>>>
>>>
>>> Git bisection log:
>>>
>>> -------------------------------------------------------------------------------
>>> git bisect start
>>> # good: [8ced7ca3570333998ad2088d5a6275701970e28e] Merge tag 'block-5.16-2021-11-25' of git://git.kernel.dk/linux-block
>>> git bisect good 8ced7ca3570333998ad2088d5a6275701970e28e
>>> # bad: [f30a24ed97b401416118756fa35fbe5d28f999e3] Add linux-next specific files for 20211126
>>> git bisect bad f30a24ed97b401416118756fa35fbe5d28f999e3
>>> # good: [d7359ce68f20102beeec57e760b746e1f76ab65c] Merge branch 'mtd/next' of git://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git
>>> git bisect good d7359ce68f20102beeec57e760b746e1f76ab65c
>>> # good: [6eb0c6fb36d50e66ca0b686ec0eebd7eddff811b] Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/ulfh/mmc.git
>>> git bisect good 6eb0c6fb36d50e66ca0b686ec0eebd7eddff811b
>>> # bad: [e62c6fa2d7d055d657c00734314d6abc664d28b5] Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git
>>> git bisect bad e62c6fa2d7d055d657c00734314d6abc664d28b5
>>> # good: [0ebaaa3b9daaf0341c75a66b8b33399e23e40a8f] Merge branch 'edac-for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/ras/ras.git
>>> git bisect good 0ebaaa3b9daaf0341c75a66b8b33399e23e40a8f
>>> # bad: [556fc834ce0be2f0dbfd8ae2bfb3840326d12cb9] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/pdx86/platform-drivers-x86.git
>>> git bisect bad 556fc834ce0be2f0dbfd8ae2bfb3840326d12cb9
>>> # good: [ad3b4071d577f2779c706a22956b6d167710459b] refscale: Simplify the errexit checkpoint
>>> git bisect good ad3b4071d577f2779c706a22956b6d167710459b
>>> # good: [24f474cf45326225f613b58b191600311109695b] rcutorture: Cause TREE02 and TREE10 scenarios to do more callback flooding
>>> git bisect good 24f474cf45326225f613b58b191600311109695b
>>> # bad: [731f2830b96ce040e52b227346087e0dc3a6f5d0] Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git
>>> git bisect bad 731f2830b96ce040e52b227346087e0dc3a6f5d0
>>> # bad: [3a528960b4b5d4069e4b066a1d9278862e902abc] Merge branch 'rcu/next' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
>>> git bisect bad 3a528960b4b5d4069e4b066a1d9278862e902abc
>>> # bad: [a106e93308b6c60944c9021fc2cad377639584a2] rcu/nocb: Remove rdp from nocb list when de-offloaded
>>> git bisect bad a106e93308b6c60944c9021fc2cad377639584a2
>>> # good: [a3d09240cbfd98525dbd7038a4ec1afb4bc896a2] rcu-tasks: Avoid raw-spinlocked wakeups from call_rcu_tasks_generic()
>>> git bisect good a3d09240cbfd98525dbd7038a4ec1afb4bc896a2
>>> # bad: [4e83e8074d198b5849926d4a7d921063b24c0250] rcutorture: Test RCU Tasks lock-contention detection
>>> git bisect bad 4e83e8074d198b5849926d4a7d921063b24c0250
>>> # bad: [dc9166f1253df20e01b0132ece22efaeb5b0a7e0] rcu-tasks: Use more callback queues if contention encountered
>>> git bisect bad dc9166f1253df20e01b0132ece22efaeb5b0a7e0
>>> # first bad commit: [dc9166f1253df20e01b0132ece22efaeb5b0a7e0] rcu-tasks: Use more callback queues if contention encountered
>>> -------------------------------------------------------------------------------
>>>
>>
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