Re: next/master bisection: baseline.dmesg.alert on qemu_arm64-virt-gicv3

[Guillaume Tucker]

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@xxxxxxxxxxxx>          *
>>> *                                                               *
>>> * 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@xxxxxxxxxx>
>>> 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@xxxxxx>
>>>     Cc: Neeraj Upadhyay <neeraj.iitr10@xxxxxxxxx>
>>>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>>>
>>> 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
>>> -------------------------------------------------------------------------------
>>>
>>