linux-kernel - Re: [rcu_sched stall] regression/miss-config ?

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Message-ID: <ae356fa9-6eb1-3e39-67ab-f1ad831205f9@oracle.com>
Date:	Tue, 17 May 2016 06:46:22 -0700
From:	"santosh.shilimkar@...cle.com" <santosh.shilimkar@...cle.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [rcu_sched stall] regression/miss-config ?

On 5/16/16 5:58 PM, Paul E. McKenney wrote:
> On Mon, May 16, 2016 at 12:49:41PM -0700, Santosh Shilimkar wrote:
>> On 5/16/2016 10:34 AM, Paul E. McKenney wrote:
>>> On Mon, May 16, 2016 at 09:33:57AM -0700, Santosh Shilimkar wrote:

[...]

>>> Are you running CONFIG_NO_HZ_FULL=y?  If so, the problem might be that
>>> you need more housekeeping CPUs than you currently have configured.
>>>
>> Yes, CONFIG_NO_HZ_FULL=y. Do you mean "CONFIG_NO_HZ_FULL_ALL=y" for
>> book keeping. Seems like without that clock-event code will just use
>> CPU0 for things like broadcasting which might become bottleneck.
>> This could explain connect the hrtimer_interrupt() path getting slowed
>> down because of book keeping bottleneck.
>>
>> $cat .config | grep NO_HZ
>> CONFIG_NO_HZ_COMMON=y
>> # CONFIG_NO_HZ_IDLE is not set
>> CONFIG_NO_HZ_FULL=y
>> # CONFIG_NO_HZ_FULL_ALL is not set
>> # CONFIG_NO_HZ_FULL_SYSIDLE is not set
>> CONFIG_NO_HZ=y
>> # CONFIG_RCU_FAST_NO_HZ is not set
>
> Yes, CONFIG_NO_HZ_FULL_ALL=y would give you only one CPU for all
> housekeeping tasks, including the RCU grace-period kthreads.  So you are
> booting without any nohz_full boot parameter?  You can end up with the
> same problem with CONFIG_NO_HZ_FULL=y and the nohz_full boot parameter
> that you can with CONFIG_NO_HZ_FULL_ALL=y.
>
I see. Yes, the systems are booting without nohz_full boot parameter.
Will try to add more CPUs to it & update the thread
after the verification since it takes time to reproduce the issue.

Thanks for discussion so far Paul. Its very insightful for me.

Regards,
Santosh