linux-kernel - Re: [PATCH RFC 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

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Date:	Fri, 02 Aug 2013 02:06:18 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Waiman Long <waiman.long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Weinberger <richard@....at>,
	Catalin Marinas <catalin.marinas@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Akinobu Mita <akinobu.mita@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Spelvin <linux@...izon.com>,
	Harvey Harrison <harvey.harrison@...il.com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH RFC 1/2] qspinlock: Introducing a 4-byte queue spinlock
 implementation

On 08/02/2013 01:40 AM, Peter Zijlstra wrote:
> On Thu, Aug 01, 2013 at 11:46:24PM +0530, Raghavendra K T wrote:
>> On 08/01/2013 11:28 PM, Waiman Long wrote:
>>> On 08/01/2013 05:40 AM, Peter Zijlstra wrote:
>>>> On Wed, Jul 31, 2013 at 10:37:10PM -0400, Waiman Long wrote:
>>>>
>> [...]
>>>>
>>>>> +         */
>>>>> +        for (qn_idx = 1; qn_idx<  MAX_QNODES; qn_idx++) {
>>>>> +            if (!node[qn_idx].used)
>>>>> +                break;
>>>>         }
>>>>
>>>>> +        if (unlikely(qn_idx == MAX_QNODES)) {
>>>>> +            /*
>>>>> +             * This shouldn't happen, print a warning message
>>>>> +             *&  busy spinning on the lock.
>>>>> +             */
>>>>> +            pr_warn("qspinlock: queue node table exhausted at "
>>>>> +                "cpu %d!\n", cpu_nr);
>>>> This could make your machine die hard.. not all contexts can printk().
>>>
>>> Do you have any suggestion? I could skip the warning and silently do the
>>> busy spinning. I just want some way to notify the user of this rare event.
>>
>> We have used debugfs in pv-spinlock to avoid that since printk uses
>> spinlock again. may be it will help to profile many other parts of
>> code too.
>
> I always use early_printk(), but that requires you set up your serial
> console properly and joe-user won't have done that.
>

Thanks Peter. Yes. this is more convenient.

/me remembers using early_printk  during pvops patch debugging without
setting up serial console ;)

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