linux-kernel - Re: [PATCH v5 1/3] smp: Print more useful debug info upon receiving IPI on an offline CPU

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Message-ID: <537516DA.9010407@linux.vnet.ibm.com>
Date:	Fri, 16 May 2014 01:04:50 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Joe Perches <joe@...ches.com>
CC:	peterz@...radead.org, tglx@...utronix.de, mingo@...nel.org,
	tj@...nel.org, rusty@...tcorp.com.au, akpm@...ux-foundation.org,
	fweisbec@...il.com, hch@...radead.org, mgorman@...e.de,
	riel@...hat.com, bp@...e.de, rostedt@...dmis.org,
	mgalbraith@...e.de, ego@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, oleg@...hat.com, rjw@...ysocki.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/3] smp: Print more useful debug info upon receiving
 IPI on an offline CPU

On 05/16/2014 12:49 AM, Joe Perches wrote:
> On Fri, 2014-05-16 at 00:43 +0530, Srivatsa S. Bhat wrote:
>> Today the smp-call-function code just prints a warning if we get an IPI on
>> an offline CPU. This info is sufficient to let us know that something went
>> wrong, but often it is very hard to debug exactly who sent the IPI and why,
>> from this info alone.
> []
>> diff --git a/kernel/smp.c b/kernel/smp.c
> []
>> @@ -185,14 +185,26 @@ void generic_smp_call_function_single_interrupt(void)
> []
>> -	entry = llist_del_all(&__get_cpu_var(call_single_queue));
>> -	entry = llist_reverse_order(entry);
>> +		/*
>> +		 * We don't have to use the _safe() variant here
>> +		 * because we are not invoking the IPI handlers yet.
>> +		 */
>> +		llist_for_each_entry(csd, entry, llist)
>> +			pr_warn("IPI callback %pS sent to offline CPU\n",
>> +				csd->func);
> 
> Perhaps add ratelimited?
> 

This entire scenario is expected to be _very_ infrequent, and even if
it happens, these prints will appear only once during the entire run
(note the use of the 'warned' variable to control that). So I don't think
ratelimiting is called for in this case.

Regards,
Srivatsa S. Bhat

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