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: <1400183029.5058.21.camel@joe-AO725>
Date:	Thu, 15 May 2014 12:43:49 -0700
From:	Joe Perches <joe@...ches.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.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 Fri, 2014-05-16 at 01:04 +0530, Srivatsa S. Bhat wrote:
> 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.

Ah, good.

I was misled a bit by the WARN_ONCE that is in the
same block.  Perhaps because there is a guard flag
above the block, maybe the WARN_ONCE should just be
WARN.


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