linux-kernel - Re: [PATCH 2/2] More precise timestamps for nested writes

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Message-ID: <552DB1BC.40103@linux.vnet.ibm.com>
Date:	Tue, 14 Apr 2015 19:33:00 -0500
From:	"Suresh E. Warrier" <warrier@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	lklml <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 2/2] More precise timestamps for nested writes

On 04/14/2015 12:13 PM, Peter Zijlstra wrote:
> On Mon, Apr 13, 2015 at 09:38:01PM -0500, Suresh E. Warrier wrote:
>> +static u64 *get_write_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
>> +				unsigned long *flags)
>> +{
>> +	if (rb_precise_nested_write_ts()) {
>> +		/*
>> +		 * Ensure that we are not preempted until after we update
>> +		 * the write timestamp.
>> +		 */
>> +		local_irq_save(*flags);
>> +		return &cpu_buffer->last_stamp;
> 
> Yeah, ever hear about NMIs? This isn't going to work.

That is a good point! If a NMI can come in and start running a handler
that can generate a trace event, this code is indeed broken.

Some architectures like PowerPC don't have NMIs like Intel and so 
I hadn't thought of that. Thanks for catching that!

Let me update the patch to handle NMIs - trace events from NMI code 
cannot be made precise (the behavior will be the same as without the
patch).

-suresh

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