linux-kernel - Re: [PATCH 1/5] perf events: add support for realtime clock attribute

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Message-ID: <alpine.LFD.2.00.1103202101270.2787@localhost6.localdomain6>
Date:	Sun, 20 Mar 2011 21:05:28 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	David Ahern <daahern@...co.com>
cc:	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	acme@...stprotocols.net, mingo@...e.hu, peterz@...radead.org,
	fweisbec@...il.com, paulus@...ba.org
Subject: Re: [PATCH 1/5] perf events: add support for realtime clock
 attribute

On Sun, 20 Mar 2011, David Ahern wrote:

> 
> 
> On 03/20/11 13:08, Thomas Gleixner wrote:
> > On Sun, 20 Mar 2011, David Ahern wrote:
> >> +
> >> +	if (sample_type & PERF_SAMPLE_REALTIME)
> >> +		data->realtime = ktime_to_ns(ktime_get_real());
> > 
> > That's going to live lock on any sample which happens to hit in a
> > xtime_lock write locked section. I knew why I suggested adding
> > tracepoints to the timekeeping code :)
> 
> Hmmm.... Can you give an example of when perf samples could be generated
> with xtime write locked?

NMI triggered samples can hit at any time.

Any tracepoint which we might add to the time keeping code inside the
xtime locked regions. And I don't want to see a restriction there.

> What are the rules for when ktime_get_real can be invoked?

Outside of xtime write locked regions.

Thanks,

	tglx

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