linux-kernel - Re: [PATCH 3/6] perf record: add time-of-day option

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Message-ID: <4D6D22A7.6000000@cisco.com>
Date:	Tue, 01 Mar 2011 09:45:27 -0700
From:	David Ahern <daahern@...co.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	acme@...stprotocols.net, mingo@...e.hu, fweisbec@...il.com,
	paulus@...ba.org, tglx@...utronix.de
Subject: Re: [PATCH 3/6] perf record: add time-of-day option



On 03/01/2011 09:37 AM, Peter Zijlstra wrote:
> On Tue, 2011-03-01 at 09:09 -0700, David Ahern wrote:
>>
>> I did not code it create samples of a realtime-clock event; I coded it
>> to add an event to the stream on demand -- whatever event the fd is
>> tied to.
>>
>>
> Well yes, but aside from you and the realtime clock stuff, who'd ever
> want to do this particular thing?

One example would be a system watchdog that decided for some reason or
another to force an event sample because of some event it detected.

> 
> I'd much rather expose the whole
> local_clock()/perf_clock()/trace_clock() (should all be the same anyway)
> as a posix clock using CLOCK_TRACING or whatever would be an appropriate
> name.
> 
> [ Since the whole thing is NMI safe it should be well possible to make a
> VDSO version as well. ]
> 
> Anyway, once its visible as a posix clock you can sync up from
> userspace. And this clock is indeed wanted for other things too, like
> user-space tracing etc.
> 

And for some silly reason I sense a time trap here (pun intended) .....

So your pushback is:
1. throw out the realtime-clock event patch,

2. add a new CLOCK_TRACING type to clock_gettime with VDSO hook,

3. and use a synthesized event from userspace for snapshotting
perf_clock to time-of-day -- similar to what I have now, but one that
will guarantee a time-of-day to perf_clock correlation (versus the
current one which hopes that perf_clock is the monotonic clock).

David
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