Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples

From: Stephane Eranian <eranian@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	"mingo@elte.hu" <mingo@elte.hu>,
	Paul Mackerras <paulus@samba.org>,
	Anton Blanchard <anton@samba.org>,
	Will Deacon <will.deacon@arm.com>,
	"ak@linux.intel.com" <ak@linux.intel.com>,
	Pekka Enberg <penberg@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Robert Richter <robert.richter@amd.com>,
	tglx <tglx@linutronix.de>, John Stultz <john.stultz@linaro.org>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples
Date: Thu, 18 Oct 2012 21:33:33 +0200	[thread overview]
Message-ID: <CABPqkBSAUQYp0n6Zwrkd7192929Bh_pWiUmi0fmNRj5hfn+V6g@mail.gmail.com> (raw)
In-Reply-To: <1350408232.2336.42.camel@laptop>

On Tue, Oct 16, 2012 at 7:23 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, 2012-10-16 at 12:13 +0200, Stephane Eranian wrote:
>> Hi,
>>
>> There are many situations where we want to correlate events happening at
>> the user level with samples recorded in the perf_event kernel sampling buffer.
>> For instance, we might want to correlate the call to a function or creation of
>> a file with samples. Similarly, when we want to monitor a JVM with jitted code,
>> we need to be able to correlate jitted code mappings with perf event samples
>> for symbolization.
>>
>> Perf_events allows timestamping of samples with PERF_SAMPLE_TIME.
>> That causes each PERF_RECORD_SAMPLE to include a timestamp
>> generated by calling the local_clock() -> sched_clock_cpu() function.
>>
>> To make correlating user vs. kernel samples easy, we would need to
>> access that sched_clock() functionality. However, none of the existing
>> clock calls permit this at this point. They all return timestamps which are
>> not using the same source and/or offset as sched_clock.
>>
>> I believe a similar issue exists with the ftrace subsystem.
>>
>> The problem needs to be adressed in a portable manner. Solutions
>> based on reading TSC for the user level to reconstruct sched_clock()
>> don't seem appropriate to me.
>>
>> One possibility to address this limitation would be to extend clock_gettime()
>> with a new clock time, e.g., CLOCK_PERF.
>>
>> However, I understand that sched_clock_cpu() provides ordering guarantees only
>> when invoked on the same CPU repeatedly, i.e., it's not globally synchronized.
>> But we already have to deal with this problem when merging samples obtained
>> from different CPU sampling buffer in per-thread mode. So this is not
>> necessarily
>> a showstopper.
>>
>> Alternatives could be to use uprobes but that's less practical to setup.
>>
>> Anyone with better ideas?
>
> You forgot to CC the time people ;-)
>
I did not know where they were.

> I've no problem with adding CLOCK_PERF (or another/better name).
>
Ok, good.

> Thomas, John?
>
Any comment?