linux-kernel - Re: [RFC][PATCH 2/2] perf: Add per event clockid support

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Message-ID: <1424446129.6259.5.camel@arm.com>
Date:	Fri, 20 Feb 2015 15:28:49 +0000
From:	Pawel Moll <pawel.moll@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"adrian.hunter@...el.com" <adrian.hunter@...el.com>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"eranian@...gle.com" <eranian@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"acme@...nel.org" <acme@...nel.org>,
	"dsahern@...il.com" <dsahern@...il.com>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"jolsa@...hat.com" <jolsa@...hat.com>,
	"namhyung@...il.com" <namhyung@...il.com>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"sonnyrao@...omium.org" <sonnyrao@...omium.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"vincent.weaver@...ne.edu" <vincent.weaver@...ne.edu>
Subject: Re: [RFC][PATCH 2/2] perf: Add per event clockid support

On Fri, 2015-02-20 at 14:29 +0000, Peter Zijlstra wrote:
> The below patch makes the distinction between these two cases by
> adding perf_event_clock() which is used for the second case. It
> further makes this configurable on a per-event basis, but adds a few
> sanity checks such that we cannot combine events with different clocks
> in confusing ways.

The idea works for me (obviously :-)

> And since we then have per-event configurability we might as well
> retain the 'legacy' behaviour as a default.

Don't mind that at all.

> @@ -334,8 +335,7 @@ struct perf_event_attr {
>  	 */
>  	__u32	sample_stack_user;
>  
> -	/* Align to u64. */
> -	__u32	__reserved_2;
> +	__u32	clockid;

I thought about it, but was sort-of-afraid to propose it :-)

Now, one thing I'm not 100% sure about it is it being unsigned, as
clockid_t is signed for a reason (negative values have meaning - eg.
dynamic clocks, which could be useful in some circumstances). Of course
casting could be an answer, but is there any reason not to make it
__s32?

> +		default:
> +			/* XXX add: clock_id_valid() && clock_gettime_ns() ? */
> +			err = -EINVAL;
> +			goto err_alloc;
> +		}

If you asked me, I'd say -EINVAL, no default.

Cheers!

Pawel

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