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

[Pawel Moll <pawel.moll@arm.com>]

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