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Date: Mon, 30 Mar 2015 11:24:12 -0600
From: David Ahern <dsahern@gmail.com>
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To: Peter Zijlstra <peterz@infradead.org>,
        Stephane Eranian <eranian@google.com>
CC: Arnaldo Carvalho de Melo <acme@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, Jiri Olsa <jolsa@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        John Stultz <john.stultz@linaro.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] perf, record: Add clockid parameter
References: <tip-34f439278cef7b1177f8ce24f9fc81dfc6221d3b@git.kernel.org> <20150327143201.GG21418@twins.programming.kicks-ass.net> <55158F25.9040100@gmail.com> <20150327172059.GK23123@twins.programming.kicks-ass.net> <551594DD.7050705@gmail.com> <20150327201534.GD6291@redhat.com> <20150327215942.GL23123@twins.programming.kicks-ass.net> <CABPqkBR_ou8E01kRmDOBb72ngcdzBkfVPNUj71XWnsYE6i0Bdg@mail.gmail.com> <20150328075549.GG27490@worktop.programming.kicks-ass.net>
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On 3/28/15 1:55 AM, Peter Zijlstra wrote:
> @@ -1085,6 +1093,8 @@ static int __perf_evsel__open(struct per
>   	}
>
>   fallback_missing_features:
> +	if (perf_missing_features.clockid)
> +		evsel->attr.use_clockid = 0;
>   	if (perf_missing_features.cloexec)
>   		flags &= ~(unsigned long)PERF_FLAG_FD_CLOEXEC;
>   	if (perf_missing_features.mmap2)
> @@ -1122,6 +1132,16 @@ static int __perf_evsel__open(struct per
>   				goto try_fallback;
>   			}
>   			set_rlimit = NO_CHANGE;
> +
> +			/*
> +			 * If we succeeded but had to kill clockid, fail and
> +			 * have perf_evsel__open_strerror() print us a nice
> +			 * error.
> +			 */
> +			if (perf_missing_features.clockid) {
> +				err = -EINVAL;
> +				goto out_close;
> +			}
>   		}
>   	}
>
> @@ -1155,7 +1175,10 @@ static int __perf_evsel__open(struct per
>   	if (err != -EINVAL || cpu > 0 || thread > 0)
>   		goto out_close;
>
> -	if (!perf_missing_features.cloexec && (flags & PERF_FLAG_FD_CLOEXEC)) {
> +	if (!perf_missing_features.clockid && evsel->attr.use_clockid) {
> +		perf_missing_features.clockid = true;
> +		goto fallback_missing_features;
> +	} else if (!perf_missing_features.cloexec && (flags & PERF_FLAG_FD_CLOEXEC)) {
>   		perf_missing_features.cloexec = true;
>   		goto fallback_missing_features;
>   	} else if (!perf_missing_features.mmap2 && evsel->attr.mmap2) {

...

> @@ -2158,6 +2188,12 @@ int perf_evsel__open_strerror(struct per
>   	"The PMU counters are busy/taken by another profiler.\n"
>   	"We found oprofile daemon running, please stop it and try again.");
>   		break;
> +
> +	case EINVAL:
> +		if (perf_missing_features.clockid)
> +			return scnprintf(msg, size, "%s", "clockid not supported.");
> +		break;
> +
>   	default:
>   		break;
>   	}
>

This works but the result is not always intuitive as to why it failed.

On a kernel that does not support the clock id you get:
     $ perf sched record -k mono -- sleep 1
     Error:
     clockid not supported.

And on a kernel that supports clockid but not for NMI:

     $ perf record -k realtime -a -- sleep 1
     Error:
     clockid not supported.

     --> H/W counters so realtime is not allowed

Same message though different root causes.