linux-kernel - Re: [PATCH 3/3 v2] perf tool: give user better message if precise is not supported

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Date:	Fri, 14 Sep 2012 07:43:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	acme@...stprotocols.net, linux-kernel@...r.kernel.org,
	peterz@...radead.org, Robert Richter <robert.richter@....com>
Subject: Re: [PATCH 3/3 v2] perf tool: give user better message if precise is
 not supported


* David Ahern <dsahern@...il.com> wrote:

> Platforms (e.g., VM's) without support for precise mode get a confusing
> error message. e.g.,
> $ perf record -e cycles:p -a -- sleep 1
> 
>   Error: sys_perf_event_open() syscall returned with 95 (Operation not
>   supported).  /bin/dmesg may provide additional information.
> 
>   No hardware sampling interrupt available. No APIC? If so then you can
>   boot the kernel with the "lapic" boot parameter to force-enable it.
>   sleep: Terminated
> 
> which is not clear that precise mode might be the root problem. With this
> patch:
> 
> $ perf record -e cycles:p -fo /tmp/perf.data -- sleep 1
>   Error:
>   'precise' request may not be supported. Try removing 'p' modifier
>   sleep: Terminated
> 
> v2: softened message to 'may not be' supported per Robert's suggestion

Well, either it's supported on this machine or it's not - why 
does the text have to be so unsure about it?

We use computers to increase determinism, not to insert extra 
uncertainty! ;-)

Thanks,

	Ingo
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