linux-kernel - Re: libperf: lack of interface

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Date:   Wed, 1 Sep 2021 11:57:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "nakamura.shun@...itsu.com" <nakamura.shun@...itsu.com>
Cc:     "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        Rob Herring <robh@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>
Subject: Re: libperf: lack of interface

On Wed, Sep 01, 2021 at 09:45:10AM +0000, nakamura.shun@...itsu.com wrote:
> Hello.
> 
> I'm trying to change rdpmc test in perf_event_tests[1] to use libperf, but libperf doesn't have enough interfaces.
> Does anyone plan to implement any of these libperf features?
> 
> - Interfaces that can run ioctl (PERF_EVENT_IOC_RESET) from userland
> - Interfaces that can run fcntl (SIGIO) from userland
> 
> [1] https://github.com/deater/perf_event_tests/tree/master/tests/rdpmc

While IOC_RESET has it's uses, it really shouldn't be used in
combination with RDPMC, it destroys the whole 'dont do
syscalls'/performance thing.

The typical RDPMC usage is:

	/* setup counter on self */

	...

	start = RDPMC
	/* your code */
	end = RDPMC
	delta = end - start;

	...

Nowhere do we need a reset...