linux-kernel - Re: Misleading hint to select CONFIG_PERF_EVENTS if driver sets PERF_PMU_CAP_NO_INTERRUPT

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Date:	Wed, 27 Apr 2016 15:13:50 +0000
From:	Lada Trimasova <Lada.Trimasova@...opsys.com>
To:	"linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	"linux-snps-arc@...ts.infradead.org" 
	<linux-snps-arc@...ts.infradead.org>
Subject: Re: Misleading hint to select CONFIG_PERF_EVENTS if driver sets
 PERF_PMU_CAP_NO_INTERRUPT

On Tue, 2016-04-26 at 12:42 +0000, Vineet Gupta wrote:

On Friday 22 April 2016 06:56 PM, Lada Trimasova wrote:
I think what we have now is sufficient - but u seem to want a prettier failure output.

Anyhow, this print is coming from util/evsel.c: perf_evsel__open_strerror()
At the very least you want another entry in switch case for ENOTSUPP and then
check if event was sampling one (
evsel->attr.sample_period) - use that as a hint for saying sampling events not
supported.

ENOTSUPP is not visible for user programs. So it's impossible to add this entry

to mentioned switch.

I think that there is no good way to make error message more understandable

without breaking existing api.

But this will print the same even if CONFIG_PERF_EVENTS=n. To really fix this you
would want to change the error code returned by SYSCALL_DEFINE5(perf_event_open
for PERF_PMU_CAP_NO_INTERRUPT to say -EOPNOTSUPP and use the sample_period to say
this was for samplign events! However this is an ABI change and might not be
acceptable as some existing scripts etc might break.

/bin/dmesg may provide additional information.
No CONFIG_PERF_EVENTS=y kernel support configured?
--------------------------------->8-----------------------------------

As you can see the root cause of this error message is not obvious.
CONFIG_PERF_EVENTS is selected but still there's a problem while
existing suggestion barely makes any sense.
So probably there could be a way to determine if CONFIG_PERF_EVENTS was
selected or not.

I am not sure about the correct way of solving this problem. Maybe I
should add some checks of syscalls return values and give user
a warning when not PMU interrupts are available.
Any suggestions are appreciated.

Regards,
Lada Trimasova.
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Regards,

Lada.

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