Re: Misleading hint to select CONFIG_PERF_EVENTS if driver sets PERF_PMU_CAP_NO_INTERRUPT

[Vineet Gupta]

On Friday 22 April 2016 06:56 PM, Lada Trimasova wrote:
> I have a question about user-space perf handling error numbers.
> The problem is that PMU interrupts are not supported in arc700
> architecture and it is impossible to evaluate `perf record` command.
> In our perf implementation we set PERF_PMU_CAP_NO_INTERRUPT flag so
> core perf infrastructure knows we don't have interrupts.
>
> Kernel `sys_perf_event_open` handler checks if PMU interrupts are
> supported and returns ENOTSUPP (524) error code.
> I'd expect that perf implementation checks the return value of syscalls
> and gives the user understandable error message.
> But now I see:
> --------------------------------->8-----------------------------------
> # perf record ls
> The sys_perf_event_open() syscall returned with 524 (Unknown error 524)
> for event (cycles:ppp).

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.

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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