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SubjectRe: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples
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On Wed, Feb 6, 2013 at 7:17 PM, Pawel Moll <pawel.moll@arm.com> wrote:
> On Wed, 2013-02-06 at 01:19 +0000, Steven Rostedt wrote:
>> If people are worried about adding a bunch of new perf syscalls, maybe
>> add a sys_perf_control() system call that works like an ioctl but
>> without a file descriptor. Something for things that don't require an
>> event attached to it, like to retrieve a time stamp counter that perf
>> uses, but done in a way that it could be used for other things perf
>> related that does not require an event.
>
> Something along these lines? (completely untested and of course missing
> all the #defines __NR_perf_control xxx)
>
> 8<-----------------
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 4f63c05..be7409b 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -322,6 +322,11 @@ enum perf_event_ioc_flags {
> };
>
> /*
> + * Command codes for ioctl-like sys_perf_control interface:
> + */
> +#define PERF_CONTROL_GET_TIME _IOR('$', 0, __u64)
> +
> +/*
> * Structure of the page that can be mapped via mmap
> */
> struct perf_event_mmap_page {
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 301079d..750404d 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6678,6 +6678,29 @@ err_fd:
> }
>
> /**
> + * sys_perf_control - ioctl-like interface to control system-wide
> + * perf behaviour
> + *
> + * @cmd: one of the PERF_CONTROL_* commands
> + * @arg: command-specific argument
> + */
> +SYSCALL_DEFINE2(perf_control, unsigned int, cmd, unsigned long, arg)
> +{
> + switch (cmd) {
> + case PERF_CONTROL_GET_TIME:
> + {
> + u64 time = perf_clock();
> + if (copy_to_user((void __user *)arg, &time, sizeof(time)))
> + return -EFAULT;
> + return 0;
> + }
> +
> + default:
> + return -ENOTTY;
> + }
> +}
> +
> +/**
> * perf_event_create_kernel_counter
> *
> * @attr: attributes of the counter to create
> 8<-----------------
>
> Cheers!

So what would be the role of this new syscall besides GET_TIME?
What other controls without a fd could be done? We are already passing
a lot of control thru the perf_event_open() some in the attr struct others
as arguments.

The only advantage of this "disguised" ioctl() is that it does not require
a fd. But it is worth adding a syscall just to avoid creating a fd?


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