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    SubjectRe: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples
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    Hello,

    I'd like to revive the topic...

    On Tue, 2012-10-16 at 18:23 +0100, Peter Zijlstra wrote:
    > On Tue, 2012-10-16 at 12:13 +0200, Stephane Eranian wrote:
    > > Hi,
    > >
    > > There are many situations where we want to correlate events happening at
    > > the user level with samples recorded in the perf_event kernel sampling buffer.
    > > For instance, we might want to correlate the call to a function or creation of
    > > a file with samples. Similarly, when we want to monitor a JVM with jitted code,
    > > we need to be able to correlate jitted code mappings with perf event samples
    > > for symbolization.
    > >
    > > Perf_events allows timestamping of samples with PERF_SAMPLE_TIME.
    > > That causes each PERF_RECORD_SAMPLE to include a timestamp
    > > generated by calling the local_clock() -> sched_clock_cpu() function.
    > >
    > > To make correlating user vs. kernel samples easy, we would need to
    > > access that sched_clock() functionality. However, none of the existing
    > > clock calls permit this at this point. They all return timestamps which are
    > > not using the same source and/or offset as sched_clock.
    > >
    > > I believe a similar issue exists with the ftrace subsystem.
    > >
    > > The problem needs to be adressed in a portable manner. Solutions
    > > based on reading TSC for the user level to reconstruct sched_clock()
    > > don't seem appropriate to me.
    > >
    > > One possibility to address this limitation would be to extend clock_gettime()
    > > with a new clock time, e.g., CLOCK_PERF.
    > >
    > > However, I understand that sched_clock_cpu() provides ordering guarantees only
    > > when invoked on the same CPU repeatedly, i.e., it's not globally synchronized.
    > > But we already have to deal with this problem when merging samples obtained
    > > from different CPU sampling buffer in per-thread mode. So this is not
    > > necessarily
    > > a showstopper.
    > >
    > > Alternatives could be to use uprobes but that's less practical to setup.
    > >
    > > Anyone with better ideas?
    >
    > You forgot to CC the time people ;-)
    >
    > I've no problem with adding CLOCK_PERF (or another/better name).
    >
    > Thomas, John?

    I've just faced the same issue - correlating an event in userspace with
    data from the perf stream, but to my mind what I want to get is a value
    returned by perf_clock() _in the current "session" context_.

    Stephane didn't like the idea of opening a "fake" perf descriptor in
    order to get the timestamp, but surely one must have the "session"
    already running to be interested in such data in the first place? So I
    think the ioctl() idea is not out of place here... How about the simple
    change below?

    Regards

    Pawel

    8<---
    From 2ad51a27fbf64bf98cee190efc3fbd7002819692 Mon Sep 17 00:00:00 2001
    From: Pawel Moll <pawel.moll@arm.com>
    Date: Fri, 1 Feb 2013 14:03:56 +0000
    Subject: [PATCH] perf: Add ioctl to return current time value

    To co-relate user space events with the perf events stream
    a current (as in: "what time(stamp) is it now?") time value
    must be made available.

    This patch adds a perf ioctl that makes this possible.

    Signed-off-by: Pawel Moll <pawel.moll@arm.com>
    ---
    include/uapi/linux/perf_event.h | 1 +
    kernel/events/core.c | 8 ++++++++
    2 files changed, 9 insertions(+)

    diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
    index 4f63c05..b745fb0 100644
    --- a/include/uapi/linux/perf_event.h
    +++ b/include/uapi/linux/perf_event.h
    @@ -316,6 +316,7 @@ struct perf_event_attr {
    #define PERF_EVENT_IOC_PERIOD _IOW('$', 4, __u64)
    #define PERF_EVENT_IOC_SET_OUTPUT _IO ('$', 5)
    #define PERF_EVENT_IOC_SET_FILTER _IOW('$', 6, char *)
    +#define PERF_EVENT_IOC_GET_TIME _IOR('$', 7, __u64)

    enum perf_event_ioc_flags {
    PERF_IOC_FLAG_GROUP = 1U << 0,
    diff --git a/kernel/events/core.c b/kernel/events/core.c
    index 301079d..4202b1c 100644
    --- a/kernel/events/core.c
    +++ b/kernel/events/core.c
    @@ -3298,6 +3298,14 @@ static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
    case PERF_EVENT_IOC_SET_FILTER:
    return perf_event_set_filter(event, (void __user *)arg);

    + case PERF_EVENT_IOC_GET_TIME:
    + {
    + u64 time = perf_clock();
    + if (copy_to_user((void __user *)arg, &time, sizeof(time)))
    + return -EFAULT;
    + return 0;
    + }
    +
    default:
    return -ENOTTY;
    }
    --
    1.7.10.4



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