linux-kernel - Re: [PATCH 1/3] timekeeping: NMI safe converter from a given time to monotonic

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Message-ID: <CABPqkBSt2WH=zo0_tXNb_Q7waDazvcquXgexXqqn=70A_f4H8Q@mail.gmail.com>
Date:   Tue, 24 Jan 2023 01:10:01 -0800
From:   Stephane Eranian <eranian@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     kan.liang@...ux.intel.com, peterz@...radead.org, mingo@...hat.com,
        jstultz@...gle.com, sboyd@...nel.org, linux-kernel@...r.kernel.org,
        namhyung@...nel.org, ak@...ux.intel.com
Subject: Re: [PATCH 1/3] timekeeping: NMI safe converter from a given time to monotonic

On Tue, Jan 24, 2023 at 12:52 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Mon, Jan 23 2023 at 10:27, kan liang wrote:
> > From: Kan Liang <kan.liang@...ux.intel.com>
> >
> > It's useful to provide a NMI safe function to convert a given time to
> > monotonic. For example, the perf_event subsystem wants to convert a TSC
> > of a PEBS record to a monotonic clock in a NMI handler.
>
> Why? That's a postprocessing problem, really.
>
Because you want to correlate samples captured by PEBS with samples
from applications timestamped with a user available clock such as
CLOCK_MONOTONIC, for instance.
When I create a perf_event event and I stipulate that
event_attr.clockid=MONOTONIC, I expect all the samples from
that event to be timestamped using the same clock source, regardless
of PEBS or IBS.



> Thanks,
>
>         tglx