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 <20230123182728.825519-2-kan.liang@linux.intel.com> <87sfg0gvrk.ffs@tglx>
In-Reply-To: <87sfg0gvrk.ffs@tglx>
From:   Stephane Eranian <eranian@google.com>
Date:   Tue, 24 Jan 2023 01:10:01 -0800
Message-ID: <CABPqkBSt2WH=zo0_tXNb_Q7waDazvcquXgexXqqn=70A_f4H8Q@mail.gmail.com>
Subject: Re: [PATCH 1/3] timekeeping: NMI safe converter from a given time to monotonic
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     kan.liang@linux.intel.com, peterz@infradead.org, mingo@redhat.com,
        jstultz@google.com, sboyd@kernel.org, linux-kernel@vger.kernel.org,
        namhyung@kernel.org, ak@linux.intel.com
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On Tue, Jan 24, 2023 at 12:52 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Mon, Jan 23 2023 at 10:27, kan liang wrote:
> > From: Kan Liang <kan.liang@linux.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