linux-kernel - Re: [RFC PATCH 0/3] Add accurate boot timing to a Linux system

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Message-ID: <CAPnjgZ2Z-nBg2m1P2zkda=nMAG0qCOzVtpAOSth6nwoGSUfrFQ@mail.gmail.com>
Date:	Tue, 27 Sep 2011 11:26:13 -0700
From:	Simon Glass <sjg@...omium.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 0/3] Add accurate boot timing to a Linux system

Hi Bjorn,

On Sun, Sep 25, 2011 at 5:54 AM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Fri, Sep 23, 2011 at 5:03 PM, Simon Glass <sjg@...omium.org> wrote:
>> This experimental patch set adds boot timing to a Linux system. The
>> timing starts with the boot loader and extends through the kernel into
>> user space to the completion of the boot process. The timing starts when
>> the system leaves reset, not later when the kernel starts.
>>
>> The concept is:
>> - Boot loader records a timestamp for key events during its operation
>> - These timestamps are passed to Linux, which adds more as it boots
>> - These timestamps are made available to user space, where more
>> timestamps are added as init does its job
>> - Finally the whole record is collected by a user-space script run at
>> the end of init. This is fed back through some mechanism to monitor
>> boot time in the field.
>
> I think this is a cool idea.  It's quite difficult to extract this
> sort of information today, and making it easily and consistently
> available should help focus attention and improve things.
>
> There are difficult issues about which clock to use, how to correlate
> bootloader & kernel timestamps, how to make sure the timestamps stay
> sensible even when we use hwclock, ntp, etc., but I think it's worth
> pushing on this to see how far you can go.

Yes it is not easy to get all of this right, especially trying to
cross two completely different projects. I think the timestamps should
be measured from reset. The kernel timestamps start not at zero, but
from the time the kernel starts. This guarantees a consistent time
base across the data. Also if there is a pre-boot program (such as a
NAND loader) then its time will be counted also.

A good timer would be an accurate monotonic wall clock, not
necessarily related to time of day. An ideal timer would be a
microsecond/nanosecond timer maintained by the hardware / SOC.

Regards,
Simon

>
> Bjorn
>
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