linux-kernel - Re: [PATCH] kdb: use ktime_get_seconds() instead of ktime_get

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Message-ID: <CAK8P3a3p69ErytBBYUv=Xo=ak5oDUxZi4Nukb6KpP8iUNfBJkA@mail.gmail.com>
Date:   Fri, 26 Jan 2018 17:01:32 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kdb: use ktime_get_seconds() instead of ktime_get_ts()

On Fri, Jan 26, 2018 at 3:20 PM, Baolin Wang <baolin.wang@...aro.org> wrote:
> On 26 January 2018 at 22:00, Daniel Thompson <daniel.thompson@...aro.org> wrote:
>> On Fri, Jan 26, 2018 at 10:21:58AM +0100, Arnd Bergmann wrote:
>>> On Fri, Jan 26, 2018 at 4:03 AM, Baolin Wang <baolin.wang@...aro.org> wrote:
>>>
>>> Using ktime_get_seconds() avoids locking problems, but I wonder what
>>> would happen if we trigger the 'WARN_ON(timekeeping_suspended)'
>>> from kdb. Is that a problem? If it is, we have to use ktime_get_mono_fast_ns()
>>> and div_u64() instead.
>>
>> Normally a WARN_ON() doesn't triggered a kgdb_breakpoint() so (apart
>> from bugs) we can start executing the warning. Unfortunately
>> kdb_trap_printk isn't set when we call ktime_get_seconds() so printing
>> the warning isn't safe.
>>
>> If we had no choice of time function we could work around by
>> enabling printk() trapping for the call but since ktime_get_mono_fast_ns()
>> already exists its probably best just to use that.
>>
>
> If timekeeping_suspended is set, which means the system had been in
> suspend state. So now we still need debugger the system? But cores
> were already powered down.

I'm not using kdb myself, but I would assume that trapping into the debugger
during a suspend/resume bug is a very important scenario.

> The ktime_get_mono_fast_ns() will access the the clocksource driver,
> if the timekeeping is suspended following system suspend and the
> clocksource is not SUSPEND_NONSTOP, we may meet some unexpected issue
> to access the timer's register without clock. So I am not sure if
> ktime_get_mono_fast_ns() can work well for this case.

I misread the code the same way before, but as Thomas Gleixner
pointed out, ktime_get_mono_fast_ns() will not call the clocksource
driver when timekeeping is suspended. See halt_fast_timekeeper().
        Arnd