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

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Date:   Mon, 29 Jan 2018 09:49:17 +0800
From:   Baolin Wang <baolin.wang@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
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 27 January 2018 at 00:01, Arnd Bergmann <arnd@...db.de> wrote:
> 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().

Ah, I missed halt_fast_timekeeper() too, thanks for pointing it out.
Now I think ktime_get_mono_fast_ns() can work for this case.

Jason, could you drop the previous patch? I will respin v2 to use
ktime_get_mono_fast_ns() as Arnd suggested. Thanks.

-- 
Baolin.wang
Best Regards