linux-kernel - Re: [PATCH v2] Add hard/soft lockup debugger entry points

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Message-ID: <CAO6TR8WaDTO=mSp07z2EbC69YOx+F0_QGLtHDXVT2n6cqktMOw@mail.gmail.com>
Date:	Tue, 26 Jan 2016 10:26:56 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Aaron Tomlin <atomlin@...hat.com>,
	Michal Hocko <mhocko@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tejun Heo <tj@...nel.org>,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>
Subject: Re: [PATCH v2] Add hard/soft lockup debugger entry points

On 1/26/16, Chris Metcalf <cmetcalf@...hip.com> wrote:
> On 01/26/2016 12:04 PM, Jeff Merkey wrote:
>> This patch adds an export which can be set by system debuggers to direct
>> the hard lockup and soft lockup detector to trigger a breakpoint
>> exception
>> and enter a debugger if one is active.  It is assumed that if someone
>> sets this variable, then an breakpoint handler of some sort will be
>> actively
>> loaded or registered via the notify die handler chain.
>>
>> This addition is extremely useful for debugging hard and soft lockups
>> real time and quickly from a console debugger.
>>
>> Signed-off-by: Jeff Merkey<linux.mdb@...il.com>
>> ---
>>   kernel/watchdog.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>
> You probably should reach out to someone who uses this stuff more
> regularly - I actually wonder if the kgdb_breakpoint() API is the
> right thing, though, not the internal arch_kgdb_breakpoint().
>
> Of course any of these strategies also assume you are building
> the kernel with CONFIG_KGDB set, and I'm pretty sure will cause
> your build to fail if it isn't.  You likely need to guard this
> stuff locally within watchdog.c for !CONFIG_KGDB.
>
> --
> Chris Metcalf, EZChip Semiconductor
> http://www.ezchip.com
>
>

I'll go review that code carefully.  What would ideal is to have the
breakpoint abstraction independent of kgdb entirely.  That would not
be a simple patch but more involved.  We really just need the
breakpoint instruction for each arch, not all of kgdb and it should
not depend on KGB being enabled.  I looked over those includes and
this may work, but you are correct, it's not squeaky clean to do it
that way.  I'll get to work on v3.

:-)

Jeff