Message-ID: <4CD2EF87.2030906@google.com>
Date:	Thu, 04 Nov 2010 10:38:15 -0700
From:	Mike Waychison <mikew@...gle.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
CC:	Matt Mackall <mpm@...enic.com>, Greg KH <greg@...ah.com>,
	simon.kagstrom@...insight.net, davem@...emloft.net,
	adurbin@...gle.com, akpm@...ux-foundation.org, chavey@...gle.com,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v1 00/12] netoops support

Américo Wang wrote:
> On Wed, Nov 03, 2010 at 06:18:41PM -0700, Mike Waychison wrote:
>> Matt Mackall wrote:
>>> On Wed, 2010-11-03 at 13:29 -0700, Mike Waychison wrote:
>>>> Mike Waychison wrote:
>>>>> FWIW, another semantic difference between netconsole and netoops (that
>>>>> I had missed in the last email) is filtering: we really do want to get
>>>>> the whole log when a crash happens, debug messages and all.
>>>>> Netconsole is subject to console filtering (which we _do_ want as
>>>>> debug messages going out the uart slows the whole world down).
>>>>>
>>>>> netconsole and netoops _do_ have bits in common, for instance the
>>>>> handling of NETDEV events and source+target configuration.  I'd rather
>>>>> those bits become common between the two than figure out how to jam
>>>>> the semantics we need into netconsole.
>>>> Hi Matt,
>>>>
>>>> I've been reading through the netconsole driver in response to
>>>> Greg's comments on this thread, and it is definitely more robust
>>>> in terms of configuration and handling of network device events
>>>> than the netoops driver I proposed.
>>> I've been following the discussion to see if it went anywhere
>>> interesting..
>>>
>>>> What are your thoughts on extending netconsole with the same sort
>>>> of semantics that are in the netoops patchset?
>>> My first thought is that it's a bit unfortunate that some of the the
>>> netconsole configgy bits weren't implemented in a generic way that would
>>> be applicable to other netpoll clients. Some people have never gotten it
>>> into their heads that netconsole isn't the only client.
>>>
>>>> I'd still like to have blit-dmesg-to-the-network-on-oops
>>>> semantics, which seems doable by having a per-target flag for
>>>> streaming of console messages (enabled by default) and a flag to
>>>> emit a structured full dmesg dump (disabled by default).
>>> I'd actually like to see you go forward with netoops. It's clear to me
>>> that it's a different beast and complexifying netconsole with a bunch of
>>> weird new options doesn't really sit well. If that means abstracting
>>> some of the sysfs crap from netconsole, great.
>> I'd be happy to take a stab at this.  This solves most of the ABI
>> reservations that I have with this v1 patchset.
>>
>> Looking at netconsole, it looks to lack some locking for data
>> consistency, and it appears that we will deadlock if we ever get a
>> NETDEV_UNREGISTER event (due to recursively grabbing the rtnl in
>> netpoll_cleanup).  I have a couple patches I've been hacking on this
>> afternoon that should clear those issues up.
>>
> 
> 
> You might want to look at net-next-2.6, it has some fixes
> from Neil.

Excellent, yes, 3b410a31 fixes the recursive rtnl deadlock I was 
referring to.

> 
> 
>> I'm thinking of pushing all the target handling options down into
>> net/core/netpoll.c.  I'll probably expose this interface as "struct
>> netpoll_targets" where ->lock and ->list could be completely exposed
>> to clients.  netconsole would then get a lot smaller as would
>> netoops.
>>
>>> That said, I don't think netoops is an ideal name, given how closely
>>> bound oops _events_ are with their textual output. Presumably it covers
>>> events other than oopsen like panics too.
>> True.  We call this code 'netdump' or 'network_dumper' internally,
>> but I figured it'd be better to follow current conventions with
>> ramoops and mtdoops already in the tree.  I don't really care what
>> it's called in the end :)
>>
> 
> 
> "netdump" was used by a utility that do crash dumping over net.
> It is deprecated now, since we have kdump.

Yup.  If you go back far enough, I think this was a gut of that code 
long long ago, hence the name.

> 
>>> Regarding rolling oopses: lots of machines regularly survive
>>> oopses, so I think you ought to consider rate-limiting them (to a
>>> configurable rate
>>> with a very low default) rather than suppressing all but the first.
>>>
>> The trouble with Oopses is just that:  We don't know whether we can
>> safely survive them or not and it's a total gamble each time we do
>> Oops.  We can't programmatically know how crapped out the machine is,
>> so historically we've erred on not allowing bad things to continue
>> happening once someone notices something wrong.
>>
>> It's easier for us to just shoot the machine in the head
>> (panic_on_oops) and move on than corrupt data or dead-lock in weird
>> ways at some later point in time.  This is definitely not the
>> behaviour I would want nor expect from my desktop or phone, but for
>> the cluster, it's just safer.
> 
> We also have pause_on_oops, or we can invent a oops_once.

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