Re: [PATCH 0/2] add new notifier function ,take3

[Neil Horman <nhorman@redhat.com>]

On Mon, Apr 14, 2008 at 10:53:23AM -0400, Vivek Goyal wrote:
> On Mon, Apr 14, 2008 at 10:42:28AM -0400, Neil Horman wrote:
> > On Mon, Apr 14, 2008 at 09:46:22AM -0400, Vivek Goyal wrote:
> > > On Fri, Apr 11, 2008 at 09:07:51PM -0700, Andrew Morton wrote:
> > > 
> > > [..]
> > > > > Kernel panic - not syncing: Panic by panic_module.
> > > > > __tunable_atomic_notifier_call_chain enter
> > > > > msg_handler:panic_event was called.
> > > > > ipmi_wdog:wdog_panic_handler was called.
> > > > > notifier_test: notifier_test_panic() is called.
> > > > > notifier_test: notifier_test_panic2() is called.
> > > > 
> > > > OK.  But I don't see anywhere in here the most important piece of
> > > > information: why do we need this feature in Linux?
> > > > 
> > > > What are the use-cases?  What is the value?  etc.
> > > > 
> > > > Often I can guess (but I like the originator to remove the guesswork).  In
> > > > this case I'm stumped - I can't see any reason why anyone would want this.
> > > > 
> > > 
> > > Hi Andrew,
> > > 
> > > To begin with, he wants kdb, kgdb etc to co-exist with kdump. He wants
> > > to put all the RAS tools (who are interested in panic event) on a list
> > > and export it to user space and let user decide in what order do the tool get
> > > executed at panic time (based on priority).
> > > 
> > > This brings in little bit reliability concerns for kdump due to notifier
> > > code being run after panic.
> > > 
> > > I think people want to use this infrastrutucure beyond RAS tools. I
> > > remember somebody wanting to send a message to remote node after a
> > > panic (before kdump kicks in)  so that remote node can initiate failover
> > > etc.
> > > 
> > I know it doesn't particularly relate to this patch, but FWIW, for cases like
> > failover, I've inserted infrastrucutre in the userspace part of kdump for
> > Fedora/RHEL to support this sort of thing.  We can run arbitrary scripts righte
> > before and after a capture so that notifications can be sent to remote nodes in
> > a much safer fashion than using the notifier chain after a panic.
> > Neil
> > 
> 
> That's great. I did not know about these. So user can write custom
> scripts/binaries which can be packed into kdump initrd and executed either
> before or after dump capture? Any idea, if somebody has started using it
> already?
> 
Thats exactly right.  I'm not sure if there is any serious use as of yet, but
I've had some interrogatories about it.  Specific cases that I recall include:

1) A set of users in japan that are using the pre-dump script to block execution
until a scsi controller detects all its drives (it apparently takes up to three
minues to scan its bus)

2) I think some people using clustering services were using the pre-script to
notify cluster peers of the failure to avoid power fencing while a node
completed the crash dump

3) A national lab had an interest in using the pre script to send an email to an
administrative address to log the failure in a cluster 

Neil

> If that's the case then only other serious user at this point of time
> is kernel debugger (kdb, kgdb), which needs to run before kdump, in case
> of panic. And Eric suggested for those cases debugger can just insert a 
> break point at panic(), instead of introducing the tunable notifier list
> infrastructure.
> 
> Thanks
> Vivek

-- 
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 *Neil Horman
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