Message-ID: <20120601225603.GD24318@sgi.com>
Date:	Fri, 1 Jun 2012 17:56:03 -0500
From:	Russ Anderson <rja@....com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	rja@...ricas.sgi.com
Subject: Re: [PATCH] x86: Avoid intermixing cpu dump_stack output on multi-processor systems

On Tue, May 29, 2012 at 07:54:07PM -0400, Don Zickus wrote:
> On Tue, May 29, 2012 at 06:11:35PM -0500, Russ Anderson wrote:
> > > > In this case, I am just using the hardware NMI, which sends the NMI
> > > > signal to each logical cpu.  Since each cpu receives the NMI at nearly
> > > > the exact same time, they end up in dump_stack() at the same time.
> > > > Without some form of locking, trace lines from different cpus end
> > > > up intermixed, making it impossible to tell what any individual 
> > > > cpu was doing.
> > > 
> > > I forgot the original reasons for having the NMI go to each CPU instead of
> > > just the boot CPU (commit 78c06176), but it seems like if you revert that
> > > patch and have the nmi handler just call trigger_all_cpu_backtrace()
> > > instead (which does stack trace locking for pretty output), that would
> > > solve your problem, no?  That locking is safe because it is only called in
> > > the NMI context.
> > 
> > We want NMI to hit all the cpus at the same time to get a coherent
> > snapshot of what is happening in the system at one point in time.
> > Sending an IPI one cpu at a time skews the results, and doesn't 
> 
> Oh, I thought it was broadcasting, but I see the apic_uv code serializes
> it.  Though getting all those hardware locks in the nmi handler has to be
> time consuming?  But I know you guys did some tricks to speed that up.
> 
> > really solve the problem of multiple cpus going into dump_stack()
> > at the same time.  NMI isn't the only possible caller of dump_stack().
> 
> I am curious, your NMI handler has locking wrapped around dump_stack,
> shouldn't that serialize the output the way you want it?  Why isn't that
> working?

Yes, you're right, it does.  It is working.  I'd forgotten that 
the community kernel has uv_nmi_lock in uv_handle_nmi.  Must 
be working too much with distro kernels.  :-)  But that doesn't
help for all the other code paths than call dump_stack.


> > FWIW, "Wait for up to 10 seconds for all CPUs to do the backtrace" on
> > a 4096 cpu system isn't long enough.  :-)
> 
> Good point. :-)
> 
> > 
> > > Whereas the lock you are proposing can be called in a mixture of NMI and
> > > IRQ which could cause deadlocks I believe.
> > 
> > Since this is a lock just around the dump_stack printk, would 
> > checking for forward progress and a timeout to catch any possible
> > deadlock be sufficient?  In the unlikely case of a deadlock the
> > lock gets broken and some of the cpu backtraces get intermixed.
> > That is still a huge improvement over the current case where 
> > all of the backtraces get intermixed.
> 
> I saw your new patch based on Frederick's input.  It seems to take care of
> deadlock situations though you run into the starving lock problem that
> ticketed spinlocks solved.  Which is why I am curious why moving the
> locking one layer up to the NMI handler (which is where it is currently),
> didn't fix your problem.

Locking in dump_stack would remove the need for uv_nmi_lock.

-- 
Russ Anderson, OS RAS/Partitioning Project Lead  
SGI - Silicon Graphics Inc          rja@....com
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