Re: [PATCH] x86: Avoid intermixing cpu dump_stack output on multi-processor systems

[Russ Anderson]

On Tue, May 29, 2012 at 06:39:23PM -0400, Don Zickus wrote:
> On Tue, May 29, 2012 at 02:19:35PM -0500, Russ Anderson wrote:
> > On Tue, May 29, 2012 at 01:53:53PM -0400, Don Zickus wrote:
> > > On Thu, May 24, 2012 at 09:42:29AM -0500, Russ Anderson wrote:
> > > > When multiple cpus on a multi-processor system call dump_stack()
> > > > at the same time, the backtrace lines get intermixed, making 
> > > > the output worthless.  Add a lock so each cpu stack dump comes
> > > > out as a coherent set.
> > > > 
> > > > For example, when a multi-processor system is NMIed, all of the
> > > > cpus call dump_stack() at the same time, resulting in output for
> > > > all of cpus getting intermixed, making it impossible to tell what
> > > > any individual cpu was doing.  With this patch each cpu prints
> > > > its stack lines as a coherent set, so one can see what each cpu
> > > > was doing.
> > > 
> > > For this particular test case, it sounds like you are doing what
> > > trigger_all_cpu_backtrace() is doing?  It doesn't solve the general
> > > problem, but probably your particular usage?
> > 
> > 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 
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().

FWIW, "Wait for up to 10 seconds for all CPUs to do the backtrace" on
a 4096 cpu system isn't long enough.  :-)

> 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.

> Cheers,
> Don

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