Re: [PATCH RFC] sysrq: rcu-ify __handle_sysrq

[Paul E. McKenney]

On Wed, Apr 23, 2014 at 11:51:55PM +0200, Jiri Kosina wrote:
> On Wed, 23 Apr 2014, Rik van Riel wrote:
> 
> > >> Echoing values into /proc/sysrq-trigger seems to be a popular way to
> > >> get information out of the kernel. However, dumping information about
> > >> thousands of processes, or hundreds of CPUs to serial console can
> > >> result in IRQs being blocked for minutes, resulting in various kinds
> > >> of cascade failures.
> > >>
> > >> The most common failure is due to interrupts being blocked for a very
> > >> long time. This can lead to things like failed IO requests, and other
> > >> things the system cannot easily recover from.
> > >>
> > >> This problem is easily fixable by making __handle_sysrq use RCU
> > >> instead of spin_lock_irqsave.
> > >>
> > >> This leaves the warning that RCU grace periods have not elapsed for a
> > >> long time, but the system will come back from that automatically.
> > > 
> > > This, however, will make RCU stall detector to send NMI to all online CPUs 
> > > so that they can dump their stacks.

Hey, if dumping the stacks once is a good idea, dumping them twice
must be twice as good, right?  ;-)

> > It already does that, since several of the longer-running
> > sysrq handlers already grab rcu_read_lock(), for example
> > show_state().
> > 
> > > IOW, this might actually make the whole sysrq dump last for much longer, 
> > > and have the log polluted with all-CPU dumps for no good reason.
> > > 
> > > I wonder whether explicitly setting rcu_cpu_stall_suppress during sysrq 
> > > handling might be a viable workaround for this.
> > 
> > I suppose that would do the trick.
> 
> I can imagine Paul opposing this though ... this variable is supposed to 
> be changed only by cmdline/modparam, not really flipped during runtime as 
> a bandaid ... let's add Paul to CC.

Well, we already crowbar it to 1 when panic starts, see rcu_panic().

How about something like the following?

	void rcu_sysrq_start(void)
	{
		rcu_cpu_stall_suppress = 2;
	}

	void rcu_sysrq_end(void)
	{
		if (rcu_cpu_stall_suppress == 2)
			rcu_cpu_stall_suppress = 0;
	}

If there get to be too many more different reasons for temporarily
suppressing RCU CPU stall warnings, I can then swap out to a better
implementation, for some definition or another of "better".

							Thanx, Paul

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