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

Jiri Kosina
2014-04-23T21:51:58Z

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.
>
> 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.

--
Jiri Kosina
SUSE Labs