Messages in this thread | | | Date | Wed, 23 Apr 2014 18:46:49 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC] sysrq: rcu-ify __handle_sysrq |
| |
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
| |