linux-kernel - Re: [PATCH RFC] sysrq: rcu-ify __handle

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Date:	Wed, 23 Apr 2014 23:51:55 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Rik van Riel <riel@...hat.com>
cc:	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org,
	joern@...fs.org, peterz@...radead.org,
	Andrew Morton <akpm@...ux-foundation.org>, cxie@...hat.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC] sysrq: rcu-ify __handle_sysrq

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

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