Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging normal ones

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 01, 2015 at 09:17:05AM -0700, Paul E. McKenney wrote:
> On Wed, Jul 01, 2015 at 04:17:10PM +0200, Peter Zijlstra wrote:

> > 74b51ee152b6 ("ACPI / osl: speedup grace period in acpi_os_map_cleanup")
> 
> Really???
> 
> I am not concerned about this one.  After all, one of the first things
> that people do for OS-jitter-sensitive workloads is to get rid of
> binary blobs.  And any runtime use of ACPI as well.  And let's face it,
> if your latency-sensitive workload is using either binary blobs or ACPI,
> you have already completely lost.  Therefore, an additional expedited
> grace period cannot possibly cause you to lose any more.

This isn't solely about rt etc.. this call is a generic facility used by
however many consumers. A normal workstation/server could run into it at
relatively high frequency depending on its workload.

Even on not latency sensitive workloads I think hammering all active
CPUs is bad behaviour. Remember that a typical server class machine
easily has more than 32 CPUs these days.

> > Also, I'm not entirely convinced things like:
> > 
> > fd2ed4d25270 ("dm: add statistics support")
> > 83d5e5b0af90 ("dm: optimize use SRCU and RCU")
> > ef3230880abd ("backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu")
> > 
> > Are in the 'never' happens category. Esp. the backing-dev one, it
> > triggers every time you unplug a USB stick or similar.
> 
> Which people should be assiduously avoiding for any sort of
> industrial-control system, especially given things like STUXNET.

USB sure, but a backing dev is involved in nfs clients, loopback and all
sorts of block/filesystem like setups.

unmount an NFS mount and voila expedited rcu, unmount a loopback, tada.

All you need is a regular server workload triggering any of that on a
semi regular basis and even !rt people might start to notice something
is up.

> > Rejigging a DM might indeed be rare enough; but then again, people use
> > DM explicitly so they can rejig while in operation.
> 
> They rejig DM when running OS-jitter-sensitive workloads?

Unlikely but who knows, I don't really know DM, so I can't even tell
what would trigger these.