Date: Wed, 1 Jul 2015 09:17:05 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: josh@joshtriplett.org, linux-kernel@vger.kernel.org, mingo@kernel.org,
        laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, tglx@linutronix.de,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com,
        bobby.prani@gmail.com
Subject: Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging
 normal ones
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On Wed, Jul 01, 2015 at 04:17:10PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 01, 2015 at 07:00:31AM -0700, Paul E. McKenney wrote:
> 
> > That is a bit extreme, Peter.
> 
> Of course; but I'm really not seeing people taking due care with them

;-)

> > Are a huge pile of them coming in this merge window or something?
> > What raised your concerns on this issue?
> 
> This is complete horse manure (breaking the nvidiot binary blob is a
> good thing):
> 
> 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.

> 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.

> 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?

> Also, they really do not explain how expedited really is the only option
> available. Why things can't be batched etc..

Fair question!

							Thanx, Paul

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