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

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Message-ID: <20150701161705.GK3717@linux.vnet.ibm.com>
Date:	Wed, 1 Jul 2015 09:17:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	josh@...htriplett.org, linux-kernel@...r.kernel.org,
	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging
 normal ones

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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