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

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Message-ID: <20150701105511.GN18673@twins.programming.kicks-ass.net>
Date:	Wed, 1 Jul 2015 12:55:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	josh@...htriplett.org
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 12:09:39PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 30, 2015 at 04:46:33PM -0700, josh@...htriplett.org wrote:
> > Consider it a fairly weak concern against.  Increasing performance seems
> > like a good thing in general; I just don't relish the future "feels less
> > responsive" bug reports that take a long time to track down and turn out
> > to be "this completely unrelated driver was loaded and started using
> > expedited grace periods".
> 
> random drivers, or for that matter, new-code of any sort. Should _NOT_
> be using expedited grace periods.
> 
> They're a horrid hack only suitable for unfixable ABI.

Let me repeat, just in case I've not been clear. Expedited grace periods
are _BAD_ and should be avoided at all costs.

They perturb the _entire_ machine.

Yes we can polish the turd, but in the end its still a turd.

Sadly people seem to have taken a liking to them, ooh a make RCU go
faster button. And there's not been much if any pushback on people using
it.


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