Date: Wed, 1 Jul 2015 12:55:11 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: josh@joshtriplett.org
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        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
Message-ID: <20150701105511.GN18673@twins.programming.kicks-ass.net>
References: <20150630214805.GA7795@linux.vnet.ibm.com>
 <20150630220014.GA10916@cloud>
 <20150630221224.GQ3717@linux.vnet.ibm.com>
 <20150630234633.GA11450@cloud>
 <20150701100939.GR19282@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150701100939.GR19282@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1254
Lines: 30

On Wed, Jul 01, 2015 at 12:09:39PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 30, 2015 at 04:46:33PM -0700, josh@joshtriplett.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.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/