Date: Wed, 18 Jun 2014 21:19:51 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Andi Kleen <ak@linux.intel.com>, Dave Hansen <dave.hansen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        "Chen, Tim C" <tim.c.chen@intel.com>, Christoph Lameter <cl@linux.com>
Subject: Re: [bisected] pre-3.16 regression on open() scalability
Message-ID: <20140619041951.GE4669@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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On Thu, Jun 19, 2014 at 04:50:19AM +0200, Mike Galbraith wrote:
> On Wed, 2014-06-18 at 19:13 -0700, Paul E. McKenney wrote: 
> > On Wed, Jun 18, 2014 at 06:42:00PM -0700, Andi Kleen wrote:
> > > 
> > > I still think it's totally the wrong direction to pollute so 
> > > many fast paths with this obscure debugging check workaround
> > > unconditionally.
> > 
> > OOM prevention should count for something, I would hope.
> > 
> > > cond_resched() is in EVERY sleeping lock and in EVERY memory allocation!
> > > And these are really critical paths for many workloads.
> > > 
> > > If you really wanted to do this I think you would first need
> > > to define a cond_resched_i_am_not_fast() or somesuch.
> > > 
> > > Or put it all behind some debugging ifdef.
> > 
> > My first thought was to put it behind CONFIG_NO_HZ_FULL, but everyone
> > seems to be enabling that one.
> 
> Not everybody, SUSE doesn't even have it enabled in factory.

OK, apologies for the over-generalization.

But you would think that I would have learned this lesson with
CONFIG_RCU_FAST_NO_HZ, wouldn't you?  :-/

							Thanx, Paul

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