Date: Wed, 18 Jun 2014 22:23:37 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Eric Dumazet <eric.dumazet@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: <20140619052337.GG4669@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20140618001836.GV4669@linux.vnet.ibm.com>
 <53A132D4.60408@intel.com>
 <20140618125831.GB4669@linux.vnet.ibm.com>
 <53A1CE19.7040103@intel.com>
 <20140618203052.GT4669@linux.vnet.ibm.com>
 <20140618235131.GA25946@linux.vnet.ibm.com>
 <20140619014200.GO8178@tassilo.jf.intel.com>
 <20140619021337.GA4669@linux.vnet.ibm.com>
 <20140619033816.GQ8178@tassilo.jf.intel.com>
 <1403153545.1225.8.camel@edumazet-glaptop2.roam.corp.google.com>
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On Wed, Jun 18, 2014 at 09:52:25PM -0700, Eric Dumazet wrote:
> On Wed, 2014-06-18 at 20:38 -0700, Andi Kleen wrote:
> > On Wed, Jun 18, 2014 at 07:13:37PM -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.
> > 
> > OOM in what scenario? This is getting bizarre.
> > 
> > If something keeps looping forever in the kernel creating 
> > RCU callbacks without any real quiescent states it's simply broken.
> 
> Typical problem we faced in the past is in exit() path when multi
> thousands of files/sockets are rcu-freed, and qhimark is hit.
> 
> Huge latency alerts, as freeing 10000+ items takes a while (about 70 ns
> per item...)
> 
> Maybe close_files() should use a
> cond_resched_and_keep_rcu_queues_small_please() ;)

That sort of approach would work for me.  Over time, I would guess
that the cond_resched_and_keep_rcu_queues_small_please() function would
find its way to where it needed to be.  ;-)

							Thanx, Paul

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