linux-kernel - Re: [bisected] pre-3.16 regression on open() scalability

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Message-ID: <20140619052337.GG4669@linux.vnet.ibm.com>
Date:	Wed, 18 Jun 2014 22:23:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Dave Hansen <dave.hansen@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Josh Triplett <josh@...htriplett.org>,
	"Chen, Tim C" <tim.c.chen@...el.com>,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [bisected] pre-3.16 regression on open() scalability

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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