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

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Date:	Thu, 19 Jun 2014 16:13:12 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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>
Subject: Re: [bisected] pre-3.16 regression on open() scalability

On Thu, 19 Jun 2014, Paul E. McKenney wrote:

> On Thu, Jun 19, 2014 at 03:31:56PM -0500, Christoph Lameter wrote:
> > On Thu, 19 Jun 2014, Paul E. McKenney wrote:
> >
> > > That is a separate issue, but unnecessary calls to cond_resched()
> > > should of course be removed -- no argument there.
> >
> > It looks like we are fighting higher latencies by adding calls in most of
> > the critical sections which will in turn increase the latencies that we
> > are trying to reduce.
>
> That is on battle in a larger war.  Another battle is to keep big systems
> from choking and dying.  We need to win both battles, though I understand
> that you are focused primarily on latency reduction.  I don't get that
> luxury because RCU is used across the board.

At least reduce the increased latencies to a processor or put it somewhere
outside of the regular execution paths. Sprinkling this throughout the
kernel via a inline function that did not generate code in the non debug
situation before is not that nice.

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