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

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Message-ID: <20140619022930.GA11365@linux.vnet.ibm.com>
Date:	Wed, 18 Jun 2014 19:29:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	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 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.
> 
> > 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.
> 
> As mentioned earlier, I could potentially push the check behind
> the need-resched check, which would get it off of the common case
> of the code paths you call out above.

Of course, it would also be good to measure this and see how much it
really hurts.

							Thanx, Paul

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