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

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Message-ID: <20140618124028.GN8178@tassilo.jf.intel.com>
Date:	Wed, 18 Jun 2014 05:40:28 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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>, peterz@...radead.org
Subject: Re: [bisected] pre-3.16 regression on open() scalability

On Tue, Jun 17, 2014 at 09:47:45PM -0700, Paul E. McKenney wrote:
> On Tue, Jun 17, 2014 at 07:27:31PM -0700, Andi Kleen wrote:
> > > OK.  What would you suggest instead?  If all we do is to revert the
> > 
> > Hang checker should have two timer phases:
> > 
> > Timer fires first time:
> > - Save context switch counter on that. Force a reschedule to some
> > work queue. Rearm timer
> > 
> > Timer fires again:
> > - Check reschedule count. If the reschedule count changed
> > it was a real hang, otherwise ignore.
> 
> I could take that approach, but the RT guys aren't going to thank me for
> the wakeup associated with the work queue.  I suppose that I could use

They can disable the hang timer if it's really problem.

If they cannot tolerate a single context switch they likely 
cannot tolerate a timer firing either.

-Andi
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