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

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Message-ID: <20140618125614.GA4669@linux.vnet.ibm.com>
Date:	Wed, 18 Jun 2014 05:56:14 -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>, peterz@...radead.org
Subject: Re: [bisected] pre-3.16 regression on open() scalability

On Wed, Jun 18, 2014 at 05:40:28AM -0700, Andi Kleen wrote:
> 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.

Ah, but I make the timer fire on some other CPU.

							Thanx, Paul

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