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

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Message-ID: <20140618044745.GZ4669@linux.vnet.ibm.com>
Date:	Tue, 17 Jun 2014 21:47:45 -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 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
one approach for real-time workloads and your workqueue-base approach
for other workloads.

Still, I bet I can drop the common-case cond_resched() overhead to a
single read of a per-CPU variable and a branch.  But yes, that would be
in response to the second phase, FWIW.  If that measurably too much
overhead, then one thing for realtime and another otherwise.

							Thanx, Paul

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