Date: Tue, 17 Jun 2014 21:47:45 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Andi Kleen <ak@linux.intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>, LKML <linux-kernel@vger.kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        "Chen, Tim C" <tim.c.chen@intel.com>, Christoph Lameter <cl@linux.com>,
        peterz@infradead.org
Subject: Re: [bisected] pre-3.16 regression on open() scalability
Message-ID: <20140618044745.GZ4669@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <539B594C.8070004@intel.com>
 <20140613224519.GV4581@linux.vnet.ibm.com>
 <53A0CAE5.9000702@intel.com>
 <20140618001517.GL8178@tassilo.jf.intel.com>
 <20140618010423.GW4669@linux.vnet.ibm.com>
 <20140618022731.GM8178@tassilo.jf.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20140618022731.GM8178@tassilo.jf.intel.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/