linux-kernel - Re: [PATCH tip/core/rcu 01/45] rcutorture: Add forward-progress checking for writer

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Date:	Fri, 9 May 2014 08:52:31 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, oleg@...hat.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 01/45] rcutorture: Add forward-progress
 checking for writer

On Wed, May 07, 2014 at 04:43:13PM -0700, Paul E. McKenney wrote:
> On Wed, May 07, 2014 at 02:16:49PM -0700, josh@...htriplett.org wrote:
> > On Mon, Apr 28, 2014 at 05:24:49PM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > 
> > > The rcutorture output currently does not distinguish between stalls in
> > > the RCU implementation and stalls in the rcu_torture_writer() kthreads.
> > > This commit therefore adds some diagnostics to help distinguish between
> > > these two conditions, at least for the non-SRCU implementations.  (SRCU
> > > does not provide evidence of update-side forward progress by design.)
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > The concept makes sense, and the writer state annotations seem like a
> > useful debugging mechanism, but having RCU know about RCU torture types
> > seems fundamentally wrong.  This mechanism accesses rcu_state, which is
> > already implementation-specific, so why not just only define the
> > function for the RCU implementations that support it, and then have a
> > function pointer in the torture-test structure to report a stall?
> 
> Ouch.  It is worse than that!  When running RCU-bh or RCU-sched,
> the current code incorrectly returns the statistics for RCU.
> So I do need some way for rcutorture to tell RCU which flavor
> it is testing.
> 
> One thing I could do would be to pass in a pointer to the call_rcu()
> function (cur_ops->call from rcutorture's viewpoint), then scan the
> rcu_state structures looking for the selected flavor (rsp->call from
> tree.c's viewpoint).  In the SRCU and RCU-busted cases, the flavor would
> not be found, and I could then just set everything to zero.
> 
> Does that seem reasonable, or is there a better way to do this?

That search seems rather too hackish; why not just declare one
stats-returning function per RCU flavor, and put the pointer to the
corresponding function in the structure for each test type?

- Josh Triplett
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