Date:	Fri, 9 May 2014 15:55:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
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 Fri, May 09, 2014 at 03:40:52PM -0700, Josh Triplett wrote:
> On Fri, May 09, 2014 at 03:30:04PM -0700, Paul E. McKenney wrote:
> > On Fri, May 09, 2014 at 12:32:46PM -0700, Josh Triplett wrote:
> > > On Fri, May 09, 2014 at 10:36:58AM -0700, Paul E. McKenney wrote:
> > > > On Fri, May 09, 2014 at 08:52:31AM -0700, Josh Triplett wrote:
> > > > > 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?
> > > > 
> > > > The problem is that rcutorture doesn't know anything about the structures,
> > > > as those are internal to the implementation.  All it knows is which
> > > > functions it is using.  I -could- EXPORT_SYMBOL_GPL() the rcu_state
> > > > structures to modules (they are already non-static), then rename
> > > > TINY_RCU's rcu_ctrlblk to rcu_state to allow the needed type punning,
> > > > then do some special-case thing for SRCU, and put a pointer to whatever
> > > > in rcu_torture_ops, but that was feeling a bit hackish as well.
> > > > 
> > > > So what did you have in mind to allow rcutorture to communicate the
> > > > rcuflavor to the underlying RCU implementation?
> > > 
> > > Rather than EXPORT_SYMBOL_GPLing the rcu_state structures, just
> > > EXPORT_SYMBOL_GPL one version of rcutorture_get_gp_data per RCU flavor.
> > > (And hide them all behind #ifdef CONFIG_RCU_TORTURE_TEST.)  Then add a
> > > get_gp_data field to rcu_torture_ops; if NULL, skip the stats.  (Or put
> > > a no-op version in rcutorture.)
> > 
> > But that would require me to provide these same exports from TINY_RCU,
> > which does not need them.
> > 
> > How about exporting integers identifying the flavors of RCU to rcutorture,
> > which rcutorture can then pass to rcutorture_get_gp_data()?  This allows
> > TINY_RCU to provide a trivial static inline function.  TREE_RCU and
> > TREE_PREEMPT_RCU can keep an array of pointers to the corresponding
> > rcu_state structure, with NULL pointers for flavors of RCU that don't
> > have any data to provide.
> > 
> > Would that help?
> 
> Either way seems fine: a single function with an extra parameter or a
> unique function per flavor.  But if you're going to provide the
> RCU-flavor integers, they should be in RCU itself and refer to RCU
> flavors, rather than being in rcutorture and refering to test types.

Fair enough, will take that approach!

I am thinking in terms of something like the following:

RCU_FLAVOR
RCU_BH_FLAVOR
RCU_SCHED_FLAVOR
SRCU_FLAVOR
OTHER_FLAVOR

The "OTHER_FLAVOR" would be what rcutorture uses for the "busted"
incorrect-on-purpose flavor of RCU, which is local to rcutorture.

Seem reasonable, or would some other naming scheme be better?

							Thanx, Paul

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