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, j...@joshtriplett.org wrote:
> > > > > On Mon, Apr 28, 2014 at 05:24:49PM -0700, Paul E. McKenney wrote:
> > > > > > From: "Paul E. McKenney" <paul...@linux.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 <paul...@linux.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?
Thanx, Paul
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