On Fri, May 09, 2014 at 05:14:54PM -0700, Josh Triplett wrote:
> On Fri, May 09, 2014 at 03:55:14PM -0700, Paul E. McKenney wrote:
> > 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, 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?
> > >
> > > 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?
>
> Seems reasonable to me. I'd call the last one "INVALID_RCU_FLAVOR" or
> similar.
Even better!
Thanx, Paul
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