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Message-ID: <20140513182738.GL18164@linux.vnet.ibm.com> Date: Tue, 13 May 2014 11:27:38 -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 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, 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? > > Seems reasonable to me. I'd call the last one "INVALID_RCU_FLAVOR" or > similar. Done! I ended up with a switch statement rather than an array of pointers, FWIW. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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