From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753969AbaEIWaN (ORCPT ); Fri, 9 May 2014 18:30:13 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:42864 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751098AbaEIWaL (ORCPT ); Fri, 9 May 2014 18:30:11 -0400 Date: Fri, 9 May 2014 15:30:04 -0700 From: "Paul E. McKenney" To: Josh Triplett Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, oleg@redhat.com, sbw@mit.edu Subject: Re: [PATCH tip/core/rcu 01/45] rcutorture: Add forward-progress checking for writer Message-ID: <20140509223004.GZ8754@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140429002455.GA15461@linux.vnet.ibm.com> <1398731133-18925-1-git-send-email-paulmck@linux.vnet.ibm.com> <20140507211649.GA27924@cloud> <20140507234313.GP8754@linux.vnet.ibm.com> <20140509155230.GA4152@thin> <20140509173658.GV8754@linux.vnet.ibm.com> <20140509193246.GC13050@jtriplet-mobl1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140509193246.GC13050@jtriplet-mobl1> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14050922-7164-0000-0000-000001A4BDB4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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@joshtriplett.org wrote: > > > > > On Mon, Apr 28, 2014 at 05:24:49PM -0700, Paul E. McKenney wrote: > > > > > > From: "Paul E. McKenney" > > > > > > > > > > > > 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 > > > > > > > > > > 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