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Date: Wed, 2 Sep 2009 09:37:05 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Nick Piggin <npiggin@...e.de> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: tree rcu: call_rcu scalability problem? On Wed, Sep 02, 2009 at 06:24:51PM +0200, Nick Piggin wrote: > On Wed, Sep 02, 2009 at 08:19:27AM -0700, Paul E. McKenney wrote: > > On Wed, Sep 02, 2009 at 02:27:56PM +0200, Nick Piggin wrote: > > > On Wed, Sep 02, 2009 at 11:48:35AM +0200, Nick Piggin wrote: > > > > Hi Paul, > > > > > > > > I'm testing out scalability of some vfs code paths, and I'm seeing > > > > a problem with call_rcu. This is a 2s8c opteron system, so nothing > > > > crazy. > > > > > > > > I'll show you the profile results for 1-8 threads: > > > > > > > > 1: > > > > 29768 total 0.0076 > > > > 15550 default_idle 48.5938 > > > > 1340 __d_lookup 3.6413 > > > > 954 __link_path_walk 0.2559 > > > > 816 system_call_after_swapgs 8.0792 > > > > 680 kmem_cache_alloc 1.4167 > > > > 669 dput 1.1946 > > > > 591 __call_rcu 2.0521 > > > > > > > > 2: > > > > 56733 total 0.0145 > > > > 20074 default_idle 62.7313 > > > > 3075 __call_rcu 10.6771 > > > > 2650 __d_lookup 7.2011 > > > > 2019 dput 3.6054 > > > > > > > > 4: > > > > 98889 total 0.0253 > > > > 21759 default_idle 67.9969 > > > > 10994 __call_rcu 38.1736 > > > > 5185 __d_lookup 14.0897 > > > > 4475 dput 7.9911 > > > > Four threads runs on one socket but 8 threads runs on two sockets, > > I take it? > > Yes. > > > > > > 8: > > > > 170391 total 0.0437 > > > > 31815 __call_rcu 110.4688 > > > > 12958 dput 23.1393 > > > > 10417 __d_lookup 28.3071 > > > > > > > > Of course there are other scalability factors involved too, but > > > > __call_rcu is taking 54 times more CPU to do 8 times the amount > > > > of work from 1-8 threads, or a factor of 6.7 slowdown. > > > > > > > > This is with tree RCU. > > > > > > It seems like nearly 2/3 of the cost is here: > > > /* Add the callback to our list. */ > > > *rdp->nxttail[RCU_NEXT_TAIL] = head; <<< > > > rdp->nxttail[RCU_NEXT_TAIL] = &head->next; > > > > Hmmm... That certainly is not the first list of code in call_rcu() that > > would come to mind... > > It's weird. I *think* I read the asm right, but oprofile maybe is > not attributing the cost to the right instruction. > > > > > In loading the pointer to the next tail pointer. If I'm reading the profile > > > correctly. Can't see why that should be a probem though... > > > > The usual diagnosis would be false sharing. > > Hmm that's possible yes. > > > > Hmmm... What is the workload? CPU-bound? If CONFIG_PREEMPT=n, I might > > expect interference from force_quiescent_state(), except that it should > > run only every few clock ticks. So this seems quite unlikely. > > It's CPU bound and preempt=y. > > Workload is just 8 processes running a loop of close(open("file$i")) as > I said though you probably won't be able to reproduce it on a vanilla > kernel. OK, so you are executing call_rcu() a -lot-!!! Could you also please try CONFIG_RCU_TRACE=y, and send me the contents of the files in the "rcu" subdirectory in debugfs? Please take a snapshot of these files, run your test for a fixed time interval (perhaps ten seconds, but please tell me how long), then take a second snapshot. Thanx, Paul > > Could you please try padding the beginning and end of struct rcu_data > > with a few hundred bytes and rerunning? Just in case there is a shared > > per-CPU variable either before or after rcu_data in your memory layout? > > OK I'll try that. > -- 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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