Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758048AbYGOOl7 (ORCPT ); Tue, 15 Jul 2008 10:41:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752424AbYGOOlv (ORCPT ); Tue, 15 Jul 2008 10:41:51 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:43163 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751692AbYGOOlu (ORCPT ); Tue, 15 Jul 2008 10:41:50 -0400 Subject: Re: [patch 01/15] Kernel Tracepoints From: Peter Zijlstra To: Mathieu Desnoyers Cc: akpm@linux-foundation.org, Ingo Molnar , linux-kernel@vger.kernel.org, Masami Hiramatsu , "Frank Ch. Eigler" , Hideo AOKI , Takashi Nishiie , Steven Rostedt , Alexander Viro , Eduard - Gabriel Munteanu , Paul E McKenney In-Reply-To: <20080715142710.GC20037@Krystal> References: <20080709145929.352201601@polymtl.ca> <20080709150043.693920317@polymtl.ca> <1216108237.12595.122.camel@twins> <20080715132543.GB20037@Krystal> <1216130356.12595.184.camel@twins> <20080715142710.GC20037@Krystal> Content-Type: text/plain Date: Tue, 15 Jul 2008 16:42:08 +0200 Message-Id: <1216132928.12595.201.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.22.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5092 Lines: 131 On Tue, 2008-07-15 at 10:27 -0400, Mathieu Desnoyers wrote: > * Peter Zijlstra (peterz@infradead.org) wrote: > > On Tue, 2008-07-15 at 09:25 -0400, Mathieu Desnoyers wrote: > > > * Peter Zijlstra (peterz@infradead.org) wrote: > > > > On Wed, 2008-07-09 at 10:59 -0400, Mathieu Desnoyers wrote: > > > > > > > +#define __DO_TRACE(tp, proto, args) \ > > > > > + do { \ > > > > > + int i; \ > > > > > + void **funcs; \ > > > > > + preempt_disable(); \ > > > > > + funcs = (tp)->funcs; \ > > > > > + smp_read_barrier_depends(); \ > > > > > + if (funcs) { \ > > > > > + for (i = 0; funcs[i]; i++) { \ > > > > > > > > can't you get rid of 'i' and write: > > > > > > > > void **func; > > > > > > > > preempt_disable(); > > > > func = (tp)->funcs; > > > > smp_read_barrier_depends(); > > > > for (; func; func++) > > > > ((void (*)(proto))func)(args); > > > > preempt_enable(); > > > > > > > > > > Yes, I though there would be an optimization to do here, I'll use your > > > proposal. This code snippet is especially important since it will > > > generate instructions near every tracepoint side. Saving a few bytes > > > becomes important. > > > > > > Given that (tp)->funcs references an array of function pointers and that > > > it can be NULL, the if (funcs) test must still be there and we must use > > > > > > #define __DO_TRACE(tp, proto, args) \ > > > do { \ > > > void *func; \ > > > \ > > > preempt_disable(); \ > > > if ((tp)->funcs) { \ > > > func = rcu_dereference((tp)->funcs); \ > > > for (; func; func++) { \ > > > ((void(*)(proto))(func))(args); \ > > > } \ > > > } \ > > > preempt_enable(); \ > > > } while (0) > > > > > > > > > The resulting assembly is a bit more dense than my previous > > > implementation, which is good : > > > > My version also has that if ((tp)->funcs), but its hidden in the > > for (; func; func++) loop. The only thing your version does is an extra > > test of tp->funcs but without read depends barrier - not sure if that is > > ok. > > > > Hrm, you are right, the implementation I just proposed is bogus. (but so > was yours) ;) > > func is an iterator on the funcs array. My typing of func is thus wrong, > it should be void **. Otherwise I'm just incrementing the function > address which is plain wrong. > > The read barrier is included in rcu_dereference() now. But given that we > have to take a pointer to the array as an iterator, we would have to > rcu_dereference() our iterator multiple times and then have many read > barrier depends, which we don't need. This is why I would go back to a > smp_read_barrier_depends(). > > Also, I use a NULL entry at the end of the funcs array as an end of > array identifier. However, I cannot use this in the for loop both as a > check for NULL array and check for NULL array element. This is why a if > () test is needed in addition to the for loop test. (this is actually > what is wrong in the implementation you proposed : you treat func both > as a pointer to the function pointer array and as a function pointer) Ah, D'0h! Indeed. > Something like this seems better : > > #define __DO_TRACE(tp, proto, args) \ > do { \ > void **it_func; \ > \ > preempt_disable(); \ > it_func = (tp)->funcs; \ > if (it_func) { \ > smp_read_barrier_depends(); \ > for (; *it_func; it_func++) \ > ((void(*)(proto))(*it_func))(args); \ > } \ > preempt_enable(); \ > } while (0) > > What do you think ? I'm confused by the barrier games here. Why not: void **it_func; preempt_disable(); it_func = rcu_dereference((tp)->funcs); if (it_func) { for (; *it_func; it_func++) ((void(*)(proto))(*it_func))(args); } preempt_enable(); That is, why can we skip the barrier when !it_func? is that because at that time we don't actually dereference it_func and therefore cannot observe stale data? If so, does this really matter since we're already in an unlikely section? Again, if so, this deserves a comment ;-) [ still think those preempt_* calls should be called rcu_read_sched_lock() or such. ] Anyway, does this still generate better code? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/