Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754673Ab1ECT3w (ORCPT ); Tue, 3 May 2011 15:29:52 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:36286 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754501Ab1ECT3u (ORCPT ); Tue, 3 May 2011 15:29:50 -0400 Date: Mon, 2 May 2011 23:25:28 -0700 From: "Paul E. McKenney" To: Gleb Natapov Cc: linux-kernel@vger.kernel.org, avi@redhat.com, mtosatti@redhat.com, kvm@vger.kernel.org Subject: Re: [PATCH 1/2] rcu: export rcu_note_context_switch() function Message-ID: <20110503062528.GV2294@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1303984323-3203-1-git-send-email-gleb@redhat.com> <1303984323-3203-2-git-send-email-gleb@redhat.com> <20110429083618.GL2191@linux.vnet.ibm.com> <20110429083904.GA12987@linux.vnet.ibm.com> <20110429180239.GA32566@redhat.com> <20110430125928.GJ2297@linux.vnet.ibm.com> <20110502105612.GJ18376@redhat.com> <20110502133608.GA4197@linux.vnet.ibm.com> <20110502141002.GK18376@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110502141002.GK18376@redhat.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5439 Lines: 131 On Mon, May 02, 2011 at 05:10:03PM +0300, Gleb Natapov wrote: > On Mon, May 02, 2011 at 06:36:08AM -0700, Paul E. McKenney wrote: > > On Mon, May 02, 2011 at 01:56:12PM +0300, Gleb Natapov wrote: > > > On Sat, Apr 30, 2011 at 05:59:28AM -0700, Paul E. McKenney wrote: > > > > On Fri, Apr 29, 2011 at 09:02:39PM +0300, Gleb Natapov wrote: > > > > > On Fri, Apr 29, 2011 at 01:39:04AM -0700, Paul E. McKenney wrote: > > > > > > On Fri, Apr 29, 2011 at 01:36:18AM -0700, Paul E. McKenney wrote: > > > > > > > On Thu, Apr 28, 2011 at 12:52:02PM +0300, Gleb Natapov wrote: > > > > > > > > > > > > > > Hmmm.... This is interesting. KVM being a module, we either expand > > > > > > > TINY_RCU's size a bit by making rcu_note_context_switch() be a real > > > > > > > function in rcutiny.c and adding an export, or we expand it by adding > > > > > > > two exports. > > > > > > > > > > > > > > I would like to solve this without making TINY_RCU larger, and preferably > > > > > > > by making it smaller. Any ideas come to mind? (Other than making > > > > > > > KVM depend on CONFIG_SMP, which sounds too much like throwing out the > > > > > > > baby with the bathwater.) > > > > > > > > > > > > Nothing quite like hitting "send" to make an idea show up... > > > > > > > > > > > > In a UP kernel, does it actually help anything to have KVM > > > > > > tell RCU about executing in a guest? If not, could we have a > > > > > > rcu_note_context_switch_kvm() that is a static inline empty function in > > > > > > TINY_RCU and maps to rcu_note_context_switch() for TREE_RCU? > > > > > > > > > > > That will work, but does making rcu_note_context_switch() out of line > > > > > actually increase kernel size? The function is called in two places > > > > > currently, so by making it out of line we make two calling site smaller. > > > > > Will measure it next week. > > > > > > > > One thing to keep in mind... Calling an out-of-line function from > > > > KVM requires an export, each of which significantly increases TINY_RCU's > > > > memory footprint. > > > > > > > > Thanx, Paul > > > > > > > How significantly? As I wrote in other mail I compiled two TINY_RCU > > > kernel with and without the patch and I didn't see memory footprint > > > increase at all. May be I measure it incorrectly, but what I see is that > > > with out of line function + export text section becomes 64 byte bigger, but > > > data section becomes 64 byte smaller: > > > > > > text data bss dec hex filename > > > 4544134 590596 2023424 7158154 6d398a vmlinux inline > > > 4544198 590532 2023424 7158154 6d398a vmlinux.ol out of line > > > > Did you add the exports that would be needed to allow KVM to call > > the functions in the inline case? > > > Yes, this is with and without patch applied. When patch is applied the > function is out of line and exported. OK, here is what I am suggesting -- create a separate API for virtualization, make it be an empty static inline function for TINY, and make it a wrapper for TREE. This gets rid of the export in the TINY case, and takes advantage of the single-CPU constraint in the TINY case. So this gains the benefit of uninlining rcu_note_context_switch(), but avoids paying the cost of the EXPORT_SYMBOL_GPL(). Then you call rcu_virt_note_context_switch() in place of rcu_note_context_switch() from KVM. Does this make sense? Thanx, Paul ------------------------------------------------------------------------ include/linux/rcutiny.h | 8 ++++++++ include/linux/rcutree.h | 10 ++++++++++ kernel/rcutiny.c | 1 - 3 files changed, 18 insertions(+), 1 deletion(-) diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h index 8e5f7cf..3cc60c0 100644 --- a/include/linux/rcutiny.h +++ b/include/linux/rcutiny.h @@ -96,6 +96,14 @@ static inline int rcu_needs_cpu(int cpu) extern void rcu_note_context_switch(int cpu); /* + * Take advantage of the fact that there is only one CPU, which + * allows us to ignore virtualization-based context switches. + */ +static inline void rcu_virt_note_context_switch(int cpu) +{ +} + +/* * Return the number of grace periods. */ static inline long rcu_batches_completed(void) diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h index 284dad1..e65d066 100644 --- a/include/linux/rcutree.h +++ b/include/linux/rcutree.h @@ -35,6 +35,16 @@ extern void rcu_note_context_switch(int cpu); extern int rcu_needs_cpu(int cpu); extern void rcu_cpu_stall_reset(void); +/* + * Note a virtualization-based context switch. This is simply a + * wrapper around rcu_note_context_switch(), which allows TINY_RCU + * to save a few bytes. + */ +static inline void rcu_virt_note_context_switch(int cpu) +{ + rcu_note_context_switch(cpu); +} + #ifdef CONFIG_TREE_PREEMPT_RCU extern void exit_rcu(void); diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c index 44d6479..8071010 100644 --- a/kernel/rcutiny.c +++ b/kernel/rcutiny.c @@ -83,7 +83,6 @@ void rcu_note_context_switch(int cpu) rcu_sched_qs(cpu); rcu_preempt_note_context_switch(); } -EXPORT_SYMBOL_GPL(rcu_note_context_switch); /* * Helper function for rcu_sched_qs() and rcu_bh_qs(). -- 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/