Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753787AbZKVKxH (ORCPT ); Sun, 22 Nov 2009 05:53:07 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752965AbZKVKxG (ORCPT ); Sun, 22 Nov 2009 05:53:06 -0500 Received: from mail-vw0-f192.google.com ([209.85.212.192]:52106 "EHLO mail-vw0-f192.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752726AbZKVKxF convert rfc822-to-8bit (ORCPT ); Sun, 22 Nov 2009 05:53:05 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc:content-type:content-transfer-encoding; b=APHwND50/h1eCbG+NyPq6Igm+A0W2RXxNB34Y1RaQuLxVHgRSEGMJi2Bs3iJXhHlSt t8KTQms3vrurUH/GD+2qy2BiNu/919aQkqC96mlhTBAl2ENJ3AfOPk7rrQ+fseNKSgZH hc21qAbSlSrZPMIwau5tog6wzN1ECnm7r6BpI= MIME-Version: 1.0 In-Reply-To: <84144f020911220117p5c4720e0g58587b97efdbb46b@mail.gmail.com> References: <80d3c5c680300de7ebd41aba89723a5cf45396ed.1258783305.git.andre.goddard@gmail.com> <84144f020911220117p5c4720e0g58587b97efdbb46b@mail.gmail.com> From: =?ISO-8859-1?Q?Andr=E9_Goddard_Rosa?= Date: Sun, 22 Nov 2009 08:52:50 -0200 Message-ID: Subject: Re: [PATCH] pid: tighten pidmap_lock critical section To: Pekka Enberg Cc: Andrew Morton , Catalin Marinas , Oleg Nesterov , Jiri Kosina , linux-kernel@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5037 Lines: 124 Hi, Pekka! On Sun, Nov 22, 2009 at 7:17 AM, Pekka Enberg wrote: > Hi Andre, > > On Sat, Nov 21, 2009 at 8:04 AM, Andr? Goddard Rosa > wrote: >> Avoid calling kfree() under pidmap_lock and doing unnecessary work. >> It doesn't change behavior. >> >> It decreases code size by 16 bytes on my gcc 4.4.1 on Core 2: >> ? text ? ?data ? ? bss ? ? dec ? ? hex filename >> ? 4314 ? ?2216 ? ? ? 8 ? ?6538 ? ?198a kernel/pid.o-BEFORE >> ? 4298 ? ?2216 ? ? ? 8 ? ?6522 ? ?197a kernel/pid.o-AFTER >> >> Signed-off-by: Andr? Goddard Rosa > > This patch is doing a lot more than the changelog above says it does. > What exactly is the purpose of this patch? What's the upside? Purpose is to make the spinlock critical section tighter by removing unnecessary instructions from under pidmap_lock. I was getting to learn about pid.c and noticed a slightly decrease in the amount of work done with the spinlock held by checking the generated assembly before/after the changes. So I had a question: while these are very small changes, they make the code under the critical section smaller, coming at a slightly decrease in legibility (initializing variables outside the lock), but still not complex compared to other kernel code. In all kernel code I can see postponing assignments until the time it's really necessary to do it. So I thought that perhaps anticipating the assignment to make it just outside of the critical section could make a small improvement in the cases where code was contending for that lock because the critical section would be smaller by a small bit, but still. >> --- >> ?kernel/pid.c | ? 16 ++++++++-------- >> ?1 files changed, 8 insertions(+), 8 deletions(-) >> >> diff --git a/kernel/pid.c b/kernel/pid.c >> index d3f722d..ec06912 100644 >> --- a/kernel/pid.c >> +++ b/kernel/pid.c >> @@ -141,11 +141,12 @@ static int alloc_pidmap(struct pid_namespace *pid_ns) >> ? ? ? ? ? ? ? ? ? ? ? ? * installing it: >> ? ? ? ? ? ? ? ? ? ? ? ? */ >> ? ? ? ? ? ? ? ? ? ? ? ?spin_lock_irq(&pidmap_lock); >> - ? ? ? ? ? ? ? ? ? ? ? if (map->page) >> - ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? kfree(page); >> - ? ? ? ? ? ? ? ? ? ? ? else >> + ? ? ? ? ? ? ? ? ? ? ? if (!map->page) { >> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?map->page = page; >> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? page = NULL; >> + ? ? ? ? ? ? ? ? ? ? ? } >> ? ? ? ? ? ? ? ? ? ? ? ?spin_unlock_irq(&pidmap_lock); >> + ? ? ? ? ? ? ? ? ? ? ? kfree(page); > > OK, maybe. The upside seem rather small and the resulting code is IMHO > slightly less readable. Motivation is that normally I don't see many other places in the kernel where allocation/release of memory is made under spinlocks. In fact there's no need why that page is freed (somewhat complex operation) under the spinlock, so I realized that it could be postponed to just after releasing the lock, which seemed a good idea. >> ? ? ? ? ? ? ? ? ? ? ? ?if (unlikely(!map->page)) >> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?break; >> ? ? ? ? ? ? ? ?} >> @@ -225,11 +226,11 @@ static void delayed_put_pid(struct rcu_head *rhp) >> ?void free_pid(struct pid *pid) >> ?{ >> ? ? ? ?/* We can be called with write_lock_irq(&tasklist_lock) held */ >> - ? ? ? int i; >> + ? ? ? int i = 0; >> ? ? ? ?unsigned long flags; >> >> ? ? ? ?spin_lock_irqsave(&pidmap_lock, flags); >> - ? ? ? for (i = 0; i <= pid->level; i++) >> + ? ? ? for ( ; i <= pid->level; i++) >> ? ? ? ? ? ? ? ?hlist_del_rcu(&pid->numbers[i].pid_chain); >> ? ? ? ?spin_unlock_irqrestore(&pidmap_lock, flags); > > This has nothing to do with kfree(). AFAICT, it just obfuscates the > code as the initial assignment to zero is lost in the noise anyway. See comments above. If you really thinks so but agree with the other explanation, I can remove this part. >> @@ -268,12 +269,11 @@ struct pid *alloc_pid(struct pid_namespace *ns) >> ? ? ? ?for (type = 0; type < PIDTYPE_MAX; ++type) >> ? ? ? ? ? ? ? ?INIT_HLIST_HEAD(&pid->tasks[type]); >> >> + ? ? ? upid = pid->numbers + ns->level; >> ? ? ? ?spin_lock_irq(&pidmap_lock); >> - ? ? ? for (i = ns->level; i >= 0; i--) { >> - ? ? ? ? ? ? ? upid = &pid->numbers[i]; >> + ? ? ? for ( ; upid >= pid->numbers; --upid) >> ? ? ? ? ? ? ? ?hlist_add_head_rcu(&upid->pid_chain, >> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?&pid_hash[pid_hashfn(upid->nr, upid->ns)]); >> - ? ? ? } >> ? ? ? ?spin_unlock_irq(&pidmap_lock); > > Again, this has nothing to do with kfree(). I suspect this is where > most of the 16 byte text savings come from. I'm not convinced it's > worth the hit in readability, though. Yes, you're right, this is where the size reduction comes indeed. As you can see, it's a trade-off, but while kernel keeps getting bigger, there's still possibility to make it smaller sometimes. Thanks for your feedback, Andr? -- 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/