From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH, RFC] x86/HVM: batch vCPU wakeups
Date: Wed, 10 Sep 2014 11:37:25 +0100
Message-ID: <541029E5.9020809@citrix.com>
References: <540ED7810200007800032678@mail.emea.novell.com>
	<20140909212925.GC82414@deinos.phlegethon.org>
	<540F8123.7060000@citrix.com>
	<20140910102924.GA44982@deinos.phlegethon.org>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <xen-devel-bounces@lists.xen.org>
Received: from mail6.bemta3.messagelabs.com ([195.245.230.39])
	by lists.xen.org with esmtp (Exim 4.72)
	(envelope-from <Andrew.Cooper3@citrix.com>) id 1XRfHH-0005vx-5t
	for xen-devel@lists.xenproject.org; Wed, 10 Sep 2014 10:37:31 +0000
In-Reply-To: <20140910102924.GA44982@deinos.phlegethon.org>
List-Unsubscribe: <http://lists.xen.org/cgi-bin/mailman/options/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=unsubscribe>
List-Post: <mailto:xen-devel@lists.xen.org>
List-Help: <mailto:xen-devel-request@lists.xen.org?subject=help>
List-Subscribe: <http://lists.xen.org/cgi-bin/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=subscribe>
Sender: xen-devel-bounces@lists.xen.org
Errors-To: xen-devel-bounces@lists.xen.org
To: Tim Deegan <tim@xen.org>
Cc: Ian Campbell <Ian.Campbell@eu.citrix.com>, xen-devel <xen-devel@lists.xenproject.org>, Keir Fraser <keir@xen.org>, Ian Jackson <Ian.Jackson@eu.citrix.com>, Jan Beulich <JBeulich@suse.com>
List-Id: xen-devel@lists.xenproject.org

On 10/09/14 11:29, Tim Deegan wrote:
> At 23:37 +0100 on 09 Sep (1410302243), Andrew Cooper wrote:
>>>> +void cpu_raise_softirq_batch_finish(void)
>>>> +{
>>>> +    unsigned int cpu, this_cpu = smp_processor_id();
>>>> +    cpumask_t *mask = &per_cpu(batch_mask, this_cpu);
>>> Again, this_cpu()?
>> ...But disagree here.  Multiple uses of this_cpu($FOO) cannot be
>> coalesced due to RELOC_HIDE() deliberately preventing optimisation.  For
>> multiple uses, pulling it out by pointer to start with results in rather
>> more efficient code.
> I wasn't questioning the pointer, but to the use of per_cpu(...,
> this_cpu) instead of this_cpu(...).  Both of those involve a
> RELOC_HIDE().
>
> Anyway, it's pretty clear from your and Jan's replies that multiple
> this_cpu() invocations are slower -- thanks for the clarification!
>
> Tim.

The difference (if any) between per_cpu() vs this_cpu() depends on
whether the compiler decides to recalculate smp_processor_id() or not. 
The former is manual optimisation on behalf of the programmer.

I am beginning to wonder whether the use of __attribute__((const)) might
help with get_cpu_info().  Despite the explicit stack pointer reference
which is undoubtedly the source of optimisation confusion for the
compiler, inside a function, the result of get_cpu_info() is genuinely
never going to change, even though the compiler can't necessarily prove
the fact.

~Andrew