From mboxrd@z Thu Jan  1 00:00:00 1970
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU
	offline from atomic context
Date: Wed, 12 Dec 2012 19:02:48 +0100
Message-ID: <20121212180248.GA24882@redhat.com>
References: <20121211140314.23621.64088.stgit@srivatsabhat.in.ibm.com> <20121211140358.23621.97011.stgit@srivatsabhat.in.ibm.com> <20121212171720.GA22289@redhat.com> <50C8C4A5.4080104@linux.vnet.ibm.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Return-path: <linux-pm-owner@vger.kernel.org>
Received: from mx1.redhat.com ([209.132.183.28]:62170 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1752257Ab2LLSCs (ORCPT <rfc822;linux-pm@vger.kernel.org>);
	Wed, 12 Dec 2012 13:02:48 -0500
Content-Disposition: inline
In-Reply-To: <50C8C4A5.4080104@linux.vnet.ibm.com>
Sender: linux-pm-owner@vger.kernel.org
List-Id: linux-pm@vger.kernel.org
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: tglx@linutronix.de, peterz@infradead.org, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org, vincent.guittot@linaro.org, tj@kernel.org, sbw@mit.edu, amit.kucheria@linaro.org, rostedt@goodmis.org, rjw@sisk.pl, wangyun@linux.vnet.ibm.com, xiaoguangrong@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org

On 12/12, Srivatsa S. Bhat wrote:
>
> On 12/12/2012 10:47 PM, Oleg Nesterov wrote:
> >
> > Why it needs to be per-cpu? It can be global and __read_mostly to avoid
> > the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal
> > into a single cacheline...
>
> Even I realized this (that we could use a global) after posting out the
> series.. But do you think that it would be better to retain the per-cpu
> variant itself, due to the cache effects?

I don't really know, up to you. This was the question ;)

> > Do we really need local_irq_save/restore in put_ ?
> >
>
> Hmm.. good point! I don't think we need it.

And _perhaps_ get_ can avoid it too?

I didn't really try to think, probably this is not right, but can't
something like this work?

	#define XXXX	(1 << 16)
	#define MASK	(XXXX -1)

	void get_online_cpus_atomic(void)
	{
		preempt_disable();

		// only for writer
		__this_cpu_add(reader_percpu_refcnt, XXXX);

		if (__this_cpu_read(reader_percpu_refcnt) & MASK) {
			__this_cpu_inc(reader_percpu_refcnt);
		} else {
			smp_wmb();
			if (writer_active()) {
				...
			}
		}

		__this_cpu_dec(reader_percpu_refcnt, XXXX);
	}

	void put_online_cpus_atomic(void)
	{
		if (__this_cpu_read(reader_percpu_refcnt) & MASK)
			__this_cpu_dec(reader_percpu_refcnt);
		else
			read_unlock(&hotplug_rwlock);
		preempt_enable();
	}

Oleg.