linux-kernel - Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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Date:	Wed, 12 Dec 2012 19:02:48 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
	amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU
	offline from atomic context

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.

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