linux-kernel - Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu

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Date:	Thu, 24 Aug 2006 13:14:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	rusty@...tcorp.com.au, torvalds@...l.org, akpm@...l.org,
	linux-kernel@...r.kernel.org, arjan@...el.linux.com,
	davej@...hat.com, vatsa@...ibm.com, dipankar@...ibm.com,
	ashok.raj@...el.com
Subject: Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu_hotplug "locking"

* Gautham R Shenoy <ego@...ibm.com> wrote:

>  void lock_cpu_hotplug(void)
>  {

> +	DECLARE_WAITQUEUE(wait, current);
> +	spin_lock(&cpu_hotplug.lock);
> +	cpu_hotplug.reader_count++;

this should be per-CPU - lock_cpu_hotplug() should _not_ be a globally 
synchronized event.

CPU removal is such a rare event that we can easily do something like a 
global read-mostly 'CPU is locked for writes' flag (plus a completion 
queue) that the 'write' side takes atomically - combined with per-CPU 
refcount and a waitqueue that the read side increases/decreases and 
wakes. Read-locking of the CPU is much more common and should be 
fundamentally scalable: it should increase the per-CPU refcount, then 
check the global 'writer active' flag, and if the writer flag is set, it 
should wait on the global completion queue. When a reader drops the 
refcount it should wake up the per-CPU waitqueue. [in which a writer 
might be waiting for the refcount to go down to 0.]

	Ingo
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