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Date: Thu, 10 Oct 2013 20:50:32 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
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        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2
Message-ID: <20131010185032.GS13848@laptop.programming.kicks-ass.net>
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On Thu, Oct 10, 2013 at 02:43:27PM -0400, Steven Rostedt wrote:
> On Thu, 10 Oct 2013 11:10:35 -0700
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
>      .. now we can free all the percpu data and kill the CPU ..
> > 
> > without any locking anywhere - not stop-machine, not anything. If
> > somebody is doing a "for_each_cpu()" (under just a regular
> > rcu_read_lock()) and they see the bit set while it's going down, who
> > cares? The CPU is still there, the data is accessible..
> 
> The problem is that rcu_read_lock() requires preemption disabled unless
> you are using the preemptable rcu tree version. There's always
> srcu_read_lock() but that's not so free. It's basically the same as
> what Peter is doing.

No, srcu is actually more expensive in the fast path. Although possibly
we could make SRCU more complex ;-)

> There's places in the kernel that does for_each_cpu() that I'm sure you
> don't want to disable preemption for. Especially when you start having
> 4096 CPU machines!

:-)