From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [RFC][PATCH] srcu: Implement call_srcu()
Date: Wed, 01 Feb 2012 13:01:38 +0200
Message-ID: <4F291B92.8070402@redhat.com>
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Cc: Peter Zijlstra <peterz@infradead.org>, paulmck@linux.vnet.ibm.com,
	Oleg Nesterov <oleg@redhat.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	KVM list <kvm@vger.kernel.org>
To: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <4F291B56.30600@oss.ntt.co.jp>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On 02/01/2012 01:00 PM, Takuya Yoshikawa wrote:
>
>> rcu_assign_pointer), and use atomic operations to copy and clear:
>>
>>    word = bitmap[i]
>>    put_user(word)
>>    atomic_and(&bitmap[i], ~word)
>>
>>
>
> This kind of this was really slow IIRC.
>
>
> How about just doing:
>
> take a spin_lock
> copy the entire (or some portions of) bitmap locally
> clear the bitmap
> unlock
>

That means that vcpus dirtying memory also have to take that lock, and
spin while the bitmap is being copied.  So kvm_vm_ioctl_get_dirty_log()
will become faster, at the expense of vcpus, which I think is a bad
tradeoff.

> write protect the dirty pages based on the copied dirty data
>
> copy_to_user
>
>
>
> I can show you some performance numbers, this weekend, if you like.

That'll be great, numbers are better than speculation.

-- 
error compiling committee.c: too many arguments to function