linux-kernel - Re: [RFC][PATCH] srcu: Implement call

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Message-ID: <4F291B92.8070402@redhat.com>
Date:	Wed, 01 Feb 2012 13:01:38 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
CC:	Peter Zijlstra <peterz@...radead.org>, paulmck@...ux.vnet.ibm.com,
	Oleg Nesterov <oleg@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [RFC][PATCH] srcu: Implement call_srcu()

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

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