linux-kernel - Re: [RFC, PATCH] kernel/rcu: add kfree

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Date:	Sun, 04 Jan 2009 13:22:00 +0100
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
CC:	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org
Subject: Re: [RFC, PATCH] kernel/rcu: add kfree_rcu

Lai Jiangshan wrote:
> I have not posted it. -:)
>   
Could you post it?

Paul: What would break if we stop processing rcu entries in (cpu) order?

The head->func(head) in rcu_do_batch() is probably a nightmare for the 
branch target predictor.

What about:
- shrinking struct rcu_head to just a pointer (let's start with the goodie)
- Adding a register_rcu_callback() function.
It allocates the per-cpu storage for the rcu grace period lists.
Seperate lists for each registered callback - thus no need to copy the 
callback target into each rcu_head structure.
It returns a pointer/handle to these lists.
- call_rcu gets that handle instead of the plain function pointer.
- rcu_do_batch enumerates all registered callbacks. Thus first all 
callback_struct->func(head) calls for the first registered callback, 
then the calls for the 2nd callback, etc.
Better for the icache, better for the branch predictor.

Paul: Do you have a test case that is suitable for benchmarking rcu?
Any workloads were rcu appears significantly in oprofile?
And: Do you know how many rcu entries are typically alive? How much 
memory is used for the function pointers?

--
    Manfred

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