netdev - Re: [PATCH 4/7] netprio_cgroup: reimplement priomap expansion

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Date:	Tue, 20 Nov 2012 06:38:32 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Daniel Wagner <wagi@...om.org>
Cc:	serge.hallyn@...onical.com, ebiederm@...ssion.com,
	nhorman@...driver.com, tgraf@...g.ch, davem@...emloft.net,
	lizefan@...wei.com, cgroups@...r.kernel.org,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 4/7] netprio_cgroup: reimplement priomap expansion

Hello, Daniel.

On Tue, Nov 20, 2012 at 09:46:22AM +0100, Daniel Wagner wrote:
> struct netprio_map {
> 	struct rcu_head rcu;
> 	struct netprio_aux *aux;	/* auxiliary config array */
> 	u32 priomap_len;
> 	u32 priomap[];
> };
> 
> Is there a specific reason why aux and priomap is handled
> differently? Couldn't you just use same approach for both variables,
> e.g. re/allocating only them here and leave the allocation struct
> netprio_map in cgrp_css_alloc()?

->aux is no longer added, so the consistency issue doesn't exist
anymore.  The reason why they were handled differently before (or
rather why I didn't change priomap[] to be allocated separately) was
that pointer chasing tends to be more expensive than offsetting.  I
don't know how much effect it would have in this case but things
sitting in packet in/out paths can be very hot so didn't wanna disturb
it.

> Also the algorithm to figure out the size of the array might be a
> bit too aggressive in my opinion. So you always start at
> PRIOMAP_MIN_SIZE and then try to double the size until target_idx
> fits. Wouldn't it make sense to start to look for the new size
> beginning at old->priomap_len and then do the power-of-two increase?

The only downside of always starting from PRIOMAP_MIN_SIZE is
iterating several more times in the sizing loop which isn't really
anything to worry about.  The loop is structured that way because I
wanted to keep the size of the whole thing power-of-two.  Due to the
fields before priomap[], if we size priomap_len power-of-two, we'll
always end up with something slightly over power-of-two, which is
usually the worst size to allocate.

Thanks.

-- 
tejun
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