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

From: Daniel Wagner <[email-masked]>
Date: Tue, 20 Nov 2012 16:09:22 +0100

On 20.11.2012 15:38, Tejun Heo wrote:
> 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.

Right, I got confused looking at v1 and v2.

> 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.

I see.

>> 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 for the explanation. I was pondering if the new size in power of 
two could be a bit too excessive and the allocation step could be 
linear, e.g. stick at 4096. target_id will increase linear, therefore 
linear increase might also be enough, no?

cheers,
daniel