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
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