Re: [PATCH 23/32] Generic dynamic per cpu refcounting

[Kent Overstreet]

On Thu, Jan 24, 2013 at 06:09:43PM -0800, Tejun Heo wrote:
> Hello, again.
> 
> On Thu, Jan 24, 2013 at 06:03:40PM -0800, Tejun Heo wrote:
> > Yeah, if we're aiming to replace refcnts in file and kobj, dynamic
> > alloc may be justified.  Hopefully, the accounting necessary to decide
> > whethre to use percpu isn't too burdensome.
> 
> Ooh, I forgot one thing.  We might not gain much by replacing file
> refcnt w/ this.  You can't really get cheaper than fget_light().

I've seen fget() show up when profiling the aio code - it's not high
enough to be a big concern when not doing stupid stuff, but high enough
that making it percpu would be worth it if it was easy. Which it's not,
for plenty of reasons.

> Also, while kobj destruction can still be considered an infrequent
> operation, file destruction is not and people will get mighty unhappy
> if you do synchronize_sched/rcu() from fput() synchronously.
> 
> So, I'm now back to "do we need dynamic allocation".  What else do we
> have to convert?

I dunno. There's a lot of random refcounts scattered around, though.

The way I see it, if it's always percpu when joe random dev needs a
refcount, he's going to weigh whether the overhead of a percpu refcount
is worth it.

With dynamic allocation, it's 16 bytes if you don't need it to be
percpu, vs. 4 for an atomic_t - so you never need to think about it, you
can just always use this for your refcounts and never have to think
about if it's going to be a fast path thing or not.

But I really liked your idea for making dynamic allocation use a pool
that's refilled from a workqueue, then I can keep dynamic allocation
without contorting the api.
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