Dynamically allocating each one is possible but not very scalable.
The question is if there is some way, we can do this with on-stack
or a single on-heap rcu_head or equivalent that can achieve the same
effect.
If the hctx structures are guaranteed to stay put, you could count
them and then do a single allocation of an array of rcu_head structures
(or some larger structure containing an rcu_head structure, if needed).
You could then sequence through this array, consuming one rcu_head per
hctx as you processed it. Once all the callbacks had been invoked,
it would be safe to free the array.
Sounds too simple, though. So what am I missing?
We don't want higher-order allocations...
So:
(1) We don't want to embed the struct in the hctx because we allocate
so many of them that this is non-negligable to add for something we
typically never use.
(2) We don't want to allocate dynamically because it's potentially
huge.
As long as we're using srcu for blocking hctx's, I think it's "pick your
poison".
Alternatively, Ming's percpu_ref patch(*) may be worth a look.
* https://www.spinics.net/lists/linux-block/msg56976.html1
I'm not opposed to having this. Will require some more testing
as this affects pretty much every driver out there..
If we are going with a lightweight percpu_ref, can we just do
it also for non-blocking hctx and have a single code-path?
