Kernel RCU: shrink the size of the struct rcu_head

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

Hi Paul,

I noticed that you already discussed the possibility of shrinking the
struct rcu_head by removing the function pointer.
(http://kernel.org/pub/linux/kernel/people/paulmck/rcutodo.html)

The ideas brought in so far require having per-callback lists, which
involves a bit of management overhead and don't permit keeping the
call_rcu() in cpu order.

You might want to look into the Userspace RCU urcu-defer.c
implementation, where I perform pointer encoding to compact the usual
case, expected to be the same callback passed as parameter multiple
times in a row to call_rcu(). This is very typical with multiple free()
calls for different data structures next to each other.

This typically keeps the size of the information to encode per callback
down to a minimum: the size of a single pointer. It would be good to
trace the kernel usage of call_rcu() to see if my assumption holds.

I just thought I should tell you before you start looking at this
issue further.

Thanks,

Mathieu

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Mathieu Desnoyers
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