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Date:   Thu, 21 Dec 2017 09:06:28 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     rao.shoaib@...cle.com, linux-kernel@...r.kernel.org,
        brouer@...hat.com, linux-mm@...ck.org
Subject: Re: [PATCH] Move kfree_call_rcu() to slab_common.c

On Thu, Dec 21, 2017 at 07:54:34AM -0800, Paul E. McKenney wrote:
> > +/* Queue an RCU callback for lazy invocation after a grace period.
> > + * Currently there is no way of tagging the lazy RCU callbacks in the
> > + * list of pending callbacks. Until then, this function may only be
> > + * called from kfree_call_rcu().
> 
> But now we might have a way.
> 
> If the value in ->func is too small to be a valid function, RCU invokes
> a fixed function name.  This function can then look at ->func and do
> whatever it wants, for example, maintaining an array indexed by the
> ->func value that says what function to call and what else to pass it,
> including for example the slab pointer and offset.
> 
> Thoughts?

Thought 1 is that we can force functions to be quad-byte aligned on all
architectures (gcc option -falign-functions=...), so we can have more
than the 4096 different values we currently use.  We can get 63.5 bits of
information into that ->func argument if we align functions to at least
4 bytes, or 63 if we only force alignment to a 2-byte boundary.  I'm not
sure if we support any architecture other than x86 with byte-aligned
instructions.  (I'm assuming that function descriptors as used on POWER
and ia64 will also be sensibly aligned).

Thought 2 is that the slab is quite capable of getting the slab pointer
from the address of the object -- virt_to_head_page(p)->slab_cache
So sorting objects by address is as good as storing their slab caches
and offsets.

Thought 3 is that we probably don't want to overengineer this.
Just allocating a 14-entry buffer (along with an RCU head) is probably
enough to give us at least 90% of the wins that a more complex solution
would give.

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