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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
