On Thu, Dec 21, 2017 at 05:27:41PM -0800, Paul E. McKenney wrote: > On Thu, Dec 21, 2017 at 09:06:28AM -0800, Matthew Wilcox wrote: > > 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). > > I do like this approach, especially should some additional subsystems > need this sort of special handling from RCU. It is also much faster > to demultiplex than alternative schemes based on address ranges and > the like. Oh, and having four-byte alignment would allow making laziness orthogonal to special handling, which should improve energy efficiency of callback handling by allowing normal call_rcu() callbacks to invoke laziness. (And would require renaming the call_rcu_lazy() API yet again, sorry Rao!) Thanx, Paul > How many bits are required by slab? Would ~56 bits (less the bottom > bit pattern reserved for function pointers) suffice on 64-bit systems > and ~24 bits on 32-bit systems? That would allow up to 256 specially > handled situations, which should be enough. (Famous last words!) > > > 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. > > Different slabs can in some cases interleave their slabs of objects, > right? It might well be that grouping together different slabs from > the same slab cache doesn't help, but seems worth my asking the question. > > > 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. > > Can we benchmark this? After all, memory allocation can sometimes > counter one's intuition. > > One alternative approach would be to allocate such a buffer per > slab cache, and run each slab caches through RCU independently. > Seems like this should allow some savings. Might not be worthwhile, > but again seemed worth asking the question. > > Thanx, Paul -- 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>
