On Wed 13-07-16 08:47:51, Dave Hansen wrote: > On 07/13/2016 08:21 AM, Michal Hocko wrote: > >> > This adds a tiny amount of overhead to all pte_none() checks. > >> > I doubt we'll be able to measure it anywhere. > > It would be better to introduce the overhead only for the affected > > cpu models but I guess this is also acceptable. Would it be too > > complicated to use alternatives for that? > > The patch as it stands ends up doing a one-instruction change in > pte_none(). It goes from > > 64c8: 48 85 ff test %rdi,%rdi > > to > > 64a8: 48 f7 c7 9f ff ff ff test $0xffffffffffffff9f,%rdi > > So it essentially eats 4 bytes of icache more than it did before. But, > it's the same number of instructions, and I can't imagine that the CPU > will have any more trouble with a test against an immediate than a test > against 0.
I see. Thanks for the clarification. > We could theoretically do alternatives for this, but we would at *best* > end up with 4 bytes of noops. So, unless the processor likes decoding 4 > noops better than 4 bytes of immediate as part of an instruction, we'll > not win anything. *Plus* the ugliness of the assembly that we'll need > to have the compiler guarantee that the PTE ends up in %rdi. Agreed! -- Michal Hocko SUSE Labs
