On 7/11/24 15:11, Edgecombe, Rick P wrote: > On Thu, 2024-07-11 at 13:58 -0700, Dave Hansen wrote: >> So we're down to choosing between >> >> * $BYTES space in 'struct fpu' (on hardware supporting CET-S) >> >> or >> >> * ~100 loc >> >> $BYTES is 24, right? Did I get anything wrong? > > Do we know what the actual memory use is? It would increases the size asked of > of the allocator by 24 bytes, but what amount of memory actually gets reserved? > > It is sometimes a slab allocated buffer, and sometimes a vmalloc, right? I'm not > sure about slab sizes, but for vmalloc if the increase doesn't cross a page > size, it will be the same size allocation in reality. Or if it is close to a > page size already, it might use a whole extra 4096 bytes. Man, I hope I don't have this all mixed up in my head. Wouldn't be the first time. I _think_ you might be confusing thread_info and thread_struct, though. I know I've gotten them confused before. But we get to the 'struct fpu' via: current->thread.fpu Where current is a 'task_struct' which is in /proc/slabinfo and 'struct thread_struct thread' and 'struct fpu' are embedded in 'task_struct', not allocated on their own: task_struct 2958 3018 10048 3 8 ... So my current task_struct is 10048 bytes and 3 of them fit in each 8-page slab, leaving 2624 bytes to spare. I don't think we're too dainty about adding thing to task_struct. Are we? > So we might be looking at a situation where some tasks get an entire extra page > allocated per task, and some get no difference. And only the average is 24 bytes > increase. I think you're right here, at least when it comes to large weirdly-sized slabs. But _so_ many things affect task_struct that I've never seen anyone sweat it too much.
