On Mon, 21 Jul 2014, Peter Zijlstra wrote: > On Mon, Jul 21, 2014 at 10:16:37PM +0200, Thomas Gleixner wrote: > > On Mon, 21 Jul 2014, Darren Hart wrote: > > > We observed some significant improvements under some very specific use > > > cases, but a more thorough dive into performance impact in the other cases > > > as well as security implications with the vdso is still wanting. > > > > The security implication is that the feature can only be available for > > process private futexes. There is no way to expose information which > > crosses the process spaces. > > > > But the way worse issue is storage. > > > > While you can cache the namespace specific TID of a thread in the > > task_struct, you still need a O(1) zero overhead mechanism to update > > the thread state (only on/off cpu is interesting) in a per process > > shared data structure from the guts of schedule() > > > > For that you have basically two choices: > > > > 1) cpu_thread_id[NR_CPUS] > > > > Simple to update from the scheduler, and a halfways moderate > > storage size (NR_CPUS * 4 bytes) in the worst case, i.e. 16k > > today. Set to 0 on scheduling out and to the namespace specific TID > > on scheduling in. > > > > But that requires a linear search in the user space spin loop. And > > that's required for every iteration of the loop. Can you imagine > > how well that works performance wise? > > > > 2) Bitmap threads_on_cpu > > > > Again, simple to update from the scheduler, cache line bouncing > > issues aside. Clear the bit on schedule out and set it on schedule > > in. > > > > But the bitmap needs the size of PID_MAX_LIMIT, which is a whopping > > 512k per process in the worst case. > > > > Anything else would involve search/lookup schemes which are just > > overkill in both the scheduler and the user space loop. > > > > Now for enhanced fun you need immutable pages for that storage, as you > > can't have pagefaults in the guts of schedule(). > > > > So once you found a way to make that opt-in as you don't want inflict > > any of this to all processes by default, it might be a worthwhile > > optimization. So the probably tolerable impact on schedule() would be > > > > schedule_out() > > if (curr->threads_on_cpu) > > clear_bit(curr->ns_tid, curr->threads_on_cpu); > > and > > > > schedule_in() > > if (curr->threads_on_cpu) > > clear_bit(curr->ns_tid, curr->threads_on_cpu); > > > > Anything more complex is just going to defeat the whole purpose. > > All this is predicated on the fact that syscalls are 'expensive'. > Weren't syscalls only 100s of cycles? All this bitmap mucking is far > more expensive due to cacheline misses, which due to the size of the > things is almost guaranteed. I completely agree. This wants to backed by proper numbers taken from a proper implementation and not from some randomly cobbled together works for me hackery. As I said: It might be a worthwhile optimization.... Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
