On Mon, Jul 21, 2014 at 2:27 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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. 120 - 300 cycles for me, unless tracing happens, and I'm working on reducing the incidence of tracing. --Andy > -- > 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 -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
