On Tue, 27 Mar 2018 12:05:23 -0400 Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote:
> Expose a new system call allowing each thread to register one userspace > memory area to be used as an ABI between kernel and user-space for two > purposes: user-space restartable sequences and quick access to read the > current CPU number value from user-space. What is the *worst* case timing achievable by using the atomics ? What does it do to real time performance requirements ? For cpu_opv you now give an answer but your answer is assuming there isn't another thread actively thrashing the cache or store buffers, and that the user didn't sneakily pass in a page of uncacheable memory (eg framebuffer, or GPU space). I don't see anything that restricts it to cached pages. With that check in place for x86 at least it would probably be ok and I think the sneaky attacks to make it uncacheable would fail becuase you've got the pages locked so trying to give them to an accelerator will block until you are done. I still like the idea it's just the latencies concern me. > Restartable sequences are atomic with respect to preemption > (making it atomic with respect to other threads running on the > same CPU), as well as signal delivery (user-space execution > contexts nested over the same thread). CPU generally means 'big lump with legs on it'. You are not atomic to the same CPU, because that CPU may have 30+ cores with 8 threads per core. It could do with some better terminology (hardware thread, CPU context ?) > In a typical usage scenario, the thread registering the rseq > structure will be performing loads and stores from/to that > structure. It is however also allowed to read that structure > from other threads. The rseq field updates performed by the > kernel provide relaxed atomicity semantics, which guarantee > that other threads performing relaxed atomic reads of the cpu > number cache will always observe a consistent value. So what happens to your API if the kernel atomics get improved ? You are effectively exporting rseq behaviour from private to public. Alan
