On 05/01/2015 02:40 PM, Ingo Molnar wrote: > Or we could do that in the syscall path with a single store of a > constant flag to a location in the task struct. We have a number of > natural flags that get written on syscall entry, such as: > > pushq_cfi $__USER_DS /* pt_regs->ss */ > > That goes to a constant location on the kernel stack. On return from > system calls we could write 0 to that location. > > So the remote CPU would have to do a read of this location. There are > two cases: > > - If it's 0, then it has observed quiescent state on that CPU. (It > does not have to be atomics anymore, as we'd only observe the value > and MESI coherency takes care of it.)
That should do the trick. > - If it's not 0 then the remote CPU is not executing user-space code > and we can install (remotely) a TIF_NOHZ flag in it and expect it > to process it either on return to user-space or on a context > switch. I may have to think about this a little more, but it seems like it should work. Can we use a separate byte in the flags word for flags that can get set remotely, so we can do stores and clearing of local-only flags without atomic instructions? > This way, unless I'm missing something, reduces the overhead to a > single store to a hot cacheline on return-to-userspace - which > instruction if we place it well might as well be close to zero cost. > No syscall entry cost. Slow-return cost only in the (rare) case of > someone using synchronize_rcu(). I think that should take care of the RCU aspect of nohz_full. -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
