On Wed, Dec 11, 2013 at 03:08:35PM -0800, H. Peter Anvin wrote: > On 12/11/2013 09:50 AM, Ingo Molnar wrote: > > > > Well, availability could be a problem too, if some CPU (real or > > virtual) implements MWAIT but not CLFLUSH. > > > > In theory we could make mwait an alternatives variant and patch in the > > right combination of instructions? The CLFLUSH goes to the same > > address as on which the monitoring happens, so it could be considered > > one meta-instruction. > > > > The first thing to do is probably to drop the use of thread_info as a > wakeup doorbell. It seemed like a good idea at the time -- after all, > there is one for each thread -- but it is extremely likely to be dirty > in the cache, which is (presumably) what causes these kinds of bugs to > be maximally likely. Even if we don't do the CLFLUSH it is likely that > the hardware has to do something expensive behind the scenes. > > So I would like to propose that we switch to using a percpu variable > which is a single cache line of nothing at all. It would only ever be > touched by MONITOR and for explicit wakeup. Hopefully that will resolve > this problem without the need for the CLFLUSH.
The reason we use thread_info::flags is because we need to write TIF_NEED_RESCHED into it to wake up anyhow. Using another cacheline would mean the wakeup path would need to write a second cross cpu cacheline -- that is badness too. So no, I don't think we want to listen to another line. -- 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/
