----- On Aug 30, 2017, at 10:46 AM, Paul E. McKenney [email protected] wrote:
> On Tue, Aug 29, 2017 at 10:01:56PM -0700, Andy Lutomirski wrote: >> > On Aug 27, 2017, at 8:05 PM, Mathieu Desnoyers >> > <[email protected]> >> > wrote: >> > >> > ----- On Aug 27, 2017, at 3:53 PM, Andy Lutomirski [email protected] >> > wrote: >> > >> >>> On Aug 27, 2017, at 1:50 PM, Mathieu Desnoyers >> >>> <[email protected]> >> >>> wrote: >> >>> >> >>> Add a new MEMBARRIER_CMD_REGISTER_SYNC_CORE command to the membarrier >> >>> system call. It allows processes to register their intent to have their >> >>> threads issue core serializing barriers in addition to memory barriers >> >>> whenever a membarrier command is performed. >> >>> >> >> >> >> Why is this stateful? That is, why not just have a new membarrier >> >> command to >> >> sync every thread's icache? >> > >> > If we'd do it on every CPU icache, it would be as trivial as you say. The >> > concern here is sending IPIs only to CPUs running threads that belong to >> > the >> > same process, so we don't disturb unrelated processes. >> > >> > If we could just grab each CPU's runqueue lock, it would be fairly simple >> > to do. But we want to avoid hitting each runqueue with exclusive atomic >> > access associated with grabbing the lock. (cache-line bouncing) >> >> Hmm. Are there really arches where there is no clean implementation >> without this hacker? It seems rather unfortunate that munmap() can be >> done efficiently but this barrier can't be. >> >> At the very least, could there be a register command *and* a special >> sync command? I dislike the idea that the sync command does something >> different depending on some other state. Even better (IMO) would be a >> design where you ask for an isync and, if the arch can do it >> efficiently (x86), you get an efficient isync and, if the arch can't >> (arm64?) you take all the rq locks? > > In some cases I suspect that IPIs might be required. Regardless of > that, we might well need to provide a way for architectures to do > special things. > > But I must defer to Mathieu on this. Yes, I think you are both correct. It's better to expose a new command for code sync, so architectures have more freedom in how they choose to implement it. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
