On Tue, Mar 05, 2019 at 05:32:10PM -0500, Mathieu Desnoyers wrote: > >> * Adaptative mutex improvements > >> > >> I have done a prototype using rseq to implement an adaptative mutex which > >> can detect preemption using a rseq critical section. This ensures the > >> thread doesn't continue to busy-loop after it returns from preemption, and > >> calls sys_futex() instead. This is part of a user-space prototype branch [2], > >> and does not require any kernel change. > > > > I'm still not convinced that is actually the right way to go about > > things. The kernel heuristic is spin while the _owner_ runs, and we > > don't get preempted, obviously. > > > > And the only userspace spinning that makes sense is to cover the cost of > > the syscall. Now Obviously PTI wrecked everything, but before that > > syscalls were actually plenty fast and you didn't need many cmpxchg > > cycles to amortize the syscall itself -- which could then do kernel side > > adaptive spinning (when required). > > Indeed with PTI the system calls are back to their slow self. ;) > > You point about owner is interesting. Perhaps there is one tweak that I > should add in there. We could write the owner thread ID in the lock word. This is already required for PI (and I think robust) futexes. There have been proposals for FUTEX_LOCK and FUTEX_UNLOCK (!PI) primitives that require the same. Waiman had some patches; but I think all went under because 'important' stuff happened. > When trying to grab a lock, one of a few situations can happen: > > - It's unlocked, so we grab it by storing our thread ID, > - It's locked, and we can fetch the CPU number of the thread owning it > if we can access its (struct rseq *)->cpu_id through a lookup using its > thread ID, We can then check whether it's the same CPU we are running on. That might just work with threads (private futexes; which are the majority these these I think), but will obviously not work with regular (shared) futexes. > - If so, we _know_ we should let the owner run, so we call futex right away, > no spinning. We can even boost it for priority inheritance mutexes, > - If it's owned by a thread which was last running on a different CPU, > then it may make sense to actively try to grab the lock by spinning > up to a certain number of loops (which can be either fixed or adaptative). > After that limit, call futex. If preempted while looping, call futex. > > Do you see this as an improvement over what exists today, or am I > on the wrong track ? That's probably better than what they have today. Last time I looked at libc pthread I got really sad -- arguably that was a long time ago, and some of that stuff is because POSIX, but still. Some day we should redesign all that.. futex2 etc.
