On Thu, May 3, 2018 at 9:48 AM Joel Fernandes <joelaf@xxxxxxxxxx> wrote: > > > can skip the manual schedule we were going to perform. > > By the way, if we eventually find a way to enhance user-space mutexes in > the > > fashion you describe here, it would belong to another TLS area, and would > > be registered by another system call than rseq. I proposed a more generic > Right. Also I still don't see any good reason why optimistic spinning in > the kernel with FUTEX_LOCK, as Peter described, can't be used instead of > using the rseq implementation and spinning in userspace, for such a case. I > don't really fully buy that we need to design this interface assuming any > privilege transition level time. > If privilege level transitions are slow, > we're going to have bad performance anyway. That's not the case. There's a large class of program that does useful work while seldom entering the kernel: just ask the user-space network stack people. It's not wise to design interfaces around system calls being cheap. Even if system calls are currently cheap enough on some architectures some of the time, there's no guarantee that they'll stay that way, especially relative to straight-line user-mode execution. A pure user-space approach, on the other hand, involves no work in the kernel, and doing nothing is always the optimal strategy. Besides, there are environments where system calls end up being more expensive than you might think: consider strace or rr. If the kernel needs to get involved on some path, it's best that its involvement be as light as possible. > we should really stick to using FUTEX_LOCK and > reuse all the work that went into that area for Android and otherwise (and > work with Waiman and others on improving that if there are any problems > with it). FUTEX_LOCK is a return to the bad old days when systems gave you a fixed list of synchronization primitives and if you wanted something else, tough. That the latest version of the FUTEX_LOCK patch includes a separate FUTEX_LOCK_SHARED mode is concerning. The functionality the kernel provides to userspace should be more general-purpose and allow more experimentation without changes in the kernel. I see no reason to force userspace into 1) reserving 30 bits of its lockword for a TID and 2) adopting the kernel's idea of spin time heuristics and lock stealing when the same basic functionality can be provided in a generic way while reserving only one bit. That this mechanism happens to be more efficient as well is a bonus. "Mechanism not policy" is still a good design principle. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
