----- On Feb 26, 2016, at 1:01 PM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote: > On Fri, 26 Feb 2016, Mathieu Desnoyers wrote: >> ----- On Feb 26, 2016, at 11:29 AM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote: >> > Right. There is no point in having two calls and two update mechanisms for a >> > very similar purpose. >> > >> > So let userspace have one struct where cpu/seq and whatever is required for >> > rseq is located and flag at register time which parts of the struct need to be >> > updated. >> >> If we put both cpu/seq/other in that structure, why not plan ahead and make >> it extensible then ? >> >> That looks very much like the "Thread-local ABI" series I posted last year. >> See https://lkml.org/lkml/2015/12/22/464 >> >> Here is why I ended up introducing the specialized "getcpu_cache" system call >> rather than the "generic" system call (quote from the getcpu_cache changelog): >> >> Rationale for the getcpu_cache system call rather than the thread-local >> ABI system call proposed earlier: >> >> Rather than doing a "generic" thread-local ABI, specialize this system >> call for a cpu number cache only. Anyway, the thread-local ABI approach >> would have required that we introduce "feature" flags, which would have >> ended up reimplementing multiplexing of features on top of a system >> call. It seems better to introduce one system call per feature instead. >> >> If everyone end up preferring that we introduce a system call that implements >> many features at once, that's indeed something we can do, but I remember >> being told in the past that this is generally a bad idea. > > It's a bad idea if you mix stuff which does not belong together, but if you > have stuff which shares a substantial amount of things then it makes a lot of > sense. Especially if it adds similar stuff into hotpathes. > >> For one thing, it would make the interface more cumbersome to deal with >> from user-space in terms of feature detection: if we want to make this >> interface extensible, in addition to check -1, errno=ENOSYS, userspace >> would have to deal with a field containing the length of the structure >> as expected by user-space and kernel, and feature flags to see the common >> set of features supported by kernel and user-space. >> >> Having one system call per feature seems simpler to handle in terms of >> feature availability detection from a userspace point of view. > > That might well be, but that does not justify two fastpath updates, two > seperate pointers to handle, etc .... Keeping two separate pointers in the task_struct rather than a single one might indeed be unwelcome, but I'm not sure I fully grasp the fast path argument in this case: getcpu_cache only sets a notifier thread flag on thread migration, whereas AFAIU rseq adds code to context switch and signal delivery, which are prone to have a higher impact. Indeed both will have their own code in the resume notifier, but is it really a fast path ? >From my point of view, making it easy for userspace to just enable getcpu_cache without having the scheduler and signal delivery fast-path overhead of rseq seems like a good thing. I'm not all that sure that saving an extra pointer in task_struct justifies the added system call interface complexity. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- 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
