----- On Feb 26, 2016, at 11:29 AM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote: > On Fri, 26 Feb 2016, Peter Zijlstra wrote: >> On Thu, Feb 25, 2016 at 05:17:51PM +0000, Mathieu Desnoyers wrote: >> > ----- On Feb 25, 2016, at 12:04 PM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote: >> > >> > > On Thu, Feb 25, 2016 at 04:55:26PM +0000, Mathieu Desnoyers wrote: >> > >> ----- On Feb 25, 2016, at 4:56 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote: >> > >> The restartable sequences are intrinsically designed to work >> > >> on per-cpu data, so they need to fetch the current CPU number >> > >> within the rseq critical section. This is where the getcpu_cache >> > >> system call becomes very useful when combined with rseq: >> > >> getcpu_cache allows reading the current CPU number in a >> > >> fraction of cycle. >> > > >> > > Yes yes, I know how restartable sequences work. >> > > >> > > But what I worry about is that they want a cpu number and a sequence >> > > number, and for performance it would be very good if those live in the >> > > same cacheline. >> > > >> > > That means either getcpu needs to grow a seq number, or restartable >> > > sequences need to _also_ provide the cpu number. >> > >> > If we plan things well, we could have both the cpu number and the >> > seqnum in the same cache line, registered by two different system >> > calls. It's up to user-space to organize those two variables >> > to fit within the same cache-line. >> >> I feel this is more fragile than needed. Why not do a single systemcall >> that does both? > > 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. 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. Thoughts ? 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
