----- On Jun 14, 2018, at 8:27 AM, Pavel Machek pa...@ucw.cz wrote: > On Tue 2018-06-12 12:31:24, Mathieu Desnoyers wrote: >> ----- On Jun 12, 2018, at 9:11 AM, Florian Weimer fwei...@redhat.com wrote: >> >> > On 06/11/2018 10:04 PM, Mathieu Desnoyers wrote: >> >> ----- On Jun 11, 2018, at 3:55 PM, Florian Weimer fwei...@redhat.com >> >> wrote: >> >> >> >>> On 06/11/2018 09:49 PM, Mathieu Desnoyers wrote: >> >>>> It should be noted that there can be only one rseq TLS area registered >> >>>> per >> >>>> thread, >> >>>> which can then be used by many libraries and by the executable, so this >> >>>> is a >> >>>> process-wide (per-thread) resource that we need to manage carefully. >> >>> >> >>> Is it possible to resize the area after thread creation, perhaps even >> >>> from other threads? >> >> >> >> I'm not sure why we would want to resize it. The per-thread area is >> >> fixed-size. >> >> Its layout is here: include/uapi/linux/rseq.h: struct rseq >> > >> > Looks I was mistaken and this is very similar to the robust mutex list. >> > >> > Should we treat it the same way? Always allocate it for each new thread >> > and register it with the kernel? >> >> That would be an efficient way to do it, indeed. There is very little >> performance overhead to have rseq registered for all threads, whether or >> not they intend to run rseq critical sections. > > People with slow / low memory machines would prefer not to see > overhead they don't need...
In terms of memory usage, if people don't want the extra few bytes of memory used by rseq in the kernel, they should use CONFIG_RSEQ=n. In terms of overhead, let's have a closer look at what it means: when a thread is registered to rseq, but does not enter rseq critical sections, only this extra work is done by the kernel: - rseq_preempt(): on preemption, the scheduler sets the TIF_NOTIFY_RESUME thread flag, so rseq_handle_notify_resume() can check whether it's in a rseq critical section when returning to user-space, - rseq_signal_deliver(): on signal delivery, rseq_handle_notify_resume() checks whether it's in a rseq critical section, - rseq_migrate: on migration, the scheduler sets TIF_NOTIFY_RESUME as well, > >> I have a few possible approaches in mind (feel free to suggest other >> options): >> >> A) glibc exposes a strong __rseq_abi TLS symbol: >> >> - should ideally *not* be global-dynamic for performance reasons, >> - registration to kernel can either be handled explicitly by requiring >> application or libraries to call an API, or implicitly at thread >> creation, > > ...so I'd prefer explicit API call. I have use-cases where a library wants to link against librseq and have rseq critical sections, without requiring the application to explicitly add rseq registration calls on thread creation/destruction. Is there a way to register callbacks to glibc which could be invoked on thread creation/destruction ? Then if we include dynamic loading of libraries (dlopen/dlclose) in the picture, this gets even worse, as we'd need to be able to iterate on all existing threads to invoke registration/unregistration callbacks. One alternative approach would be to let the user library lazily register rseq when needed, and use a pthread_key for unregistration. However, this does not allow dlclose of the user library without figuring a way to iterate on all threads. Another alternative would be to somehow let glibc handle the registration, perhaps only doing it for applications expressing their interest for rseq. Thoughts ? Thanks, Mathieu > >> B) librseq.so exposes a strong __rseq_abi symbol: > > Works for me. > Pavel > > -- > (english) http://www.livejournal.com/~pavelmachek > (cesky, pictures) > http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
