----- On Jun 14, 2018, at 9:25 AM, Pavel Machek pa...@ucw.cz wrote: > Hi! > >> >> >>>> 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, > > Yes, this is not likely to be noticeable. > > But the proposal wanted to add a syscall to thread creation, right? > And I believe that may be noticeable.
Fair point! Do we have a standard benchmark that would stress this ? If it ends up being noticeable overhead, I wonder whether we could extend clone() with a new CLONE_RSEQ flag so glibc could pass a pointer to the rseq TLS area through an extra argument to the clone system call rather than do an extra syscall on thread creation ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
