----- On Sep 10, 2021, at 1:48 PM, Peter Oskolkov posk@xxxxxxxxxx wrote: > On Fri, Sep 10, 2021 at 10:33 AM Mathieu Desnoyers > <mathieu.desnoyers@xxxxxxxxxxxx> wrote: >> >> ----- On Sep 10, 2021, at 12:37 PM, Florian Weimer fweimer@xxxxxxxxxx wrote: >> >> > * Peter Oskolkov: >> > >> >> In short, due to the need to read/write to the userspace from >> >> non-sleepable contexts in the kernel it seems that we need to have some >> >> form of per task/thread kernel/userspace shared memory that is pinned, >> >> similar to what your sys_task_getshared does. >> > >> > In glibc, we'd also like to have this for PID and TID. Eventually, >> > rt_sigprocmask without kernel roundtrip in most cases would be very nice >> > as well. For performance and simplicity in userspace, it would be best >> > if the memory region could be at the same offset from the TCB for all >> > threads. >> > >> > For KTLS, the idea was that the auxiliary vector would contain size and >> > alignment of the KTLS. Userspace would reserve that memory, register it >> > with the kernel like rseq (or the robust list pointers), and pass its >> > address to the vDSO functions that need them. The last part ensures >> > that the vDSO functions do not need non-global data to determine the >> > offset from the TCB. Registration is still needed for the caches. >> > >> > I think previous discussions (in the KTLS and rseq context) did not have >> > the pinning constraint. >> >> If this data is per-thread, and read from user-space, why is it relevant >> to update this data from non-sleepable kernel context rather than update it as >> needed on return-to-userspace ? When returning to userspace, sleeping due to a >> page fault is entirely acceptable. This is what we currently do for rseq. >> >> In short, the data could be accessible from the task struct. Flags in the >> task struct can let return-to-userspace know that it has outdated ktls >> data. So before returning to userspace, the kernel can copy the relevant data >> from the task struct to the shared memory area, without requiring any pinning. >> >> What am I missing ? > > I can't speak about other use cases, but in the context of userspace > scheduling, the information that a task has blocked in the kernel and > is going to be removed from its runqueue cannot wait to be delivered > to the userspace until the task wakes up, as the userspace scheduler > needs to know of the even when it happened so that it can schedule > another task in place of the blocked one. See the discussion here: > > https://lore.kernel.org/lkml/CAG48ez0mgCXpXnqAUsa0TcFBPjrid-74Gj=xG8HZqj2n+OPoKw@xxxxxxxxxxxxxx/ OK, just to confirm my understanding, so the use-case here is per-thread state which can be read by other threads (in this case the userspace scheduler) ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
