[M] Change in libosmocore[master]: Automatically increase io_uring, if too small.

Attention is currently required from: jolly, pespin. laforge has posted comments on this change by jolly. ( https://gerrit.osmocom.org/c/libosmocore/+/40725?usp=email ) Change subject: Automatically increase io_uring, if too small. ...................................................................... Patch Set 6: Code-Review-1 (5 comments) File src/core/osmo_io_internal.h: https://gerrit.osmocom.org/c/libosmocore/+/40725/comment/6799a0a9_4e2cf970?… : PS6, Line 116: /* ! array of rings the submitted read SQEs have been submitted */ same comment like in the other patch regarding "*!" vs "* !" File src/core/osmo_io_uring.c: https://gerrit.osmocom.org/c/libosmocore/+/40725/comment/14dd5fc1_ad400304?… : PS2, Line 175: ok, then please mention this fact explicitly in this c ommit in a TODO-comment and also in the commit log. At least this way it's clear that there is no related cleanup yet. Implementation could then follow at a later patch, but at least it is known to everyone that this is the situation. I would consider having a single per-ring counter of the number of not-yet-completed sqe's would be sufficient. If that counter reaches zero, and we are not the currently active ring, we can destroy it? File src/core/osmo_io_uring.c: https://gerrit.osmocom.org/c/libosmocore/+/40725/comment/c5d522d0_aeb3e8ef?… : PS6, Line 172: sqe = io_uring_get_sqe(&g_ring->ring); does it matter? Do we ever care? I think there's no such guarantee even with two read sqe's for the same fd on a single ring? In the end, they are all empty msgb's waiting for some incoming data. It doesn't matter which is used/filled/reported-completed first? https://gerrit.osmocom.org/c/libosmocore/+/40725/comment/5501dd65_4c9eabef?… : PS6, Line 527: while ((sqe = io_uring_get_sqe(iofd->u.uring.read_ring[idx])) == NULL); yeah. It's one problem stalling the process itself by waiting for something to happen eventually - it's another problem to waste a full CPU core while doing that, potentially slowing down the event that we're waiting for. It seems `io_uring_sqring_wait` was introduced for this: https://man7.org/linux/man-pages/man3/io_uring_sqring_wait.3.html - we may have to check 1) at compile time that a sufficient enough liburing version is present 2) at runtime that the kernel supports it (-EINVAL is returned otherwise) It might also make sense to log something just before we call io_uiring_sqring_wait... that might help debugging if we ever ended up spending significant time in it. So my strategy would be: * fast path trying io_uring_get_sqe * only if that fails, log the situation and use io_uring_sqring_wait * if io_uring_sqring_wait fails, fall-back to busy-waiting, but maybe with some usleep in the loop Alternatively, we could try calling io_uring_sqring_wait once at ring creation time (when we know it will never block as there are always free SQEs inside), and cache whether or not it is supported (positive value) or unsupported (-EINVAL) in a global variable. The code here and below could then avoid the trial-and-error every time we unregister an fd. https://gerrit.osmocom.org/c/libosmocore/+/40725/comment/c930b6b7_3dd558ea?… : PS6, Line 543: while ((sqe = io_uring_get_sqe(iofd->u.uring.write_ring)) == NULL); same as above -- To view, visit https://gerrit.osmocom.org/c/libosmocore/+/40725?usp=email To unsubscribe, or for help writing mail filters, visit https://gerrit.osmocom.org/settings?usp=email Gerrit-MessageType: comment Gerrit-Project: libosmocore Gerrit-Branch: master Gerrit-Change-Id: Id9230146acc8d54bfd44834e783c31b37bd64bca Gerrit-Change-Number: 40725 Gerrit-PatchSet: 6 Gerrit-Owner: jolly <andreas(a)eversberg.eu> Gerrit-Reviewer: Jenkins Builder Gerrit-Reviewer: laforge <laforge(a)osmocom.org> Gerrit-CC: pespin <pespin(a)sysmocom.de> Gerrit-Attention: jolly <andreas(a)eversberg.eu> Gerrit-Attention: pespin <pespin(a)sysmocom.de> Gerrit-Comment-Date: Thu, 31 Jul 2025 14:00:21 +0000 Gerrit-HasComments: Yes Gerrit-Has-Labels: Yes Comment-In-Reply-To: jolly <andreas(a)eversberg.eu> Comment-In-Reply-To: laforge <laforge(a)osmocom.org> Comment-In-Reply-To: pespin <pespin(a)sysmocom.de>