On Wed, Feb 22, 2023 at 5:44 AM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote: > > On Wed, 2023-02-15 at 14:54 +0000, Usama Arif wrote: > > The main change over v8 is dropping the patch to avoid repeated saves of MTRR > > at boot time. It didn't make a difference to smpboot time and is independent > > of parallel CPU bringup, so if needed can be explored in a separate patchset. > > > > The patches have also been rebased to v6.2-rc8 and retested and the > > improvement in boot time is the same as v8. > > Thanks for picking this up, Usama. > > So the next thing that might be worth looking at is allowing the APs > all to be running their hotplug thread simultaneously, bringing > themselves from CPUHP_BRINGUP_CPU to CPUHP_AP_ONLINE. This series eats > the initial INIT/SIPI/SIPI latency, but if there's any significant time > in the AP hotplug thread, that could be worth parallelising. > > There may be further wins in the INIT/SIPI/SIPI too. Currently we > process each CPU at a time, sending INIT, SIPI, waiting 10µs and > sending another SIPI. > > What if we sent the first INIT+SIPI to all CPUs, then did another pass > sending another SIPI only to those which hadn't already started running > and set their bit in cpu_initialized_mask ? > > Might not be worth it, and there's an added complexity that they all > have to wait for each other (on the real mode trampoline lock) before > they can take their turn and get as far as setting their bit in > cpu_initialized_mask. So we'd probably end up sending the second SIPI > to most of them *anyway*. Speaking of next steps, I have a followup patchset ready to go that removes the global variables initial_stack, initial_gs, and early_gdt_descr. Should I send that now or wait until this patchset lands in -tip? -- Brian Gerst
