Messages in this thread | | | Date | Wed, 14 Jun 2023 10:45:22 +0100 | From | "Richard W.M. Jones" <> | Subject | Re: printk.time causes rare kernel boot hangs |
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On Wed, Jun 14, 2023 at 11:21:58AM +0200, Peter Zijlstra wrote: > On Tue, Jun 13, 2023 at 02:41:05PM +0100, Richard W.M. Jones wrote: > > [Being tracked in this bug which contains much more detail: > > https://gitlab.com/qemu-project/qemu/-/issues/1696 ] > > Can I please just get the detail in mail instead of having to go look at > random websites?
Sure, the kernel hangs after printing:
[ 0.070120] x86/cpu: User Mode Instruction Prevention (UMIP) activated [ 0.070120] Last level iTLB entries: 4KB 512, 2MB 255, 4MB 127 [ 0.070120] Last level dTLB entries: 4KB 512, 2MB 255, 4MB 127, 1GB 0 [ 0.070120] Spectre V1 : Mitigation: usercopy/swapgs barriers and __user pointer sanitization [ 0.070120] Spectre V2 : Mitigation: Retpolines [ 0.070120] Spectre V2 : Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch [ 0.070120] Spectre V2 : Spectre v2 / SpectreRSB : Filling RSB on VMEXIT [ 0.070120] Spectre V2 : Enabling Speculation Barrier for firmware calls [ 0.070120] RETBleed: Mitigation: untrained return thunk [ 0.070120] Spectre V2 : mitigation: Enabling conditional Indirect Branch Prediction Barrier [ 0.070120] Speculative Store Bypass: Mitigation: Speculative Store Bypass disabled via prctl [ 0.070120] Freeing SMP alternatives memory: 48K
The next message we'd expect here would be:
[ 0.070794] smpboot: CPU0: AMD Ryzen 9 3900X 12-Core Processor (family: 0x17, model: 0x71, stepping: 0x0)
I believe this bug would affect baremetal too, basically any kernel compiled with CONFIG_PRINTK_TIME. However the hang is very rare.
> > Recent kernels hang rarely when booted on qemu. Usually you need to > > boot 100s or 1,000s of times to see the hang, compared to 292,612 [sic] > > successful boots which I was able to do before the problematic commit. > > > > A reproducer (you'll probably need to use Fedora) is: > > Debian only shop here... in fact, I still have machines without systemd.
Debian should work too actually, just run the following command until it hangs:
> > $ while guestfish -a /dev/null -v run >& /tmp/log; do echo -n . ; done > > > > You will need to leave it running for probably several hours, and > > examine the /tmp/log file at the end. > > > > I tracked this down to the following commit: > > > > commit f31dcb152a3d0816e2f1deab4e64572336da197d > > Author: Aaron Thompson <dev@aaront.org> > > Date: Thu Apr 13 17:50:12 2023 +0000 > > > > sched/clock: Fix local_clock() before sched_clock_init() > > > > Have local_clock() return sched_clock() if sched_clock_init() has not > > yet run. sched_clock_cpu() has this check but it was not included in the > > new noinstr implementation of local_clock(). > > > > (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f31dcb152a3d0816e2f1deab4e64572336da197d) > > > > Reverting this commit fixes the problem. > > > > I don't know _why_ this commit is wrong, but can we revert it as it > > causes serious problems with libguestfs hanging randomly. > > > > Or if there's anything you want me to try out then let me know, > > because I can reproduce the problem locally quite easily. > > Well, since it's virt and all, can you attach gdb to the gdb-stub and > see where it's at? Any clue is better than no clue.
I'll see if this is possible, but I didn't have much luck with gdb on qemu guests in the past.
Rich.
-- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org
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