Steps To Eliminate Pmap_flush_tlbs Timeout On Kernel Panic

Over the past few days, a number of readers have reported kernel panic over pmap_flush_tlbs timeout.

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    I recently had to fix an issue where macOS virtual machines on an overloaded Proxmox server (constantly over 100% CPU, load> 100) panicked and rebooted every 13 minutes or so. All the VMs at the top of the box were continuous CI workloads, so Proxmox was actually a massive CPU torture test, much like Chrome’s closed loop. However, only the guest macOS was hit by the kernel panic.

    Since multiple virtual machines were running concurrently with high CPU utilization and the host cores were overcrowded, there was fierce competition for computing resources on the Internet, so every guest was familiar with high and variable latency. This was not seen as a hindrance because the product being executed was not an interactive batch job and therefore throughput was more important than latency.

    I have collected 5 records of macOS panic activity in the kernel and compared them. You were really consistent, the main complaint will still be type 3, this one:

    kernel panic pmap_flush_tlbs timeout

     Panic (precious cpu time 12345678910): NMIPI for unresponsive cpu: TLB flush timeout, TLB status: 0x0 

    The stack traces are a bit multidimensional, but I found a specific trace that contained a call to _panic:

     panic (cpu 7, caller 0x ....): "Uninterrupted processor (s): Bitmap CPU: 0x800, NMIPI 0x0, acks: now: 0x1, Deadline = xxx" @ / AppleInternal / BuildRoot / Library / Caches /com.apple.xbs/Sources/xnu/xnu-6153.121.2/osfmk/x86_64/pmap.c:2696mach_kernel: _panic_trap_to_debugger + 0x277mach_kernel: _panik + 0x54mach_kernel: _pmap_flush + 0x4a6mach_kernel: _vm_page_sleep + 0x9e2mach_kernel: _vm_map_msync + 0x18cmach_kernel: _madvise + 0xcemach_kernel: _unix_syscall64 + 0x287mach_kernel: _hndl_unix_scall64 + 0x16 

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  • The XNU core is open source, so I was able to take a closer look at the execution of the _pmap_flush panic in pmap.c.

    This function clears the TLB of the current kernel, signals from all other kernels to be the same, and then waits on them to satisfy them. If most people don’t catch up, this error will fire user:

     if (cpus_to_respond && (mach_absolute_time ()> Deadline)) 
    in cases where (machine_timeout_suspended ())
    continues;

    whenever (TLBTimeOut == 0)
    also if (is_timeout_traced)
    continue;


    PMAP_TRACE_CONSTANT (PMAP_CODE (PMAP__FLUSH_TLBS_TO),


    NULL, cpus_to_signal, cpus_to_respond);

    is_timeout_traced = TRUE;




    orig_acks implies NMIPI_acks;
    NMIPI_panic (cpus_to_respond, TLB_FLUSH_TIMEOUT);
    panic ("Uninterrupted processor (s): CPU bitmap: 0x% llx, NMIPI acks: 0x% lx, now: 0x% lx, deadline:% llu",
    cpus_to_respond, orig_acks, NMIPI_acks, extreme term);

    I assumed that the host, also known as the emulated CPU, simply did not shutdown, and the timeout was most likely caused by an overload on the remote computer that caused many guest threads to not be scheduled. final target flush time. So maybe I could just turn off tlbtimeout so I don’t panic? You can see strategies for changing this setting in machine_routines.c:

    / * * TLBTimeOut defines the menstrual cycle after tlb timeout. this is the default * LockTimeOut can be overwritten separately. In particular * Zero value usually suppresses timeout panic and interrupts event search instead * - see pmap_flush_tlbs (). * /if (PE_parse_boot_argn ("tlbto_us", & slto, sizeof (slto)))default_timeout_ns means slto * NSEC_PER_USEC;nanoseconds_to_absolutetime (default_timeout_ns, & abstime);TLBTimeOut equals (uint32_t) abstime; otherwise =tlbtimeout LockTimeOut;

    So I added tlbto_us = 0 to the load argumentsOpenCore kernels to significantly reduce TLB dump timeout panic. This fixed this crash! But what actually exists is that the kernel has made some mistakes against various other spinlocks :(.

     Panic (CPU 9, time 12345678910): NMIPI spinlock for data collection timeout, spinlock: 0xffffff12345678 ... 

    It was clear that it made sense to increase the lock timeout globally to keep the kernel happy. Fortunately, I noticed that this kernel is pretty common:

      virtualized ((cpuid_features () & CPUID_FEATURE_VMM)! = 0);if (virtualized)int vti;if (! PE_parse_boot_argn ("vti", & vti, sizeof (vti)))vti is 6;printf ("Adjusted virtualization timeouts (<<% d)  n", vti);kprintf ("Virtualization timeouts changed (<<% d):  n", vti);VIRTUAL_TIMEOUT_INFLATE32 (LockTimeOutUsec);VIRTUAL_TIMEOUT_INFLATE64 (LockTimeOut);VIRTUAL_TIMEOUT_INFLATE64 (LockTimeOutTSC);VIRTUAL_TIMEOUT_INFLATE64 (TLBTimeOut);VIRTUAL_TIMEOUT_INFLATE64 (MutexSpin);VIRTUAL_TIMEOUT_INFLATE64 (reportphyreaddelayabs); 

    The real goal of this procedure was to increase kernel timeouts when macOS is running as a virtual machine, until that would include both TLBTimeOut and timeouts. generic that were inventedat the time of receiving the twistlock!

    I assumed that the "VMM" CPU personal identification number was already correctly recognized by the guest, which caused the timeouts to drift to the left of all values ​​in vti "" Virtualization timeout inflation " you keep it at level 6. In other words, kernel timeouts were definitely 2 ^ 6 = 64 times longer than macOS on bare metal.

    kernel panic pmap_flush_tlbs timeout

    Because this Proxmox machine was so overloaded, I made the decision to vti increase this boot arguments parameter to # 1 by 3 (i.e. kernel timeouts have a different factor of 8). So my last boot arguments were:

     keepyms = 1  tlbto_us = 0  vti = 9 

    You can check how the macOS kernel recognized your changes or applied a new value given vti by checking the macOS kernel log for the last day as follows:

     # log show --predicate "processID == 0" --start $ (date "+% Y-% m-% d") --debug | grep "adjusted timeouts"Adjusted virtualization timeouts (<< 9) 

    This completely solved the kernel panic problem! Ultimately, some of these tThe virtual machine types will either be redesigned so you have different hosts, or their core cores will be reduced in number to reduce the overall load on your Proxmox. This will reduce the latency experienced by virtual machines to what they expect from a machine without an operating system. However, this setting of macOS boot parameters is a great and simple solution to improve the stability of the macOS virtual machine in the event of a load spike on a particular host.

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