[RFC PATCH 00/14] context_tracking,x86: Defer some IPIs until a user->kernel transition

Valentin Schneider
2023-07-05T18:16:34Z

Context
=======

We've observed within Red Hat that isolated, NOHZ_FULL CPUs running a
pure-userspace application get regularly interrupted by IPIs sent from
housekeeping CPUs. Those IPIs are caused by activity on the housekeeping CPUs
leading to various on_each_cpu() calls, e.g.:

64359.052209596 NetworkManager 0 1405 smp_call_function_many_cond (cpu=0, func=do_kernel_range_flush)
smp_call_function_many_cond+0x1
smp_call_function+0x39
on_each_cpu+0x2a
flush_tlb_kernel_range+0x7b
__purge_vmap_area_lazy+0x70
_vm_unmap_aliases.part.42+0xdf
change_page_attr_set_clr+0x16a
set_memory_ro+0x26
bpf_int_jit_compile+0x2f9
bpf_prog_select_runtime+0xc6
bpf_prepare_filter+0x523
sk_attach_filter+0x13
sock_setsockopt+0x92c
__sys_setsockopt+0x16a
__x64_sys_setsockopt+0x20
do_syscall_64+0x87
entry_SYSCALL_64_after_hwframe+0x65

The heart of this series is the thought that while we cannot remove NOHZ_FULL
CPUs from the list of CPUs targeted by these IPIs, they may not have to execute
the callbacks immediately. Anything that only affects kernelspace can wait
until the next user->kernel transition, providing it can be executed "early
enough" in the entry code.

The original implementation is from Peter [1]. Nicolas then added kernel TLB
invalidation deferral to that [2], and I picked it up from there.

Deferral approach
=================

Storing each and every callback, like a secondary call_single_queue turned out
to be a no-go: the whole point of deferral is to keep NOHZ_FULL CPUs in
userspace for as long as possible - no signal of any form would be sent when
deferring an IPI. This means that any form of queuing for deferred callbacks
would end up as a convoluted memory leak.

Deferred IPIs must thus be coalesced, which this series achieves by assigning
IPIs a "type" and having a mapping of IPI type to callback, leveraged upon
kernel entry.

What about IPIs whose callback take a parameter, you may ask?

Peter suggested during OSPM23 [3] that since on_each_cpu() targets
housekeeping CPUs *and* isolated CPUs, isolated CPUs can access either global or
housekeeping-CPU-local state to "reconstruct" the data that would have been sent
via the IPI.

This series does not affect any IPI callback that requires an argument, but the
approach would remain the same (one coalescable callback executed on kernel
entry).

Kernel entry vs execution of the deferred operation
===================================================

There is a non-zero length of code that is executed upon kernel entry before the
deferred operation can be itself executed (i.e. before we start getting into
context_tracking.c proper).

This means one must take extra care to what can happen in the early entry code,
and that <bad things> cannot happen. For instance, we really don't want to hit
instructions that have been modified by a remote text_poke() while we're on our
way to execute a deferred sync_core().

Patches
=======

o Patches 1-5 have been submitted previously and are included for the sake of
testing

o Patches 6-10 focus on having objtool detect problematic static key usage in early
entry

The context_tracking_key one causes a page_fault_oops() on my RHEL userspace
due to the KVM module, cf. changelog.

o Patch 11 adds the infrastructure for IPI deferral.
o Patch 12 adds text_poke() IPI deferral.

This one I'm fairly confident about, we "just" need to do something about the
__ro_after_init key vs module loading issue.

o Patches 13-14 add vunmap() flush_tlb_kernel_range() IPI deferral

These ones I'm a lot less confident about, mostly due to lacking
instrumentation/verification.

The actual deferred callback is also incomplete as it's not properly noinstr:
vmlinux.o: warning: objtool: __flush_tlb_all_noinstr+0x19: call to native_write_cr4() leaves .noinstr.text section
and it doesn't support PARAVIRT - it's going to need a pv_ops.mmu entry, but I
have *no idea* what a sane implementation would be for Xen so I haven't
touched that yet.

Patches are also available at:

https://gitlab.com/vschneid/linux.git -b redhat/isolirq/defer/v1

Testing
=======

Xeon Platinum 8380 system with SMToff, NOHZ_FULL, isolated CPUs.
RHEL9 userspace.

Workload is using rteval (kernel compilation + hackbench) on housekeeping CPUs
and a dummy stay-in-userspace loop on the isolated CPUs. The main invocation is:

$ trace-cmd record -e "ipi_send_cpumask" -f "cpumask & MASK{$ISOL_CPUS}" \
-e "ipi_send_cpu" -f "cpu & MASK{$ISOL_CPUS}" \
rteval --onlyload --loads-cpulist=$HK_CPUS \
--hackbench-runlowmem=True --duration=$DURATION

This only records IPIs sent to isolated CPUs, so any event there is interference
(with a bit of fuzz at the start/end of the workload when spawning the
processes). All tests were done with a duration of 20 minutes.

v6.4 (+ cpumask filtering patches):
$ trace-cmd report | grep callback | awk '{ print $NF }' | sort | uniq -c
236 callback=do_flush_tlb_all+0x0
576 callback=do_sync_core+0x0
814 callback=generic_smp_call_function_single_interrupt+0x0
309 callback=nohz_full_kick_func+0x0

v6.4 + patches:
$ trace-cmd report | grep callback | awk '{ print $NF }' | sort | uniq -c
22 callback=do_flush_tlb_all+0x0
24 callback=generic_smp_call_function_single_interrupt+0x0
307 callback=nohz_full_kick_func+0x0

o IPIs from instruction patching are entirely gone.

o Some TLB flushes remain as I only patched the vunmap cases:

kworker/2:0-13856 [002] 3517.445719: ipi_send_cpumask: cpumask=0-1,3-79 callsite=on_each_cpu_cond_mask+0x20
callback=do_flush_tlb_all+0x0
kworker/2:0-13856 [002] 3517.445722: kernel_stack: <stack trace >
=> trace_event_raw_event_ipi_send_cpumask (ffffffffa974a050)
=> smp_call_function_many_cond (ffffffffa97fb0a7)
=> on_each_cpu_cond_mask (ffffffffa97fb1f0)
=> pcpu_reclaim_populated (ffffffffa996b451)
=> pcpu_balance_workfn (ffffffffa996c399)
=> process_one_work (ffffffffa9730e14)
=> worker_thread (ffffffffa9731440)
=> kthread (ffffffffa973984e)

o The nohz_full_kick_func() ones seem to come from the dev_watchdog() but are
anyway consistent across revisions

<...>-3734 [042] 392.890491: ipi_send_cpu: cpu=42 callsite=irq_work_queue_on+0x77 callback=nohz_full_kick_func+0x0
<...>-3734 [042] 392.890497: kernel_stack: <stack trace >
=> trace_event_raw_event_ipi_send_cpu (ffffffff901492d8)
=> __irq_work_queue_local (ffffffff902acb3d)
=> irq_work_queue_on (ffffffff902acc47)
=> __mod_timer (ffffffff901dcd81)
=> dev_watchdog (ffffffff90a75310)
=> call_timer_fn (ffffffff901dc174)
=> __run_timers.part.0 (ffffffff901dc47e)
=> run_timer_softirq (ffffffff901dc546)
=> __do_softirq (ffffffff90c52348)
=> __irq_exit_rcu (ffffffff90113329)
=> sysvec_apic_timer_interrupt (ffffffff90c3c895)
=> asm_sysvec_apic_timer_interrupt (ffffffff90e00d86)

Acknowledgements
================

Special thanks to:
o Clark Williams for listening to my ramblings about this and throwing ideas my way
o Josh Poimboeuf for his guidance regarding objtool and hinting at the
.data..ro_after_init section.

Links
=====

[1]: https://lore.kernel.org/all/20210929151723.162004989@infradead.org/
[2]: https://github.com/vianpl/linux.git -b ct-work-defer-wip
[3]: https://youtu.be/0vjE6fjoVVE

Valentin Schneider (14):
tracing/filters: Dynamically allocate filter_pred.regex
tracing/filters: Enable filtering a cpumask field by another cpumask
tracing/filters: Enable filtering a scalar field by a cpumask
tracing/filters: Enable filtering the CPU common field by a cpumask
tracing/filters: Document cpumask filtering
objtool: Flesh out warning related to pv_ops[] calls
objtool: Warn about non __ro_after_init static key usage in .noinstr
BROKEN: context_tracking: Make context_tracking_key __ro_after_init
x86/kvm: Make kvm_async_pf_enabled __ro_after_init
x86/sev-es: Make sev_es_enable_key __ro_after_init
context-tracking: Introduce work deferral infrastructure
context_tracking,x86: Defer kernel text patching IPIs
context_tracking,x86: Add infrastructure to defer kernel TLBI
x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ_FULL
CPUs

Documentation/trace/events.rst | 14 ++
arch/Kconfig | 9 +
arch/x86/Kconfig | 1 +
arch/x86/include/asm/context_tracking_work.h | 20 ++
arch/x86/include/asm/text-patching.h | 1 +
arch/x86/include/asm/tlbflush.h | 2 +
arch/x86/kernel/alternative.c | 24 +-
arch/x86/kernel/kprobes/core.c | 4 +-
arch/x86/kernel/kprobes/opt.c | 4 +-
arch/x86/kernel/kvm.c | 2 +-
arch/x86/kernel/module.c | 2 +-
arch/x86/kernel/sev.c | 2 +-
arch/x86/mm/tlb.c | 40 +++-
include/linux/context_tracking.h | 1 +
include/linux/context_tracking_state.h | 1 +
include/linux/context_tracking_work.h | 30 +++
include/linux/trace_events.h | 1 +
kernel/context_tracking.c | 65 +++++-
kernel/time/Kconfig | 5 +
kernel/trace/trace_events_filter.c | 228 ++++++++++++++++---
mm/vmalloc.c | 15 +-
tools/objtool/check.c | 22 +-
tools/objtool/include/objtool/check.h | 1 +
tools/objtool/include/objtool/special.h | 2 +
tools/objtool/special.c | 3 +
25 files changed, 451 insertions(+), 48 deletions(-)
create mode 100644 arch/x86/include/asm/context_tracking_work.h
create mode 100644 include/linux/context_tracking_work.h

--
2.31.1