[RFC,v2,0/7] tracing/user_events: Remote write ABI

Message

Beau Belgrave Nov. 15, 2022, 12:58 a.m. UTC

As part of the discussions for user_events aligned with user space
tracers, it was determined that user programs should register a aligned
value to set or clear a bit when an event becomes enabled. Currently a
shared page is being used that requires mmap(). Remove the shared page
implementation and move to a user registered address implementation.

In this new model during the event registration from user programs 3 new
values are specified. The first is the address to update when the event
is either enabled or disabled. The second is the bit to set/clear to
reflect the event being enabled. The third is the size of the value at
the specified address.

This allows for a local 32/64-bit value in user programs to support
both kernel and user tracers. As an example, setting bit 31 for kernel
tracers when the event becomes enabled allows for user tracers to use
the other bits for ref counts or other flags. The kernel side updates
the bit atomically, user programs need to also update these values
atomically.

User provided addresses must be aligned on a natural boundary, this
allows for single page checking and prevents odd behaviors such as a
enable value straddling 2 pages instead of a single page. Currently
page faults are only logged, future patches will handle these.

When page faults are encountered they are done asyncly via a workqueue.
If the page faults back in, the write update is attempted again. If the
page cannot fault-in, then we log and wait until the next time the event
is enabled/disabled. This is to prevent possible infinite loops resulting
from bad user processes unmapping or changing protection values after
registering the address.

Change history

V2:
Rebase to 6.1-rc5.

Added various comments based on feedback.

Added enable_size to register struct, allows 32/64 bit addresses
as long as the enable_bit fits and the address is naturally aligned.

Changed user_event_enabler_write to accept a new flag indicating if a
fault fixup should be done or not. This allows user_event_enabler_create
to return back failures to the user ioctl reg call and retry to fault
in data.

Added tracking fork/exec/exit of tasks to have the user_event_mm lifetime
tied more to the task than the file. This came with extra requirements
around when you can lock, such as softirq cases, as well as a RCU
pattern to ensure fork/exec/exit take minimal lock times.

Changed enablers to use a single word-aligned value for saving the bit
to set and any flags, such as faulting asyncly or being freed. This was
required to ensure atomic bit set/test for fork cases where taking the
event_mutex is not a good scalability decision.

Added unregister IOCTL, since file lifetime no longer limits the enable
time for any events (the mm does).

Updated sample code to reflect the new remote write based ABI.

Updated self-test code to reflect the new remote write based ABI.

Beau Belgrave (7):
  tracing/user_events: Split header into uapi and kernel
  tracing/user_events: Track fork/exec/exit for mm lifetime
  tracing/user_events: Use remote writes for event enablement
  tracing/user_events: Fixup enable faults asyncly
  tracing/user_events: Add ioctl for disabling addresses
  tracing/user_events: Update self-tests to write ABI
  tracing/user_events: Use write ABI in example

 fs/exec.c                                     |   2 +
 include/linux/sched.h                         |   5 +
 include/linux/user_events.h                   | 101 ++-
 include/uapi/linux/user_events.h              |  81 ++
 kernel/exit.c                                 |   2 +
 kernel/fork.c                                 |   3 +
 kernel/trace/trace_events_user.c              | 757 ++++++++++++++----
 samples/user_events/example.c                 |  47 +-
 .../testing/selftests/user_events/dyn_test.c  |   2 +-
 .../selftests/user_events/ftrace_test.c       | 162 ++--
 .../testing/selftests/user_events/perf_test.c |  39 +-
 11 files changed, 889 insertions(+), 312 deletions(-)
 create mode 100644 include/uapi/linux/user_events.h


base-commit: 094226ad94f471a9f19e8f8e7140a09c2625abaa