[PATCH v4 0/2] tracing: fgraph: fix the quadratic shadow stack walk

[Vineet Gupta <vineet.gupta@linux.dev>]

alloc_retstack_tasklist() assigns a shadow stack to every thread when
fgraph is enabled. It did so FTRACE_RETSTACK_ALLOC_SIZE (32) tasks at a
time, and since for_each_process_thread() has no cursor, every sweep
restarted from init_task and re-walked the tasks already served, making
the ftrace_graph_active 0 -> 1 transition quadratic in the thread count.
On a 60-core Sapphire Rapids machine with 400000 idle threads it took
227 s, inside a single bpf() syscall for the kprobe_multi case.

 - 1/2 allocates inline with GFP_NOWAIT, which removes the batching:
   a single sweep serves every task, 227 s -> 0.092 s. It keeps the
   open-coded rcu_read_lock() for ease of backport.

 - 2/2 is the cleanup: convert that same walk to scoped_guard(rcu).
   No functional change, and not needed by stable.

Both were tested on bare metal and in a VM, including forcing the
GFP_NOWAIT failure path with failslab injection.

v1: https://lore.kernel.org/all/20260922225526.1554758-1-vineet.gupta@linux.dev/
v2: https://lore.kernel.org/all/20260929005411.4105448-1-vineet.gupta@linux.dev/
v3: https://lore.kernel.org/all/20261001102909.1152572-1-vineet.gupta@linux.dev/

Changes since v3:
 - split into two patches: the fix keeps goto-based unwinding so it
   backports cleanly, and scoped_guard(rcu) moves to a separate
   cleanup, per Masami and Steve:
   https://lore.kernel.org/all/20261013194542.7e036b03@226.227.38.69.naturalwireless.com/
 - in the cleanup, -EAGAIN now exits via the existing free label
   rather than returning from inside the guard, so there is no longer
   a path that skips the free loop

Changes since v2:
 - update the stale alloc_retstack_tasklist() comment, which still
   described the old batch-of-32 behaviour (bot+bpf-ci)

Changes since v1:
 - replace both v1 patches with Peter's inline GFP_NOWAIT approach,
   which removes the quadratic behaviour rather than dividing it by a
   constant. With a single sweep there is no retry loop left, so the
   v1 cond_resched() patch has nothing to attach to and is dropped.

Vineet Gupta (2):
  tracing: fgraph: allocate shadow stacks inline with GFP_NOWAIT
  tracing: fgraph: use scoped_guard() for the task list walk

 kernel/trace/fgraph.c | 38 +++++++++++++++++++++++---------------
 1 file changed, 23 insertions(+), 15 deletions(-)


base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222
-- 
2.53.0-Meta