Turning on function-graph tracing, or attaching a kprobe_multi return
probe, hands every thread a shadow stack. alloc_retstack_tasklist()
does that 32 tasks at a time, and since for_each_process_thread() has
no cursor, every sweep restarts from init_task and re-walks the tasks
already served. Total work is quadratic O(N^2) on thread count.

On a 60-core Sapphire Rapids machine with 400000 idle threads the 0 -> 1
transition takes 227 s, inside a single bpf() syscall for the kprobe_multi
case. On Meta fleet this showed up as RCU stalls and softlockup panics.

Patch 1 raises the batch to 1024, dividing the sweeps by 32:
227 s -> 7.2 s at 400000 threads. It helps on every preemption model.

Patch 2 adds a cond_resched() between sweeps. It is supplementary and
separable: a no-op on current x86 and arm64, but on !CONFIG_PREEMPTION
builds it takes soft lockups from 3-of-3 runs to 0-of-3. Dropping it
leaves patch 1 intact.

Neither changes the O(N^2) shape; a cursor-based walk would, but
task_struct lifetime makes that considerably more involved.

Vineet Gupta (2):
  tracing: fgraph: Raise FTRACE_RETSTACK_ALLOC_SIZE to 1024
  tracing: fgraph: Add a cond_resched() to the shadow stack retry loop

 include/linux/ftrace.h | 7 ++++++-
 kernel/trace/fgraph.c  | 8 ++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

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2.53.0-Meta