Re: [PATCH 0/2] tracing: fgraph: cut the cost of the shadow stack retry loop

[Steven Rostedt <rostedt@goodmis.org>]

Just an FYI. My wife flew into Europe today and I'll be touring Europe
with her until Oct 3rd. I may get time to look at patches, but it will
be very limited. And then LPC starts and who knows when I'll get a
chance.

Thus I may not be very responsive for the next couple of weeks.

-- Steve


On Tue, 22 Sep 2026 15:55:24 -0700
Vineet Gupta <vineet.gupta@linux.dev> wrote:

> 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(-)
>