Re: [PATCH v3] tracing: fgraph: fix the quadratic shadow stack walk

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

On 10/1/26 15:59, Vineet Gupta wrote:
> 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.
>
> Allocating inline with GFP_NOWAIT removes the batching: a single sweep
> serves every task, 227 s -> 0.092 s.
>
> 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/
>
> Changes since v2:
>   - update the stale alloc_retstack_tasklist() comment, which still
>     described the old batch-of-32 behaviour (bot+bpf-ci)
>
> Sashiko flagged the mix of goto-based cleanup and scoped_guard() in
> this function. Peter and Andrii both considered it fine as is, so it
> is unchanged:
> https://lore.kernel.org/all/20260929080346.GR4120091@noisy.programming.kicks-ass.net/
>
> 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.
>   - comment why returning from inside scoped_guard() skips the free
>     loop safely
>
> Vineet Gupta (1):
>    tracing: fgraph: allocate shadow stacks inline with GFP_NOWAIT
>
>   kernel/trace/fgraph.c | 44 ++++++++++++++++++++++++++++---------------
>   1 file changed, 29 insertions(+), 15 deletions(-)

Gentle ping !

I appreciate folks might still be on the way home and the mayhem of 
merge window.

Thx,
-Vineet