Re: [PATCH v5 13/17] perf: Support deferred user callchains

[Namhyung Kim]

Hello,

On Thu, Apr 24, 2025 at 12:25:42PM -0400, Steven Rostedt wrote:
> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> 
> Use the new unwind_deferred_trace() interface (if available) to defer
> unwinds to task context.  This will allow the use of .sframe (when it
> becomes available) and also prevents duplicate userspace unwinds.
> 
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Co-developed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> ---
[SNIP]
> +/*
> + * Returns:
> +*     > 0 : if already queued.
> + *      0 : if it performed the queuing
> + *    < 0 : if it did not get queued.
> + */
> +static int deferred_request(struct perf_event *event)
> +{
> +	struct callback_head *work = &event->pending_unwind_work;
> +	int pending;
> +	int ret;

I'm not sure if it works for per-CPU events.  The event is shared so any
task can request the deferred callchains.  Does it handle if task A
requests one and scheduled out before going to the user mode, and task B
on the CPU also requests another after that?  I'm afraid not..

> +
> +	if (!current->mm || !user_mode(task_pt_regs(current)))
> +		return -EINVAL;

Does it mean it cannot use deferred callstack when it's in the kernel
mode like during a syscall?

Thanks,
Namhyung

> +
> +	if (in_nmi())
> +		return deferred_request_nmi(event);
> +
> +	guard(irqsave)();
> +
> +        /* callback already pending? */
> +        pending = READ_ONCE(event->pending_unwind_callback);
> +        if (pending)
> +                return 1;
> +
> +        /* Claim the work unless an NMI just now swooped in to do so. */
> +        if (!try_cmpxchg(&event->pending_unwind_callback, &pending, 1))
> +                return 1;
> +
> +        /* The work has been claimed, now schedule it. */
> +        ret = task_work_add(current, work, TWA_RESUME);
> +        if (WARN_ON_ONCE(ret)) {
> +                WRITE_ONCE(event->pending_unwind_callback, 0);
> +                return ret;
> +        }
> +	return 0;
> +}