Re: [PATCH v2 09/11] perf: Introduce deferred user callchains

[Namhyung Kim <namhyung@kernel.org>]

On Sat, Sep 14, 2024 at 01:02:11AM +0200, Josh Poimboeuf wrote:
> Instead of attempting to unwind user space from the NMI handler, defer
> it to run in task context by sending a self-IPI and then scheduling the
> unwind to run in the IRQ's exit task work before returning to user space.
> 
> This allows the user stack page to be paged in if needed, avoids
> duplicate unwinds for kernel-bound workloads, and prepares for SFrame
> unwinding (so .sframe sections can be paged in on demand).
> 
> Suggested-by: Steven Rostedt <rostedt@goodmis.org>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
[SNIP]  
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 19fd7bd38ecf..5fc7c5156287 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6854,11 +6860,70 @@ static void perf_pending_irq(struct irq_work *entry)
>  		perf_swevent_put_recursion_context(rctx);
>  }
>  
> +struct perf_callchain_deferred_event {
> +	struct perf_event_header	header;
> +	struct perf_callchain_entry	callchain;
> +};
> +
> +#define PERF_CALLCHAIN_DEFERRED_EVENT_SIZE				\
> +	sizeof(struct perf_callchain_deferred_event) +			\
> +	(sizeof(__u64) * 1) + /* PERF_CONTEXT_USER */			\
> +	(sizeof(__u64) * PERF_MAX_STACK_DEPTH)
> +
> +static void perf_event_callchain_deferred(struct perf_event *event)
> +{
> +	struct pt_regs *regs = task_pt_regs(current);
> +	struct perf_callchain_entry *callchain;
> +	struct perf_output_handle handle;
> +	struct perf_sample_data data;
> +	unsigned char buf[PERF_CALLCHAIN_DEFERRED_EVENT_SIZE];
> +	struct perf_callchain_entry_ctx ctx;
> +	struct perf_callchain_deferred_event *deferred_event;
> +
> +	deferred_event = (void *)&buf;
> +
> +	callchain = &deferred_event->callchain;
> +	callchain->nr = 0;
> +
> +	ctx.entry		= callchain;
> +	ctx.max_stack		= MIN(event->attr.sample_max_stack,
> +				      PERF_MAX_STACK_DEPTH);
> +	ctx.nr			= 0;
> +	ctx.contexts		= 0;
> +	ctx.contexts_maxed	= false;
> +
> +	perf_callchain_store_context(&ctx, PERF_CONTEXT_USER);
> +	perf_callchain_user_deferred(&ctx, regs);
> +
> +	deferred_event->header.type = PERF_RECORD_CALLCHAIN_DEFERRED;
> +	deferred_event->header.misc = 0;

I think we can use PERF_RECORD_MISC_USER here as it's about user
callchains.

> +	deferred_event->header.size = sizeof(*deferred_event) +
> +				      (callchain->nr * sizeof(u64));
> +
> +	perf_event_header__init_id(&deferred_event->header, &data, event);
> +
> +	if (perf_output_begin(&handle, &data, event,
> +			      deferred_event->header.size))
> +		return;
> +
> +	perf_output_copy(&handle, deferred_event, deferred_event->header.size);

You should not copy the whole event size because it also contains the
id_sample parts in the below.  Maybe something like this instead?

	perf_output_put(&handle, *deferred_event);
	__output_copy(&handle, callchain->ip, callchain->nr * sizeof(u64));

Thanks,
Namhyung


> +	perf_event__output_id_sample(event, &handle, &data);
> +	perf_output_end(&handle);
> +}
> +
>  static void perf_pending_task(struct callback_head *head)
>  {
>  	struct perf_event *event = container_of(head, struct perf_event, pending_task);
>  	int rctx;
>  
> +	if (!is_software_event(event)) {
> +		if (event->pending_callchain) {
> +			perf_event_callchain_deferred(event);
> +			event->pending_callchain = 0;
> +		}
> +		return;
> +	}
> +
>  	/*
>  	 * All accesses to the event must belong to the same implicit RCU read-side
>  	 * critical section as the ->pending_work reset. See comment in
> @@ -7688,6 +7753,8 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
>  	bool user   = !event->attr.exclude_callchain_user;
>  	const u32 max_stack = event->attr.sample_max_stack;
>  	struct perf_callchain_entry *callchain;
> +	bool defer_user = IS_ENABLED(CONFIG_HAVE_PERF_CALLCHAIN_DEFERRED) &&
> +			  event->attr.defer_callchain;
>  
>  	/* Disallow cross-task user callchains. */
>  	user &= !event->ctx->task || event->ctx->task == current;
> @@ -7695,7 +7762,14 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
>  	if (!kernel && !user)
>  		return &__empty_callchain;
>  
> -	callchain = get_perf_callchain(regs, kernel, user, max_stack, true);
> +	callchain = get_perf_callchain(regs, kernel, user, max_stack, true,
> +				       defer_user);
> +
> +	if (user && defer_user && !event->pending_callchain) {
> +		event->pending_callchain = 1;
> +		irq_work_queue(&event->pending_irq);
> +	}
> +
>  	return callchain ?: &__empty_callchain;
>  }
>  
> -- 
> 2.46.0
>