linux-kernel - Re: [PATCH 06/12] perf: Return empty callchain instead of NULL

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Date:   Mon, 8 Jan 2018 13:15:16 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: Re: [PATCH 06/12] perf: Return empty callchain instead of NULL

On Sun, Jan 07, 2018 at 05:03:50PM +0100, Jiri Olsa wrote:
> It simplifies the code a bit, because we dump the callchain
> even if it's empty. With 'empty' callchain we can remove
> all the NULL-checking code paths.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

> 
> Original-patch-from: Peter Zijlstra <peterz@...radead.org>
> Link: http://lkml.kernel.org/n/tip-n4t869o2qc1b31v4wwvmxznb@git.kernel.org
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/events/core.c | 30 ++++++++++++------------------
>  1 file changed, 12 insertions(+), 18 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 5fc1ded4b450..4e1a1bf8d867 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5815,19 +5815,11 @@ void perf_output_sample(struct perf_output_handle *handle,
>  		perf_output_read(handle, event);
>  
>  	if (sample_type & PERF_SAMPLE_CALLCHAIN) {
> -		if (data->callchain) {
> -			int size = 1;
> -
> -			if (data->callchain)
> -				size += data->callchain->nr;
> -
> -			size *= sizeof(u64);
> +		int size = 1;
>  
> -			__output_copy(handle, data->callchain, size);
> -		} else {
> -			u64 nr = 0;
> -			perf_output_put(handle, nr);
> -		}
> +		size += data->callchain->nr;
> +		size *= sizeof(u64);
> +		__output_copy(handle, data->callchain, size);
>  	}
>  
>  	if (sample_type & PERF_SAMPLE_RAW) {
> @@ -5980,6 +5972,8 @@ static u64 perf_virt_to_phys(u64 virt)
>  	return phys_addr;
>  }
>  
> +static struct perf_callchain_entry __empty_callchain = { .nr = 0, };
> +
>  static struct perf_callchain_entry *
>  perf_callchain(struct perf_event *event, struct pt_regs *regs)
>  {
> @@ -5988,12 +5982,14 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
>  	/* Disallow cross-task user callchains. */
>  	bool crosstask = event->ctx->task && event->ctx->task != current;
>  	const u32 max_stack = event->attr.sample_max_stack;
> +	struct perf_callchain_entry *callchain;
>  
>  	if (!kernel && !user)
> -		return NULL;
> +		return &__empty_callchain;
>  
> -	return get_perf_callchain(regs, 0, kernel, user,
> -				  max_stack, crosstask, true);
> +	callchain = get_perf_callchain(regs, 0, kernel, user,
> +				       max_stack, crosstask, true);
> +	return callchain ?: &__empty_callchain;
>  }
>  
>  void perf_prepare_sample(struct perf_event_header *header,
> @@ -6018,9 +6014,7 @@ void perf_prepare_sample(struct perf_event_header *header,
>  		int size = 1;
>  
>  		data->callchain = perf_callchain(event, regs);
> -
> -		if (data->callchain)
> -			size += data->callchain->nr;
> +		size += data->callchain->nr;
>  
>  		header->size += size * sizeof(u64);
>  	}
> -- 
> 2.13.6
>