linux-kernel - Re: [PATCH v3 05/15] perf: Merge consecutive conditionals in perf

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Message-ID: <f51fa4b7-58b2-4e0b-9e6d-0d5bb032b0c4@lucifer.local>
Date: Wed, 13 Aug 2025 06:41:50 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: tglx@...utronix.de, linux-kernel@...r.kernel.org,
        torvalds@...uxfoundation.org, mingo@...nel.org, namhyung@...nel.org,
        acme@...hat.com, kees@...nel.org
Subject: Re: [PATCH v3 05/15] perf: Merge consecutive conditionals in
 perf_mmap()

On Tue, Aug 12, 2025 at 12:39:03PM +0200, Peter Zijlstra wrote:
>   if (cond) {
>     A;
>   } else {
>     B;
>   }
>
>   if (cond) {
>     C;
>   } else {
>     D;
>   }
>
> into:
>
>   if (cond) {
>     A;
>     C;
>   } else {
>     B;
>     D;
>   }
>
> Notably the conditions are not identical in form, but are equivalent.

Good to point out.

>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Seems logical and reduces complexity, so LGTM and:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  kernel/events/core.c |   41 +++++++++++++++++++----------------------
>  1 file changed, 19 insertions(+), 22 deletions(-)
>
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7064,6 +7064,25 @@ static int perf_mmap(struct file *file,
>  		if (vma->vm_flags & VM_WRITE)
>  			flags |= RING_BUFFER_WRITABLE;
>
> +		rb = rb_alloc(nr_pages,
> +			      event->attr.watermark ? event->attr.wakeup_watermark : 0,
> +			      event->cpu, flags);
> +
> +		if (!rb) {
> +			ret = -ENOMEM;
> +			goto unlock;
> +		}
> +
> +		atomic_set(&rb->mmap_count, 1);
> +		rb->mmap_user = get_current_user();
> +		rb->mmap_locked = extra;
> +
> +		ring_buffer_attach(event, rb);
> +
> +		perf_event_update_time(event);
> +		perf_event_init_userpage(event);
> +		perf_event_update_userpage(event);
> +		ret = 0;
>  	} else {
>  		/*
>  		 * AUX area mapping: if rb->aux_nr_pages != 0, it's already
> @@ -7120,29 +7139,7 @@ static int perf_mmap(struct file *file,
>
>  		if (vma->vm_flags & VM_WRITE)
>  			flags |= RING_BUFFER_WRITABLE;
> -	}
>
> -	if (!rb) {
> -		rb = rb_alloc(nr_pages,
> -			      event->attr.watermark ? event->attr.wakeup_watermark : 0,
> -			      event->cpu, flags);
> -
> -		if (!rb) {
> -			ret = -ENOMEM;
> -			goto unlock;
> -		}
> -
> -		atomic_set(&rb->mmap_count, 1);
> -		rb->mmap_user = get_current_user();
> -		rb->mmap_locked = extra;
> -
> -		ring_buffer_attach(event, rb);
> -
> -		perf_event_update_time(event);
> -		perf_event_init_userpage(event);
> -		perf_event_update_userpage(event);
> -		ret = 0;
> -	} else {
>  		ret = rb_alloc_aux(rb, event, vma->vm_pgoff, nr_pages,
>  				   event->attr.aux_watermark, flags);
>  		if (!ret) {
>
>