linux-kernel - Re: [PATCH v3 06/15] perf: Move common code into both rb and aux branches

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Message-ID: <20250813082524.GJ4067720@noisy.programming.kicks-ass.net>
Date: Wed, 13 Aug 2025 10:25:24 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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 06/15] perf: Move common code into both rb and aux
 branches

On Wed, Aug 13, 2025 at 06:55:51AM +0100, Lorenzo Stoakes wrote:
> On Tue, Aug 12, 2025 at 12:39:04PM +0200, Peter Zijlstra wrote:
> >   if (cond) {
> >     A;
> >   } else {
> >     B;
> >   }
> >   C;
> >
> > into
> >
> >   if (cond) {
> >     A;
> >     C;
> >   } else {
> >     B;
> >     C;
> >   }
> >
> 
> Hm we're doing more than that here though, we're refactoring other stuff at
> the same time.
> 
> I guess you're speaking broad strokes here, but maybe worth mentioniing the
> tricksy hobbitses around the rb_has_aux() bit.

Does something like so clarify:

Notably C has a success branch and both A and B have two places for
success. For A (rb case), duplicate the success case because later
patches will result in them no longer being identical. For B (aux
case), share using goto (cleaned up later).

> Anyway LGTM so:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> 
> > Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  kernel/events/core.c |   25 +++++++++++++++----------
> >  1 file changed, 15 insertions(+), 10 deletions(-)
> >
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -7043,6 +7043,8 @@ static int perf_mmap(struct file *file,
> >  				ret = 0;
> >  				/* We need the rb to map pages. */
> >  				rb = event->rb;
> > +				perf_mmap_account(vma, user_extra, extra);
> > +				atomic_inc(&event->mmap_count);
> >  				goto unlock;
> >  			}
> >
> > @@ -7083,6 +7085,9 @@ static int perf_mmap(struct file *file,
> >  		perf_event_init_userpage(event);
> >  		perf_event_update_userpage(event);
> >  		ret = 0;
> > +
> > +		perf_mmap_account(vma, user_extra, extra);
> > +		atomic_inc(&event->mmap_count);
> >  	} else {
> >  		/*
> >  		 * AUX area mapping: if rb->aux_nr_pages != 0, it's already
> > @@ -7127,11 +7132,12 @@ static int perf_mmap(struct file *file,
> >  		if (rb_has_aux(rb)) {
> >  			atomic_inc(&rb->aux_mmap_count);
> >  			ret = 0;
> > -			goto unlock;
> > +			goto aux_success;
> >  		}
> >
> >  		if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
> >  			ret = -EPERM;
> > +			atomic_dec(&rb->mmap_count);
> >  			goto unlock;
> >  		}
> >
> > @@ -7142,20 +7148,19 @@ static int perf_mmap(struct file *file,
> >
> >  		ret = rb_alloc_aux(rb, event, vma->vm_pgoff, nr_pages,
> >  				   event->attr.aux_watermark, flags);
> > -		if (!ret) {
> > -			atomic_set(&rb->aux_mmap_count, 1);
> > -			rb->aux_mmap_locked = extra;
> > +		if (ret) {
> 
> You dropped the 'AUX allocation failed' comment here, but honestly I think
> that's fine, it's pretty obvious that's the case given the literal previous
> line is you trying the AUX allocation... :)

Yes that :-)