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Groups > linux.kernel > #1698952 > unrolled thread

[PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members long

Started byWill Deacon <will.deacon@arm.com>
First post2017-07-28 19:00 +0200
Last post2017-08-04 17:00 +0200
Articles 3 — 2 participants

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  [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members long Will Deacon <will.deacon@arm.com> - 2017-07-28 19:00 +0200
    Re: [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members long Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2017-07-31 12:40 +0200
      Re: [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members  long Will Deacon <will.deacon@arm.com> - 2017-08-04 17:00 +0200

#1698952 — [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members long

FromWill Deacon <will.deacon@arm.com>
Date2017-07-28 19:00 +0200
Subject[PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members long
Message-ID<u8gMh-6gp-7@gated-at.bofh.it>
The aux_head and aux_wakeup members of struct ring_buffer are defined
using the local_t type, despite the fact that they are only accessed via
the perf_aux_output_* functions, which cannot race with each other for a
given ring buffer.

This patch changes the type of the members to long, so we can avoid
using the local_* API where it isn't needed.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 kernel/events/internal.h    |  4 ++--
 kernel/events/ring_buffer.c | 27 +++++++++++++--------------
 2 files changed, 15 insertions(+), 16 deletions(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 486fd78eb8d5..2941b868353c 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -38,9 +38,9 @@ struct ring_buffer {
 	struct user_struct		*mmap_user;
 
 	/* AUX area */
-	local_t				aux_head;
+	long				aux_head;
 	local_t				aux_nest;
-	local_t				aux_wakeup;
+	long				aux_wakeup;
 	unsigned long			aux_pgoff;
 	int				aux_nr_pages;
 	int				aux_overwrite;
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 14a240ff439d..330df5a7f762 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -367,7 +367,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
 	if (WARN_ON_ONCE(local_xchg(&rb->aux_nest, 1)))
 		goto err_put;
 
-	aux_head = local_read(&rb->aux_head);
+	aux_head = rb->aux_head;
 
 	handle->rb = rb;
 	handle->event = event;
@@ -382,7 +382,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
 	 */
 	if (!rb->aux_overwrite) {
 		aux_tail = ACCESS_ONCE(rb->user_page->aux_tail);
-		handle->wakeup = local_read(&rb->aux_wakeup) + rb->aux_watermark;
+		handle->wakeup = rb->aux_wakeup + rb->aux_watermark;
 		if (aux_head - aux_tail < perf_aux_size(rb))
 			handle->size = CIRC_SPACE(aux_head, aux_tail, perf_aux_size(rb));
 
@@ -434,12 +434,12 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
 		handle->aux_flags |= PERF_AUX_FLAG_OVERWRITE;
 
 		aux_head = handle->head;
-		local_set(&rb->aux_head, aux_head);
+		rb->aux_head = aux_head;
 	} else {
 		handle->aux_flags &= ~PERF_AUX_FLAG_OVERWRITE;
 
-		aux_head = local_read(&rb->aux_head);
-		local_add(size, &rb->aux_head);
+		aux_head = rb->aux_head;
+		rb->aux_head += size;
 	}
 
 	if (size || handle->aux_flags) {
@@ -451,11 +451,11 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
 		                     handle->aux_flags);
 	}
 
-	aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
+	aux_head = rb->user_page->aux_head = rb->aux_head;
 
-	if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
+	if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {
 		wakeup = true;
-		local_add(rb->aux_watermark, &rb->aux_wakeup);
+		rb->aux_wakeup += rb->aux_watermark;
 	}
 
 	if (wakeup) {
@@ -485,14 +485,13 @@ int perf_aux_output_skip(struct perf_output_handle *handle, unsigned long size)
 	if (size > handle->size)
 		return -ENOSPC;
 
-	local_add(size, &rb->aux_head);
+	rb->aux_head += size;
 
-	aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
-	if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
+	aux_head = rb->user_page->aux_head = rb->aux_head;
+	if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {
 		perf_output_wakeup(handle);
-		local_add(rb->aux_watermark, &rb->aux_wakeup);
-		handle->wakeup = local_read(&rb->aux_wakeup) +
-				 rb->aux_watermark;
+		rb->aux_wakeup += rb->aux_watermark;
+		handle->wakeup = rb->aux_wakeup + rb->aux_watermark;
 	}
 
 	handle->head = aux_head;
-- 
2.1.4

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#1699915

FromAlexander Shishkin <alexander.shishkin@linux.intel.com>
Date2017-07-31 12:40 +0200
Message-ID<u9ghb-57H-1@gated-at.bofh.it>
In reply to#1698952
Will Deacon <will.deacon@arm.com> writes:

> The aux_head and aux_wakeup members of struct ring_buffer are defined
> using the local_t type, despite the fact that they are only accessed via
> the perf_aux_output_* functions, which cannot race with each other for a
> given ring buffer.
>
> This patch changes the type of the members to long, so we can avoid
> using the local_* API where it isn't needed.

Thanks for digging this up! Some minor nits below.

> @@ -434,12 +434,12 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
>  		handle->aux_flags |= PERF_AUX_FLAG_OVERWRITE;
>  
>  		aux_head = handle->head;
> -		local_set(&rb->aux_head, aux_head);
> +		rb->aux_head = aux_head;
>  	} else {
>  		handle->aux_flags &= ~PERF_AUX_FLAG_OVERWRITE;
>  
> -		aux_head = local_read(&rb->aux_head);
> -		local_add(size, &rb->aux_head);
> +		aux_head = rb->aux_head;
> +		rb->aux_head += size;
>  	}
>  
>  	if (size || handle->aux_flags) {
> @@ -451,11 +451,11 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
>  		                     handle->aux_flags);
>  	}
>  
> -	aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
> +	aux_head = rb->user_page->aux_head = rb->aux_head;
>  
> -	if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
> +	if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {

Can't we just do away with aux_head here:

  rb->user_page->aux_head = rb->aux_head;
  if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) { ...

?

> @@ -485,14 +485,13 @@ int perf_aux_output_skip(struct perf_output_handle *handle, unsigned long size)
>  	if (size > handle->size)
>  		return -ENOSPC;
>  
> -	local_add(size, &rb->aux_head);
> +	rb->aux_head += size;
>  
> -	aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
> -	if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
> +	aux_head = rb->user_page->aux_head = rb->aux_head;
> +	if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {
>  		perf_output_wakeup(handle);
> -		local_add(rb->aux_watermark, &rb->aux_wakeup);
> -		handle->wakeup = local_read(&rb->aux_wakeup) +
> -				 rb->aux_watermark;
> +		rb->aux_wakeup += rb->aux_watermark;
> +		handle->wakeup = rb->aux_wakeup + rb->aux_watermark;
>  	}
>  
>  	handle->head = aux_head;

And here I think we don't need aux_head at all.

Thanks,
--
Alex

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#1704053 — Re: [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members long

FromWill Deacon <will.deacon@arm.com>
Date2017-08-04 17:00 +0200
SubjectRe: [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members long
Message-ID<uaMf0-8ke-13@gated-at.bofh.it>
In reply to#1699915
On Mon, Jul 31, 2017 at 01:28:01PM +0300, Alexander Shishkin wrote:
> Will Deacon <will.deacon@arm.com> writes:
> 
> > The aux_head and aux_wakeup members of struct ring_buffer are defined
> > using the local_t type, despite the fact that they are only accessed via
> > the perf_aux_output_* functions, which cannot race with each other for a
> > given ring buffer.
> >
> > This patch changes the type of the members to long, so we can avoid
> > using the local_* API where it isn't needed.
> 
> Thanks for digging this up! Some minor nits below.
> 
> > @@ -434,12 +434,12 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
> >  		handle->aux_flags |= PERF_AUX_FLAG_OVERWRITE;
> >  
> >  		aux_head = handle->head;
> > -		local_set(&rb->aux_head, aux_head);
> > +		rb->aux_head = aux_head;
> >  	} else {
> >  		handle->aux_flags &= ~PERF_AUX_FLAG_OVERWRITE;
> >  
> > -		aux_head = local_read(&rb->aux_head);
> > -		local_add(size, &rb->aux_head);
> > +		aux_head = rb->aux_head;
> > +		rb->aux_head += size;
> >  	}
> >  
> >  	if (size || handle->aux_flags) {
> > @@ -451,11 +451,11 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
> >  		                     handle->aux_flags);
> >  	}
> >  
> > -	aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
> > +	aux_head = rb->user_page->aux_head = rb->aux_head;
> >  
> > -	if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
> > +	if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {
> 
> Can't we just do away with aux_head here:
> 
>   rb->user_page->aux_head = rb->aux_head;
>   if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) { ...
> 
> ?
> 
> > @@ -485,14 +485,13 @@ int perf_aux_output_skip(struct perf_output_handle *handle, unsigned long size)
> >  	if (size > handle->size)
> >  		return -ENOSPC;
> >  
> > -	local_add(size, &rb->aux_head);
> > +	rb->aux_head += size;
> >  
> > -	aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
> > -	if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
> > +	aux_head = rb->user_page->aux_head = rb->aux_head;
> > +	if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {
> >  		perf_output_wakeup(handle);
> > -		local_add(rb->aux_watermark, &rb->aux_wakeup);
> > -		handle->wakeup = local_read(&rb->aux_wakeup) +
> > -				 rb->aux_watermark;
> > +		rb->aux_wakeup += rb->aux_watermark;
> > +		handle->wakeup = rb->aux_wakeup + rb->aux_watermark;
> >  	}
> >  
> >  	handle->head = aux_head;
> 
> And here I think we don't need aux_head at all.

Agreed on both counts. Will fix for v2.

Will

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