Re: [PATCH] ring-buffer: Fix wrong stat of cpu_buffer->read

[Steven Rostedt]

Do you folks find it fun to post a fix right after I send Linus a pull request? ;-)

Thanks, I'll queue it up (but I will wait a bit before posting in case
more fixes trickle in)

-- Steve


On Mon, 24 Jul 2023 13:40:40 +0800
Zheng Yejian <zhengyejian1@xxxxxxxxxx> wrote:

> When pages are removed in rb_remove_pages(), 'cpu_buffer->read' is set
> to 0 in order to make sure any read iterators reset themselves. However,
> this will mess 'entries' stating, see following steps:
> 
>   # cd /sys/kernel/tracing/
>   # 1. Enlarge ring buffer prepare for later reducing:
>   # echo 20 > per_cpu/cpu0/buffer_size_kb
>   # 2. Write a log into ring buffer of cpu0:
>   # taskset -c 0 echo "hello1" > trace_marker
>   # 3. Read the log:
>   # cat per_cpu/cpu0/trace_pipe
>        <...>-332     [000] .....    62.406844: tracing_mark_write: hello1
>   # 4. Stop reading and see the stats, now 0 entries, and 1 event readed:
>   # cat per_cpu/cpu0/stats
>    entries: 0
>    [...]
>    read events: 1
>   # 5. Reduce the ring buffer
>   # echo 7 > per_cpu/cpu0/buffer_size_kb
>   # 6. Now entries became unexpected 1 because actually no entries!!!
>   # cat per_cpu/cpu0/stats
>    entries: 1
>    [...]
>    read events: 0
> 
> To fix it, introduce 'page_removed' field to count total removed pages
> since last reset, then use it to let read iterators reset themselves
> instead of changing the 'read' pointer.
> 
> Fixes: 83f40318dab0 ("ring-buffer: Make removal of ring buffer pages atomic")
> Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
> ---
>  kernel/trace/ring_buffer.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index de061dd47313..46b4a3c7c3bf 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -523,6 +523,8 @@ struct ring_buffer_per_cpu {
>  	rb_time_t			before_stamp;
>  	u64				event_stamp[MAX_NEST];
>  	u64				read_stamp;
> +	/* pages removed since last reset */
> +	unsigned long			pages_removed;
>  	/* ring buffer pages to update, > 0 to add, < 0 to remove */
>  	long				nr_pages_to_update;
>  	struct list_head		new_pages; /* new pages to add */
> @@ -559,6 +561,7 @@ struct ring_buffer_iter {
>  	struct buffer_page		*head_page;
>  	struct buffer_page		*cache_reader_page;
>  	unsigned long			cache_read;
> +	unsigned long			cache_pages_removed;
>  	u64				read_stamp;
>  	u64				page_stamp;
>  	struct ring_buffer_event	*event;
> @@ -1957,6 +1960,8 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages)
>  		to_remove = rb_list_head(to_remove)->next;
>  		head_bit |= (unsigned long)to_remove & RB_PAGE_HEAD;
>  	}
> +	/* Read iterators need to reset themselves when some pages removed */
> +	cpu_buffer->pages_removed += nr_removed;
>  
>  	next_page = rb_list_head(to_remove)->next;
>  
> @@ -1978,12 +1983,6 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages)
>  		cpu_buffer->head_page = list_entry(next_page,
>  						struct buffer_page, list);
>  
> -	/*
> -	 * change read pointer to make sure any read iterators reset
> -	 * themselves
> -	 */
> -	cpu_buffer->read = 0;
> -
>  	/* pages are removed, resume tracing and then free the pages */
>  	atomic_dec(&cpu_buffer->record_disabled);
>  	raw_spin_unlock_irq(&cpu_buffer->reader_lock);
> @@ -4395,6 +4394,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter)
>  
>  	iter->cache_reader_page = iter->head_page;
>  	iter->cache_read = cpu_buffer->read;
> +	iter->cache_pages_removed = cpu_buffer->pages_removed;
>  
>  	if (iter->head) {
>  		iter->read_stamp = cpu_buffer->read_stamp;
> @@ -4849,12 +4849,13 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
>  	buffer = cpu_buffer->buffer;
>  
>  	/*
> -	 * Check if someone performed a consuming read to
> -	 * the buffer. A consuming read invalidates the iterator
> -	 * and we need to reset the iterator in this case.
> +	 * Check if someone performed a consuming read to the buffer
> +	 * or removed some pages from the buffer. In these cases,
> +	 * iterator was invalidated and we need to reset it.
>  	 */
>  	if (unlikely(iter->cache_read != cpu_buffer->read ||
> -		     iter->cache_reader_page != cpu_buffer->reader_page))
> +		     iter->cache_reader_page != cpu_buffer->reader_page ||
> +		     iter->cache_pages_removed != cpu_buffer->pages_removed))
>  		rb_iter_reset(iter);
>  
>   again:
> @@ -5298,6 +5299,7 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
>  	cpu_buffer->last_overrun = 0;
>  
>  	rb_head_page_activate(cpu_buffer);
> +	cpu_buffer->pages_removed = 0;
>  }
>  
>  /* Must have disabled the cpu buffer then done a synchronize_rcu */