Commit 45d99ea4 authored by Zheng Yejian's avatar Zheng Yejian Committed by Steven Rostedt (Google)
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ring-buffer: Fix bytes info in per_cpu buffer stats 

The 'bytes' info in file 'per_cpu/cpu<X>/stats' means the number of
bytes in cpu buffer that have not been consumed. However, currently
after consuming data by reading file 'trace_pipe', the 'bytes' info
was not changed as expected.

  # cat per_cpu/cpu0/stats
  entries: 0
  overrun: 0
  commit overrun: 0
  bytes: 568             <--- 'bytes' is problematical !!!
  oldest event ts:  8651.371479
  now ts:  8653.912224
  dropped events: 0
  read events: 8

The root cause is incorrect stat on cpu_buffer->read_bytes. To fix it:
  1. When stat 'read_bytes', account consumed event in rb_advance_reader();
  2. When stat 'entries_bytes', exclude the discarded padding event which
     is smaller than minimum size because it is invisible to reader. Then
     use rb_page_commit() instead of BUF_PAGE_SIZE at where accounting for
     page-based read/remove/overrun.

Also correct the comments of ring_buffer_bytes_cpu() in this patch.

Link: https://lore.kernel.org/linux-trace-kernel/20230921125425.1708423-1-zhengyejian1@huawei.com



Cc: stable@vger.kernel.org
Fixes: c64e148a ("trace: Add ring buffer stats to measure rate of events")
Signed-off-by: default avatarZheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent ce9ecca0

kernel/trace/ring_buffer.c

+15 −13
Original line number Diff line number Diff line
@@ -354,6 +354,11 @@ static void rb_init_page(struct buffer_data_page *bpage) 
	local_set(&bpage->commit, 0);

}



static __always_inline unsigned int rb_page_commit(struct buffer_page *bpage)

{

	return local_read(&bpage->page->commit);

}



static void free_buffer_page(struct buffer_page *bpage)

{

	free_page((unsigned long)bpage->page);
@@ -2003,7 +2008,7 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages) 
			 * Increment overrun to account for the lost events.

			 */

			local_add(page_entries, &cpu_buffer->overrun);

			local_sub(BUF_PAGE_SIZE, &cpu_buffer->entries_bytes);

			local_sub(rb_page_commit(to_remove_page), &cpu_buffer->entries_bytes);

			local_inc(&cpu_buffer->pages_lost);

		}
@@ -2367,11 +2372,6 @@ rb_reader_event(struct ring_buffer_per_cpu *cpu_buffer) 
			       cpu_buffer->reader_page->read);

}



static __always_inline unsigned rb_page_commit(struct buffer_page *bpage)

{

	return local_read(&bpage->page->commit);

}



static struct ring_buffer_event *

rb_iter_head_event(struct ring_buffer_iter *iter)

{
@@ -2517,7 +2517,7 @@ rb_handle_head_page(struct ring_buffer_per_cpu *cpu_buffer, 
		 * the counters.

		 */

		local_add(entries, &cpu_buffer->overrun);

		local_sub(BUF_PAGE_SIZE, &cpu_buffer->entries_bytes);

		local_sub(rb_page_commit(next_page), &cpu_buffer->entries_bytes);

		local_inc(&cpu_buffer->pages_lost);



		/*
@@ -2660,9 +2660,6 @@ rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer, 


	event = __rb_page_index(tail_page, tail);



	/* account for padding bytes */

	local_add(BUF_PAGE_SIZE - tail, &cpu_buffer->entries_bytes);



	/*

	 * Save the original length to the meta data.

	 * This will be used by the reader to add lost event
@@ -2676,7 +2673,8 @@ rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer, 
	 * write counter enough to allow another writer to slip

	 * in on this page.

	 * We put in a discarded commit instead, to make sure

	 * that this space is not used again.

	 * that this space is not used again, and this space will

	 * not be accounted into 'entries_bytes'.

	 *

	 * If we are less than the minimum size, we don't need to

	 * worry about it.
@@ -2701,6 +2699,9 @@ rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer, 
	/* time delta must be non zero */

	event->time_delta = 1;



	/* account for padding bytes */

	local_add(BUF_PAGE_SIZE - tail, &cpu_buffer->entries_bytes);



	/* Make sure the padding is visible before the tail_page->write update */

	smp_wmb();
@@ -4215,7 +4216,7 @@ u64 ring_buffer_oldest_event_ts(struct trace_buffer *buffer, int cpu) 
EXPORT_SYMBOL_GPL(ring_buffer_oldest_event_ts);



/**

 * ring_buffer_bytes_cpu - get the number of bytes consumed in a cpu buffer

 * ring_buffer_bytes_cpu - get the number of bytes unconsumed in a cpu buffer

 * @buffer: The ring buffer

 * @cpu: The per CPU buffer to read from.

 */
@@ -4723,6 +4724,7 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer) 


	length = rb_event_length(event);

	cpu_buffer->reader_page->read += length;

	cpu_buffer->read_bytes += length;

}



static void rb_advance_iter(struct ring_buffer_iter *iter)
@@ -5816,7 +5818,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer, 
	} else {

		/* update the entry counter */

		cpu_buffer->read += rb_page_entries(reader);

		cpu_buffer->read_bytes += BUF_PAGE_SIZE;

		cpu_buffer->read_bytes += rb_page_commit(reader);



		/* swap the pages */

		rb_init_page(bpage);