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ring-buffer: Check for empty ring-buffer with rb_num_of_entries()

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Currently there are two ways of identifying an empty ring-buffer. One
relying on the current status of the commit / reader page
(rb_per_cpu_empty()) and the other on the write and read counters
(rb_num_of_entries() used in rb_get_reader_page()).

with rb_num_of_entries(). This intends to ease later
introduction of ring-buffer writers which are out of the kernel control
and with whom, the only information available is through the meta-page
counters.

Link: https://lore.kernel.org/20250108114536.627715-2-vdonnefort@google.com
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Vincent Donnefort 2025-01-08 11:45:25 +00:00 committed by Steven Rostedt (Google)
parent 5bc55a333a
commit 0568c6ebf0
1 changed files with 14 additions and 45 deletions
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59
kernel/trace/ring_buffer.c
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@ -4682,40 +4682,22 @@ int ring_buffer_write(struct trace_buffer *buffer,
}
EXPORT_SYMBOL_GPL(ring_buffer_write);
/*
* The total entries in the ring buffer is the running counter
* of entries entered into the ring buffer, minus the sum of
* the entries read from the ring buffer and the number of
* entries that were overwritten.
*/
static inline unsigned long
rb_num_of_entries(struct ring_buffer_per_cpu *cpu_buffer)
{
return local_read(&cpu_buffer->entries) -
(local_read(&cpu_buffer->overrun) + cpu_buffer->read);
}
static bool rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer)
{
struct buffer_page *reader = cpu_buffer->reader_page;
struct buffer_page *head = rb_set_head_page(cpu_buffer);
struct buffer_page *commit = cpu_buffer->commit_page;
/* In case of error, head will be NULL */
if (unlikely(!head))
return true;
/* Reader should exhaust content in reader page */
if (reader->read != rb_page_size(reader))
return false;
/*
* If writers are committing on the reader page, knowing all
* committed content has been read, the ring buffer is empty.
*/
if (commit == reader)
return true;
/*
* If writers are committing on a page other than reader page
* and head page, there should always be content to read.
*/
if (commit != head)
return false;
/*
* Writers are committing on the head page, we just need
* to care about there're committed data, and the reader will
* swap reader page with head page when it is to read data.
*/
return rb_page_commit(commit) == 0;
return !rb_num_of_entries(cpu_buffer);
}
/**
@ -4861,19 +4843,6 @@ void ring_buffer_record_enable_cpu(struct trace_buffer *buffer, int cpu)
}
EXPORT_SYMBOL_GPL(ring_buffer_record_enable_cpu);
/*
* The total entries in the ring buffer is the running counter
* of entries entered into the ring buffer, minus the sum of
* the entries read from the ring buffer and the number of
* entries that were overwritten.
*/
static inline unsigned long
rb_num_of_entries(struct ring_buffer_per_cpu *cpu_buffer)
{
return local_read(&cpu_buffer->entries) -
(local_read(&cpu_buffer->overrun) + cpu_buffer->read);
}
/**
* ring_buffer_oldest_event_ts - get the oldest event timestamp from the buffer
* @buffer: The ring buffer