• Steven Rostedt's avatar
    tracing: unified trace buffer · 7a8e76a3
    Steven Rostedt authored
    This is a unified tracing buffer that implements a ring buffer that
    hopefully everyone will eventually be able to use.
    
    The events recorded into the buffer have the following structure:
    
      struct ring_buffer_event {
    	u32 type:2, len:3, time_delta:27;
    	u32 array[];
      };
    
    The minimum size of an event is 8 bytes. All events are 4 byte
    aligned inside the buffer.
    
    There are 4 types (all internal use for the ring buffer, only
    the data type is exported to the interface users).
    
     RINGBUF_TYPE_PADDING: this type is used to note extra space at the end
    	of a buffer page.
    
     RINGBUF_TYPE_TIME_EXTENT: This type is used when the time between events
    	is greater than the 27 bit delta can hold. We add another
    	32 bits, and record that in its own event (8 byte size).
    
     RINGBUF_TYPE_TIME_STAMP: (Not implemented yet). This will hold data to
    	help keep the buffer timestamps in sync.
    
    RINGBUF_TYPE_DATA: The event actually holds user data.
    
    The "len" field is only three bits. Since the data must be
    4 byte aligned, this field is shifted left by 2, giving a
    max length of 28 bytes. If the data load is greater than 28
    bytes, the first array field holds the full length of the
    data load and the len field is set to zero.
    
    Example, data size of 7 bytes:
    
    	type = RINGBUF_TYPE_DATA
    	len = 2
    	time_delta: <time-stamp> - <prev_event-time-stamp>
    	array[0..1]: <7 bytes of data> <1 byte empty>
    
    This event is saved in 12 bytes of the buffer.
    
    An event with 82 bytes of data:
    
    	type = RINGBUF_TYPE_DATA
    	len = 0
    	time_delta: <time-stamp> - <prev_event-time-stamp>
    	array[0]: 84 (Note the alignment)
    	array[1..14]: <82 bytes of data> <2 bytes empty>
    
    The above event is saved in 92 bytes (if my math is correct).
    82 bytes of data, 2 bytes empty, 4 byte header, 4 byte length.
    
    Do not reference the above event struct directly. Use the following
    functions to gain access to the event table, since the
    ring_buffer_event structure may change in the future.
    
    ring_buffer_event_length(event): get the length of the event.
    	This is the size of the memory used to record this
    	event, and not the size of the data pay load.
    
    ring_buffer_time_delta(event): get the time delta of the event
    	This returns the delta time stamp since the last event.
    	Note: Even though this is in the header, there should
    		be no reason to access this directly, accept
    		for debugging.
    
    ring_buffer_event_data(event): get the data from the event
    	This is the function to use to get the actual data
    	from the event. Note, it is only a pointer to the
    	data inside the buffer. This data must be copied to
    	another location otherwise you risk it being written
    	over in the buffer.
    
    ring_buffer_lock: A way to lock the entire buffer.
    ring_buffer_unlock: unlock the buffer.
    
    ring_buffer_alloc: create a new ring buffer. Can choose between
    	overwrite or consumer/producer mode. Overwrite will
    	overwrite old data, where as consumer producer will
    	throw away new data if the consumer catches up with the
    	producer.  The consumer/producer is the default.
    
    ring_buffer_free: free the ring buffer.
    
    ring_buffer_resize: resize the buffer. Changes the size of each cpu
    	buffer. Note, it is up to the caller to provide that
    	the buffer is not being used while this is happening.
    	This requirement may go away but do not count on it.
    
    ring_buffer_lock_reserve: locks the ring buffer and allocates an
    	entry on the buffer to write to.
    ring_buffer_unlock_commit: unlocks the ring buffer and commits it to
    	the buffer.
    
    ring_buffer_write: writes some data into the ring buffer.
    
    ring_buffer_peek: Look at a next item in the cpu buffer.
    ring_buffer_consume: get the next item in the cpu buffer and
    	consume it. That is, this function increments the head
    	pointer.
    
    ring_buffer_read_start: Start an iterator of a cpu buffer.
    	For now, this disables the cpu buffer, until you issue
    	a finish. This is just because we do not want the iterator
    	to be overwritten. This restriction may change in the future.
    	But note, this is used for static reading of a buffer which
    	is usually done "after" a trace. Live readings would want
    	to use the ring_buffer_consume above, which will not
    	disable the ring buffer.
    
    ring_buffer_read_finish: Finishes the read iterator and reenables
    	the ring buffer.
    
    ring_buffer_iter_peek: Look at the next item in the cpu iterator.
    ring_buffer_read: Read the iterator and increment it.
    ring_buffer_iter_reset: Reset the iterator to point to the beginning
    	of the cpu buffer.
    ring_buffer_iter_empty: Returns true if the iterator is at the end
    	of the cpu buffer.
    
    ring_buffer_size: returns the size in bytes of each cpu buffer.
    	Note, the real size is this times the number of CPUs.
    
    ring_buffer_reset_cpu: Sets the cpu buffer to empty
    ring_buffer_reset: sets all cpu buffers to empty
    
    ring_buffer_swap_cpu: swaps a cpu buffer from one buffer with a
    	cpu buffer of another buffer. This is handy when you
    	want to take a snap shot of a running trace on just one
    	cpu. Having a backup buffer, to swap with facilitates this.
    	Ftrace max latencies use this.
    
    ring_buffer_empty: Returns true if the ring buffer is empty.
    ring_buffer_empty_cpu: Returns true if the cpu buffer is empty.
    
    ring_buffer_record_disable: disable all cpu buffers (read only)
    ring_buffer_record_disable_cpu: disable a single cpu buffer (read only)
    ring_buffer_record_enable: enable all cpu buffers.
    ring_buffer_record_enabl_cpu: enable a single cpu buffer.
    
    ring_buffer_entries: The number of entries in a ring buffer.
    ring_buffer_overruns: The number of entries removed due to writing wrap.
    
    ring_buffer_time_stamp: Get the time stamp used by the ring buffer
    ring_buffer_normalize_time_stamp: normalize the ring buffer time stamp
    	into nanosecs.
    
    I still need to implement the GTOD feature. But we need support from
    the cpu frequency infrastructure.  But this can be done at a later
    time without affecting the ring buffer interface.
    Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    7a8e76a3
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