From 77ae365eca895061c8bf2b2e3ae1d9ea62869739 Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Fri, 27 Mar 2009 11:00:29 -0400 Subject: ring-buffer: make lockless This patch converts the ring buffers into a completely lockless buffer recording system. The read side still takes locks since we still serialize readers. But the writers are the ones that must be lockless (those can happen in NMIs). The main change is to the "head_page" pointer. We write to the tail, and read from the head. The "head_page" pointer in the cpu buffer is now just a reference to where to look. The real head page is now kept in the head_page->list->prev->next pointer. That is, in the list head of the previous page we set flags. The list pages are allocated to be aligned such that the lowest significant bits are always zero pointing to the list. This gives us play to put in flags to their pointers. bit 0: set when the page is a head page bit 1: set when the writer is moving the page (for overwrite mode) cmpxchg is used to update the pointer. When the writer wraps the buffer and the tail meets the head, in overwrite mode, the writer must move the head page forward. It first uses cmpxchg to change the pointer flag from 1 to 2. Once this is done, the reader on another CPU will not take the page from the buffer. The writers need to protect against interrupts (we don't bother with disabling interrupts because NMIs are allowed to write too). After the writer sets the pointer flag to 2, it takes care to manage interrupts coming in. This is discribed in detail within the comments of the code. Changes in version 2: - Let reader reset entries value of header page. - Fix tail page passing commit page on reader page test. - Always increment entries and write counter in rb_tail_page_update - Add safety check in rb_set_commit_to_write to break out of infinite loop - add mask in rb_is_reader_page [ Impact: lock free writing to the ring buffer ] Signed-off-by: Steven Rostedt --- include/linux/ring_buffer.h | 1 - 1 file changed, 1 deletion(-) (limited to 'include/linux/ring_buffer.h') diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h index 29f8599e6bea..7fca71693ae7 100644 --- a/include/linux/ring_buffer.h +++ b/include/linux/ring_buffer.h @@ -170,7 +170,6 @@ unsigned long ring_buffer_overruns(struct ring_buffer *buffer); unsigned long ring_buffer_entries_cpu(struct ring_buffer *buffer, int cpu); unsigned long ring_buffer_overrun_cpu(struct ring_buffer *buffer, int cpu); unsigned long ring_buffer_commit_overrun_cpu(struct ring_buffer *buffer, int cpu); -unsigned long ring_buffer_nmi_dropped_cpu(struct ring_buffer *buffer, int cpu); u64 ring_buffer_time_stamp(struct ring_buffer *buffer, int cpu); void ring_buffer_normalize_time_stamp(struct ring_buffer *buffer, -- cgit From dc892f7339af2d125478b800edb9081d6149665b Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Thu, 3 Sep 2009 15:33:41 -0400 Subject: ring-buffer: remove ring_buffer_event_discard The function ring_buffer_event_discard can be used on any item in the ring buffer, even after the item was committed. This function provides no safety nets and is very race prone. An item may be safely removed from the ring buffer before it is committed with the ring_buffer_discard_commit. Since there are currently no users of this function, and because this function is racey and error prone, this patch removes it altogether. Note, removing this function also allows the counters to ignore all discarded events (patches will follow). Signed-off-by: Steven Rostedt --- include/linux/ring_buffer.h | 14 -------------- 1 file changed, 14 deletions(-) (limited to 'include/linux/ring_buffer.h') diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h index 7fca71693ae7..e061b4ecdc3a 100644 --- a/include/linux/ring_buffer.h +++ b/include/linux/ring_buffer.h @@ -74,20 +74,6 @@ ring_buffer_event_time_delta(struct ring_buffer_event *event) return event->time_delta; } -/* - * ring_buffer_event_discard can discard any event in the ring buffer. - * it is up to the caller to protect against a reader from - * consuming it or a writer from wrapping and replacing it. - * - * No external protection is needed if this is called before - * the event is commited. But in that case it would be better to - * use ring_buffer_discard_commit. - * - * Note, if an event that has not been committed is discarded - * with ring_buffer_event_discard, it must still be committed. - */ -void ring_buffer_event_discard(struct ring_buffer_event *event); - /* * ring_buffer_discard_commit will remove an event that has not * ben committed yet. If this is used, then ring_buffer_unlock_commit -- cgit From 85bac32c4a52c592b857f2c360cc5ec93a097d70 Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Fri, 4 Sep 2009 14:24:40 -0400 Subject: ring-buffer: only enable ring_buffer_swap_cpu when needed Since the ability to swap the cpu buffers adds a small overhead to the recording of a trace, we only want to add it when needed. Only the irqsoff and preemptoff tracers use this feature, and both are not recommended for production kernels. This patch disables its use when neither irqsoff nor preemptoff is configured. Signed-off-by: Steven Rostedt --- include/linux/ring_buffer.h | 9 +++++++++ 1 file changed, 9 insertions(+) (limited to 'include/linux/ring_buffer.h') diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h index e061b4ecdc3a..5fcc31ed5771 100644 --- a/include/linux/ring_buffer.h +++ b/include/linux/ring_buffer.h @@ -140,8 +140,17 @@ unsigned long ring_buffer_size(struct ring_buffer *buffer); void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu); void ring_buffer_reset(struct ring_buffer *buffer); +#ifdef CONFIG_RING_BUFFER_ALLOW_SWAP int ring_buffer_swap_cpu(struct ring_buffer *buffer_a, struct ring_buffer *buffer_b, int cpu); +#else +static inline int +ring_buffer_swap_cpu(struct ring_buffer *buffer_a, + struct ring_buffer *buffer_b, int cpu) +{ + return -ENODEV; +} +#endif int ring_buffer_empty(struct ring_buffer *buffer); int ring_buffer_empty_cpu(struct ring_buffer *buffer, int cpu); -- cgit