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Diffstat (limited to 'Documentation/trace/ring-buffer-design.txt')
| -rw-r--r-- | Documentation/trace/ring-buffer-design.txt | 56 |
1 files changed, 28 insertions, 28 deletions
diff --git a/Documentation/trace/ring-buffer-design.txt b/Documentation/trace/ring-buffer-design.txt index 5b1d23d604c..d299ff31df5 100644 --- a/Documentation/trace/ring-buffer-design.txt +++ b/Documentation/trace/ring-buffer-design.txt @@ -33,9 +33,9 @@ head_page - a pointer to the page that the reader will use next tail_page - a pointer to the page that will be written to next -commit_page - a pointer to the page with the last finished non nested write. +commit_page - a pointer to the page with the last finished non-nested write. -cmpxchg - hardware assisted atomic transaction that performs the following: +cmpxchg - hardware-assisted atomic transaction that performs the following: A = B iff previous A == C @@ -52,15 +52,15 @@ The Generic Ring Buffer The ring buffer can be used in either an overwrite mode or in producer/consumer mode. -Producer/consumer mode is where the producer were to fill up the +Producer/consumer mode is where if the producer were to fill up the buffer before the consumer could free up anything, the producer will stop writing to the buffer. This will lose most recent events. -Overwrite mode is where the produce were to fill up the buffer +Overwrite mode is where if the producer were to fill up the buffer before the consumer could free up anything, the producer will overwrite the older data. This will lose the oldest events. -No two writers can write at the same time (on the same per cpu buffer), +No two writers can write at the same time (on the same per-cpu buffer), but a writer may interrupt another writer, but it must finish writing before the previous writer may continue. This is very important to the algorithm. The writers act like a "stack". The way interrupts works @@ -79,16 +79,16 @@ the interrupt doing a write as well. Readers can happen at any time. But no two readers may run at the same time, nor can a reader preempt/interrupt another reader. A reader -can not preempt/interrupt a writer, but it may read/consume from the +cannot preempt/interrupt a writer, but it may read/consume from the buffer at the same time as a writer is writing, but the reader must be on another processor to do so. A reader may read on its own processor and can be preempted by a writer. -A writer can preempt a reader, but a reader can not preempt a writer. +A writer can preempt a reader, but a reader cannot preempt a writer. But a reader can read the buffer at the same time (on another processor) as a writer. -The ring buffer is made up of a list of pages held together by a link list. +The ring buffer is made up of a list of pages held together by a linked list. At initialization a reader page is allocated for the reader that is not part of the ring buffer. @@ -102,7 +102,7 @@ the head page. The reader has its own page to use. At start up time, this page is allocated but is not attached to the list. When the reader wants -to read from the buffer, if its page is empty (like it is on start up) +to read from the buffer, if its page is empty (like it is on start-up), it will swap its page with the head_page. The old reader page will become part of the ring buffer and the head_page will be removed. The page after the inserted page (old reader_page) will become the @@ -206,7 +206,7 @@ The main pointers: commit page - the page that last finished a write. -The commit page only is updated by the outer most writer in the +The commit page only is updated by the outermost writer in the writer stack. A writer that preempts another writer will not move the commit page. @@ -281,7 +281,7 @@ with the previous write. The commit pointer points to the last write location that was committed without preempting another write. When a write that preempted another write is committed, it only becomes a pending commit -and will not be a full commit till all writes have been committed. +and will not be a full commit until all writes have been committed. The commit page points to the page that has the last full commit. The tail page points to the page with the last write (before @@ -292,7 +292,7 @@ be several pages ahead. If the tail page catches up to the commit page then no more writes may take place (regardless of the mode of the ring buffer: overwrite and produce/consumer). -The order of pages are: +The order of pages is: head page commit page @@ -311,7 +311,7 @@ Possible scenario: There is a special case that the head page is after either the commit page and possibly the tail page. That is when the commit (and tail) page has been swapped with the reader page. This is because the head page is always -part of the ring buffer, but the reader page is not. When ever there +part of the ring buffer, but the reader page is not. Whenever there has been less than a full page that has been committed inside the ring buffer, and a reader swaps out a page, it will be swapping out the commit page. @@ -338,7 +338,7 @@ and a reader swaps out a page, it will be swapping out the commit page. In this case, the head page will not move when the tail and commit move back into the ring buffer. -The reader can not swap a page into the ring buffer if the commit page +The reader cannot swap a page into the ring buffer if the commit page is still on that page. If the read meets the last commit (real commit not pending or reserved), then there is nothing more to read. The buffer is considered empty until another full commit finishes. @@ -395,7 +395,7 @@ The main idea behind the lockless algorithm is to combine the moving of the head_page pointer with the swapping of pages with the reader. State flags are placed inside the pointer to the page. To do this, each page must be aligned in memory by 4 bytes. This will allow the 2 -least significant bits of the address to be used as flags. Since +least significant bits of the address to be used as flags, since they will always be zero for the address. To get the address, simply mask out the flags. @@ -460,7 +460,7 @@ When the reader tries to swap the page with the ring buffer, it will also use cmpxchg. If the flag bit in the pointer to the head page does not have the HEADER flag set, the compare will fail and the reader will need to look for the new head page and try again. -Note, the flag UPDATE and HEADER are never set at the same time. +Note, the flags UPDATE and HEADER are never set at the same time. The reader swaps the reader page as follows: @@ -539,7 +539,7 @@ updated to the reader page. | +-----------------------------+ | +------------------------------------+ -Another important point. The page that the reader page points back to +Another important point: The page that the reader page points back to by its previous pointer (the one that now points to the new head page) never points back to the reader page. That is because the reader page is not part of the ring buffer. Traversing the ring buffer via the next pointers @@ -572,7 +572,7 @@ not be able to swap the head page from the buffer, nor will it be able to move the head page, until the writer is finished with the move. This eliminates any races that the reader can have on the writer. The reader -must spin, and this is why the reader can not preempt the writer. +must spin, and this is why the reader cannot preempt the writer. tail page | @@ -659,9 +659,9 @@ before pushing the head page. If it is, then it can be assumed that the tail page wrapped the buffer, and we must drop new writes. This is not a race condition, because the commit page can only be moved -by the outter most writer (the writer that was preempted). +by the outermost writer (the writer that was preempted). This means that the commit will not move while a writer is moving the -tail page. The reader can not swap the reader page if it is also being +tail page. The reader cannot swap the reader page if it is also being used as the commit page. The reader can simply check that the commit is off the reader page. Once the commit page leaves the reader page it will never go back on it unless a reader does another swap with the @@ -733,7 +733,7 @@ The write converts the head page pointer to UPDATE. --->| |<---| |<---| |<---| |<--- +---+ +---+ +---+ +---+ -But if a nested writer preempts here. It will see that the next +But if a nested writer preempts here, it will see that the next page is a head page, but it is also nested. It will detect that it is nested and will save that information. The detection is the fact that it sees the UPDATE flag instead of a HEADER or NORMAL @@ -761,7 +761,7 @@ to NORMAL. --->| |<---| |<---| |<---| |<--- +---+ +---+ +---+ +---+ -After the nested writer finishes, the outer most writer will convert +After the nested writer finishes, the outermost writer will convert the UPDATE pointer to NORMAL. @@ -812,7 +812,7 @@ head page. +---+ +---+ +---+ +---+ The nested writer moves the tail page forward. But does not set the old -update page to NORMAL because it is not the outer most writer. +update page to NORMAL because it is not the outermost writer. tail page | @@ -892,7 +892,7 @@ It will return to the first writer. --->| |<---| |<---| |<---| |<--- +---+ +---+ +---+ +---+ -The first writer can not know atomically test if the tail page moved +The first writer cannot know atomically if the tail page moved while it updates the HEAD page. It will then update the head page to what it thinks is the new head page. @@ -923,9 +923,9 @@ if the tail page is either where it use to be or on the next page: --->| |<---| |<---| |<---| |<--- +---+ +---+ +---+ +---+ -If tail page != A and tail page does not equal B, then it must reset the -pointer back to NORMAL. The fact that it only needs to worry about -nested writers, it only needs to check this after setting the HEAD page. +If tail page != A and tail page != B, then it must reset the pointer +back to NORMAL. The fact that it only needs to worry about nested +writers means that it only needs to check this after setting the HEAD page. (first writer) @@ -939,7 +939,7 @@ nested writers, it only needs to check this after setting the HEAD page. +---+ +---+ +---+ +---+ Now the writer can update the head page. This is also why the head page must -remain in UPDATE and only reset by the outer most writer. This prevents +remain in UPDATE and only reset by the outermost writer. This prevents the reader from seeing the incorrect head page. |