In a private email conversation I explained how the ring buffer page worked by using silly ASCII art. Ingo suggested that I add that to the comments of the code. Here it is.
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| @@ -21,6 +21,74 @@ · 20 unmodified lines | |||
| 21 | 21 | ||
| 22 | #include "trace.h" | 22 | #include "trace.h" |
| 23 | 23 | ||
| 24 | /* | ||
| 25 | * The ring buffer is made up of a list of pages. A separate list of pages is | ||
| 26 | * allocated for each CPU. A writer may only write to a buffer that is | ||
| 27 | * associated with the CPU it is currently executing on. A reader may read | ||
| 28 | * from any per cpu buffer. | ||
| 29 | * | ||
| 30 | * The reader is special. For each per cpu buffer, the reader has its own | ||
| 31 | * reader page. When a reader has read the entire reader page, this reader | ||
| 32 | * page is swapped with another page in the ring buffer. | ||
| 33 | * | ||
| 34 | * Now, as long as the writer is off the reader page, the reader can do what | ||
| 35 | * ever it wants with that page. The writer will never write to that page | ||
| 36 | * again (as long as it is out of the ring buffer). | ||
| 37 | * | ||
| 38 | * Here's some silly ASCII art. | ||
| 39 | * | ||
| 40 | * +------+ | ||
| 41 | * |reader| RING BUFFER | ||
| 42 | * |page | | ||
| 43 | * +------+ +---+ +---+ +---+ | ||
| 44 | * | |-->| |-->| | | ||
| 45 | * +---+ +---+ +---+ | ||
| 46 | * ^ | | ||
| 47 | * | | | ||
| 48 | * +---------------+ | ||
| 49 | * | ||
| 50 | * | ||
| 51 | * +------+ | ||
| 52 | * |reader| RING BUFFER | ||
| 53 | * |page |------------------v | ||
| 54 | * +------+ +---+ +---+ +---+ | ||
| 55 | * | |-->| |-->| | | ||
| 56 | * +---+ +---+ +---+ | ||
| 57 | * ^ | | ||
| 58 | * | | | ||
| 59 | * +---------------+ | ||
| 60 | * | ||
| 61 | * | ||
| 62 | * +------+ | ||
| 63 | * |reader| RING BUFFER | ||
| 64 | * |page |------------------v | ||
| 65 | * +------+ +---+ +---+ +---+ | ||
| 66 | * ^ | |-->| |-->| | | ||
| 67 | * | +---+ +---+ +---+ | ||
| 68 | * | | | ||
| 69 | * | | | ||
| 70 | * +------------------------------+ | ||
| 71 | * | ||
| 72 | * | ||
| 73 | * +------+ | ||
| 74 | * |buffer| RING BUFFER | ||
| 75 | * |page |------------------v | ||
| 76 | * +------+ +---+ +---+ +---+ | ||
| 77 | * ^ | | | |-->| | | ||
| 78 | * | New +---+ +---+ +---+ | ||
| 79 | * | Reader------^ | | ||
| 80 | * | page | | ||
| 81 | * +------------------------------+ | ||
| 82 | * | ||
| 83 | * | ||
| 84 | * After we make this swap, the reader can hand this page off to the splice | ||
| 85 | * code and be done with it. It can even allocate a new page if it needs to | ||
| 86 | * and swap that into the ring buffer. | ||
| 87 | * | ||
| 88 | * We will be using cmpxchg soon to make all this lockless. | ||
| 89 | * | ||
| 90 | */ | ||
| 91 | |||
| 24 | /* | 92 | /* |
| 25 | * A fast way to enable or disable all ring buffers is to | 93 | * A fast way to enable or disable all ring buffers is to |
| 26 | * call tracing_on or tracing_off. Turning off the ring buffers | 94 | * call tracing_on or tracing_off. Turning off the ring buffers |