1relay interface (formerly relayfs) 2================================== 3 4The relay interface provides a means for kernel applications to 5efficiently log and transfer large quantities of data from the kernel 6to userspace via user-defined 'relay channels'. 7 8A 'relay channel' is a kernel->user data relay mechanism implemented 9as a set of per-cpu kernel buffers ('channel buffers'), each 10represented as a regular file ('relay file') in user space. Kernel 11clients write into the channel buffers using efficient write 12functions; these automatically log into the current cpu's channel 13buffer. User space applications mmap() or read() from the relay files 14and retrieve the data as it becomes available. The relay files 15themselves are files created in a host filesystem, e.g. debugfs, and 16are associated with the channel buffers using the API described below. 17 18The format of the data logged into the channel buffers is completely 19up to the kernel client; the relay interface does however provide 20hooks which allow kernel clients to impose some structure on the 21buffer data. The relay interface doesn't implement any form of data 22filtering - this also is left to the kernel client. The purpose is to 23keep things as simple as possible. 24 25This document provides an overview of the relay interface API. The 26details of the function parameters are documented along with the 27functions in the relay interface code - please see that for details. 28 29Semantics 30========= 31 32Each relay channel has one buffer per CPU, each buffer has one or more 33sub-buffers. Messages are written to the first sub-buffer until it is 34too full to contain a new message, in which case it is written to 35the next (if available). Messages are never split across sub-buffers. 36At this point, userspace can be notified so it empties the first 37sub-buffer, while the kernel continues writing to the next. 38 39When notified that a sub-buffer is full, the kernel knows how many 40bytes of it are padding i.e. unused space occurring because a complete 41message couldn't fit into a sub-buffer. Userspace can use this 42knowledge to copy only valid data. 43 44After copying it, userspace can notify the kernel that a sub-buffer 45has been consumed. 46 47A relay channel can operate in a mode where it will overwrite data not 48yet collected by userspace, and not wait for it to be consumed. 49 50The relay channel itself does not provide for communication of such 51data between userspace and kernel, allowing the kernel side to remain 52simple and not impose a single interface on userspace. It does 53provide a set of examples and a separate helper though, described 54below. 55 56The read() interface both removes padding and internally consumes the 57read sub-buffers; thus in cases where read(2) is being used to drain 58the channel buffers, special-purpose communication between kernel and 59user isn't necessary for basic operation. 60 61One of the major goals of the relay interface is to provide a low 62overhead mechanism for conveying kernel data to userspace. While the 63read() interface is easy to use, it's not as efficient as the mmap() 64approach; the example code attempts to make the tradeoff between the 65two approaches as small as possible. 66 67klog and relay-apps example code 68================================ 69 70The relay interface itself is ready to use, but to make things easier, 71a couple simple utility functions and a set of examples are provided. 72 73The relay-apps example tarball, available on the relay sourceforge 74site, contains a set of self-contained examples, each consisting of a 75pair of .c files containing boilerplate code for each of the user and 76kernel sides of a relay application. When combined these two sets of 77boilerplate code provide glue to easily stream data to disk, without 78having to bother with mundane housekeeping chores. 79 80The 'klog debugging functions' patch (klog.patch in the relay-apps 81tarball) provides a couple of high-level logging functions to the 82kernel which allow writing formatted text or raw data to a channel, 83regardless of whether a channel to write into exists or not, or even 84whether the relay interface is compiled into the kernel or not. These 85functions allow you to put unconditional 'trace' statements anywhere 86in the kernel or kernel modules; only when there is a 'klog handler' 87registered will data actually be logged (see the klog and kleak 88examples for details). 89 90It is of course possible to use the relay interface from scratch, 91i.e. without using any of the relay-apps example code or klog, but 92you'll have to implement communication between userspace and kernel, 93allowing both to convey the state of buffers (full, empty, amount of 94padding). The read() interface both removes padding and internally 95consumes the read sub-buffers; thus in cases where read(2) is being 96used to drain the channel buffers, special-purpose communication 97between kernel and user isn't necessary for basic operation. Things 98such as buffer-full conditions would still need to be communicated via 99some channel though. 100 101klog and the relay-apps examples can be found in the relay-apps 102tarball on http://relayfs.sourceforge.net 103 104The relay interface user space API 105================================== 106 107The relay interface implements basic file operations for user space 108access to relay channel buffer data. Here are the file operations 109that are available and some comments regarding their behavior: 110 111open() enables user to open an _existing_ channel buffer. 112 113mmap() results in channel buffer being mapped into the caller's 114 memory space. Note that you can't do a partial mmap - you 115 must map the entire file, which is NRBUF * SUBBUFSIZE. 116 117read() read the contents of a channel buffer. The bytes read are 118 'consumed' by the reader, i.e. they won't be available 119 again to subsequent reads. If the channel is being used 120 in no-overwrite mode (the default), it can be read at any 121 time even if there's an active kernel writer. If the 122 channel is being used in overwrite mode and there are 123 active channel writers, results may be unpredictable - 124 users should make sure that all logging to the channel has 125 ended before using read() with overwrite mode. Sub-buffer 126 padding is automatically removed and will not be seen by 127 the reader. 128 129sendfile() transfer data from a channel buffer to an output file 130 descriptor. Sub-buffer padding is automatically removed 131 and will not be seen by the reader. 132 133poll() POLLIN/POLLRDNORM/POLLERR supported. User applications are 134 notified when sub-buffer boundaries are crossed. 135 136close() decrements the channel buffer's refcount. When the refcount 137 reaches 0, i.e. when no process or kernel client has the 138 buffer open, the channel buffer is freed. 139 140In order for a user application to make use of relay files, the 141host filesystem must be mounted. For example, 142 143 mount -t debugfs debugfs /sys/kernel/debug 144 145NOTE: the host filesystem doesn't need to be mounted for kernel 146 clients to create or use channels - it only needs to be 147 mounted when user space applications need access to the buffer 148 data. 149 150 151The relay interface kernel API 152============================== 153 154Here's a summary of the API the relay interface provides to in-kernel clients: 155 156TBD(curr. line MT:/API/) 157 channel management functions: 158 159 relay_open(base_filename, parent, subbuf_size, n_subbufs, 160 callbacks, private_data) 161 relay_close(chan) 162 relay_flush(chan) 163 relay_reset(chan) 164 165 channel management typically called on instigation of userspace: 166 167 relay_subbufs_consumed(chan, cpu, subbufs_consumed) 168 169 write functions: 170 171 relay_write(chan, data, length) 172 __relay_write(chan, data, length) 173 relay_reserve(chan, length) 174 175 callbacks: 176 177 subbuf_start(buf, subbuf, prev_subbuf, prev_padding) 178 buf_mapped(buf, filp) 179 buf_unmapped(buf, filp) 180 create_buf_file(filename, parent, mode, buf, is_global) 181 remove_buf_file(dentry) 182 183 helper functions: 184 185 relay_buf_full(buf) 186 subbuf_start_reserve(buf, length) 187 188 189Creating a channel 190------------------ 191 192relay_open() is used to create a channel, along with its per-cpu 193channel buffers. Each channel buffer will have an associated file 194created for it in the host filesystem, which can be and mmapped or 195read from in user space. The files are named basename0...basenameN-1 196where N is the number of online cpus, and by default will be created 197in the root of the filesystem (if the parent param is NULL). If you 198want a directory structure to contain your relay files, you should 199create it using the host filesystem's directory creation function, 200e.g. debugfs_create_dir(), and pass the parent directory to 201relay_open(). Users are responsible for cleaning up any directory 202structure they create, when the channel is closed - again the host 203filesystem's directory removal functions should be used for that, 204e.g. debugfs_remove(). 205 206In order for a channel to be created and the host filesystem's files 207associated with its channel buffers, the user must provide definitions 208for two callback functions, create_buf_file() and remove_buf_file(). 209create_buf_file() is called once for each per-cpu buffer from 210relay_open() and allows the user to create the file which will be used 211to represent the corresponding channel buffer. The callback should 212return the dentry of the file created to represent the channel buffer. 213remove_buf_file() must also be defined; it's responsible for deleting 214the file(s) created in create_buf_file() and is called during 215relay_close(). 216 217Here are some typical definitions for these callbacks, in this case 218using debugfs: 219 220/* 221 * create_buf_file() callback. Creates relay file in debugfs. 222 */ 223static struct dentry *create_buf_file_handler(const char *filename, 224 struct dentry *parent, 225 umode_t mode, 226 struct rchan_buf *buf, 227 int *is_global) 228{ 229 return debugfs_create_file(filename, mode, parent, buf, 230 &relay_file_operations); 231} 232 233/* 234 * remove_buf_file() callback. Removes relay file from debugfs. 235 */ 236static int remove_buf_file_handler(struct dentry *dentry) 237{ 238 debugfs_remove(dentry); 239 240 return 0; 241} 242 243/* 244 * relay interface callbacks 245 */ 246static struct rchan_callbacks relay_callbacks = 247{ 248 .create_buf_file = create_buf_file_handler, 249 .remove_buf_file = remove_buf_file_handler, 250}; 251 252And an example relay_open() invocation using them: 253 254 chan = relay_open("cpu", NULL, SUBBUF_SIZE, N_SUBBUFS, &relay_callbacks, NULL); 255 256If the create_buf_file() callback fails, or isn't defined, channel 257creation and thus relay_open() will fail. 258 259The total size of each per-cpu buffer is calculated by multiplying the 260number of sub-buffers by the sub-buffer size passed into relay_open(). 261The idea behind sub-buffers is that they're basically an extension of 262double-buffering to N buffers, and they also allow applications to 263easily implement random-access-on-buffer-boundary schemes, which can 264be important for some high-volume applications. The number and size 265of sub-buffers is completely dependent on the application and even for 266the same application, different conditions will warrant different 267values for these parameters at different times. Typically, the right 268values to use are best decided after some experimentation; in general, 269though, it's safe to assume that having only 1 sub-buffer is a bad 270idea - you're guaranteed to either overwrite data or lose events 271depending on the channel mode being used. 272 273The create_buf_file() implementation can also be defined in such a way 274as to allow the creation of a single 'global' buffer instead of the 275default per-cpu set. This can be useful for applications interested 276mainly in seeing the relative ordering of system-wide events without 277the need to bother with saving explicit timestamps for the purpose of 278merging/sorting per-cpu files in a postprocessing step. 279 280To have relay_open() create a global buffer, the create_buf_file() 281implementation should set the value of the is_global outparam to a 282non-zero value in addition to creating the file that will be used to 283represent the single buffer. In the case of a global buffer, 284create_buf_file() and remove_buf_file() will be called only once. The 285normal channel-writing functions, e.g. relay_write(), can still be 286used - writes from any cpu will transparently end up in the global 287buffer - but since it is a global buffer, callers should make sure 288they use the proper locking for such a buffer, either by wrapping 289writes in a spinlock, or by copying a write function from relay.h and 290creating a local version that internally does the proper locking. 291 292The private_data passed into relay_open() allows clients to associate 293user-defined data with a channel, and is immediately available 294(including in create_buf_file()) via chan->private_data or 295buf->chan->private_data. 296 297Buffer-only channels 298-------------------- 299 300These channels have no files associated and can be created with 301relay_open(NULL, NULL, ...). Such channels are useful in scenarios such 302as when doing early tracing in the kernel, before the VFS is up. In these 303cases, one may open a buffer-only channel and then call 304relay_late_setup_files() when the kernel is ready to handle files, 305to expose the buffered data to the userspace. 306 307Channel 'modes' 308--------------- 309 310relay channels can be used in either of two modes - 'overwrite' or 311'no-overwrite'. The mode is entirely determined by the implementation 312of the subbuf_start() callback, as described below. The default if no 313subbuf_start() callback is defined is 'no-overwrite' mode. If the 314default mode suits your needs, and you plan to use the read() 315interface to retrieve channel data, you can ignore the details of this 316section, as it pertains mainly to mmap() implementations. 317 318In 'overwrite' mode, also known as 'flight recorder' mode, writes 319continuously cycle around the buffer and will never fail, but will 320unconditionally overwrite old data regardless of whether it's actually 321been consumed. In no-overwrite mode, writes will fail, i.e. data will 322be lost, if the number of unconsumed sub-buffers equals the total 323number of sub-buffers in the channel. It should be clear that if 324there is no consumer or if the consumer can't consume sub-buffers fast 325enough, data will be lost in either case; the only difference is 326whether data is lost from the beginning or the end of a buffer. 327 328As explained above, a relay channel is made of up one or more 329per-cpu channel buffers, each implemented as a circular buffer 330subdivided into one or more sub-buffers. Messages are written into 331the current sub-buffer of the channel's current per-cpu buffer via the 332write functions described below. Whenever a message can't fit into 333the current sub-buffer, because there's no room left for it, the 334client is notified via the subbuf_start() callback that a switch to a 335new sub-buffer is about to occur. The client uses this callback to 1) 336initialize the next sub-buffer if appropriate 2) finalize the previous 337sub-buffer if appropriate and 3) return a boolean value indicating 338whether or not to actually move on to the next sub-buffer. 339 340To implement 'no-overwrite' mode, the userspace client would provide 341an implementation of the subbuf_start() callback something like the 342following: 343 344static int subbuf_start(struct rchan_buf *buf, 345 void *subbuf, 346 void *prev_subbuf, 347 unsigned int prev_padding) 348{ 349 if (prev_subbuf) 350 *((unsigned *)prev_subbuf) = prev_padding; 351 352 if (relay_buf_full(buf)) 353 return 0; 354 355 subbuf_start_reserve(buf, sizeof(unsigned int)); 356 357 return 1; 358} 359 360If the current buffer is full, i.e. all sub-buffers remain unconsumed, 361the callback returns 0 to indicate that the buffer switch should not 362occur yet, i.e. until the consumer has had a chance to read the 363current set of ready sub-buffers. For the relay_buf_full() function 364to make sense, the consumer is responsible for notifying the relay 365interface when sub-buffers have been consumed via 366relay_subbufs_consumed(). Any subsequent attempts to write into the 367buffer will again invoke the subbuf_start() callback with the same 368parameters; only when the consumer has consumed one or more of the 369ready sub-buffers will relay_buf_full() return 0, in which case the 370buffer switch can continue. 371 372The implementation of the subbuf_start() callback for 'overwrite' mode 373would be very similar: 374 375static int subbuf_start(struct rchan_buf *buf, 376 void *subbuf, 377 void *prev_subbuf, 378 size_t prev_padding) 379{ 380 if (prev_subbuf) 381 *((unsigned *)prev_subbuf) = prev_padding; 382 383 subbuf_start_reserve(buf, sizeof(unsigned int)); 384 385 return 1; 386} 387 388In this case, the relay_buf_full() check is meaningless and the 389callback always returns 1, causing the buffer switch to occur 390unconditionally. It's also meaningless for the client to use the 391relay_subbufs_consumed() function in this mode, as it's never 392consulted. 393 394The default subbuf_start() implementation, used if the client doesn't 395define any callbacks, or doesn't define the subbuf_start() callback, 396implements the simplest possible 'no-overwrite' mode, i.e. it does 397nothing but return 0. 398 399Header information can be reserved at the beginning of each sub-buffer 400by calling the subbuf_start_reserve() helper function from within the 401subbuf_start() callback. This reserved area can be used to store 402whatever information the client wants. In the example above, room is 403reserved in each sub-buffer to store the padding count for that 404sub-buffer. This is filled in for the previous sub-buffer in the 405subbuf_start() implementation; the padding value for the previous 406sub-buffer is passed into the subbuf_start() callback along with a 407pointer to the previous sub-buffer, since the padding value isn't 408known until a sub-buffer is filled. The subbuf_start() callback is 409also called for the first sub-buffer when the channel is opened, to 410give the client a chance to reserve space in it. In this case the 411previous sub-buffer pointer passed into the callback will be NULL, so 412the client should check the value of the prev_subbuf pointer before 413writing into the previous sub-buffer. 414 415Writing to a channel 416-------------------- 417 418Kernel clients write data into the current cpu's channel buffer using 419relay_write() or __relay_write(). relay_write() is the main logging 420function - it uses local_irqsave() to protect the buffer and should be 421used if you might be logging from interrupt context. If you know 422you'll never be logging from interrupt context, you can use 423__relay_write(), which only disables preemption. These functions 424don't return a value, so you can't determine whether or not they 425failed - the assumption is that you wouldn't want to check a return 426value in the fast logging path anyway, and that they'll always succeed 427unless the buffer is full and no-overwrite mode is being used, in 428which case you can detect a failed write in the subbuf_start() 429callback by calling the relay_buf_full() helper function. 430 431relay_reserve() is used to reserve a slot in a channel buffer which 432can be written to later. This would typically be used in applications 433that need to write directly into a channel buffer without having to 434stage data in a temporary buffer beforehand. Because the actual write 435may not happen immediately after the slot is reserved, applications 436using relay_reserve() can keep a count of the number of bytes actually 437written, either in space reserved in the sub-buffers themselves or as 438a separate array. See the 'reserve' example in the relay-apps tarball 439at http://relayfs.sourceforge.net for an example of how this can be 440done. Because the write is under control of the client and is 441separated from the reserve, relay_reserve() doesn't protect the buffer 442at all - it's up to the client to provide the appropriate 443synchronization when using relay_reserve(). 444 445Closing a channel 446----------------- 447 448The client calls relay_close() when it's finished using the channel. 449The channel and its associated buffers are destroyed when there are no 450longer any references to any of the channel buffers. relay_flush() 451forces a sub-buffer switch on all the channel buffers, and can be used 452to finalize and process the last sub-buffers before the channel is 453closed. 454 455Misc 456---- 457 458Some applications may want to keep a channel around and re-use it 459rather than open and close a new channel for each use. relay_reset() 460can be used for this purpose - it resets a channel to its initial 461state without reallocating channel buffer memory or destroying 462existing mappings. It should however only be called when it's safe to 463do so, i.e. when the channel isn't currently being written to. 464 465Finally, there are a couple of utility callbacks that can be used for 466different purposes. buf_mapped() is called whenever a channel buffer 467is mmapped from user space and buf_unmapped() is called when it's 468unmapped. The client can use this notification to trigger actions 469within the kernel application, such as enabling/disabling logging to 470the channel. 471 472 473Resources 474========= 475 476For news, example code, mailing list, etc. see the relay interface homepage: 477 478 http://relayfs.sourceforge.net 479 480 481Credits 482======= 483 484The ideas and specs for the relay interface came about as a result of 485discussions on tracing involving the following: 486 487Michel Dagenais <michel.dagenais@polymtl.ca> 488Richard Moore <richardj_moore@uk.ibm.com> 489Bob Wisniewski <bob@watson.ibm.com> 490Karim Yaghmour <karim@opersys.com> 491Tom Zanussi <zanussi@us.ibm.com> 492 493Also thanks to Hubertus Franke for a lot of useful suggestions and bug 494reports. 495