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diff --git a/doc/README.trace b/doc/README.trace new file mode 100644 index 0000000..b535c06 --- /dev/null +++ b/doc/README.trace @@ -0,0 +1,361 @@ +# +# Copyright (c) 2013 The Chromium OS Authors. +# +# This program is free software; you can redistribute it and/or +# modify it under the terms of the GNU General Public License as +# published by the Free Software Foundatio; either version 2 of +# the License, or (at your option) any later version. +# +# This program is distributed in the hope that it will be useful, +# but WITHOUT ANY WARRANTY; without even the implied warranty of +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +# GNU General Public License for more details. +# +# You should have received a copy of the GNU General Public License +# along with this program; if not, write to the Free Software +# Foundation, Inc., 59 Temple Place, Suite 330, Boston, +# MA 02111-1307 USA +# + +Tracing in U-Boot +================= + +U-Boot supports a simple tracing feature which allows a record of excecution +to be collected and sent to a host machine for analysis. At present the +main use for this is to profile boot time. + + +Overview +-------- + +The trace feature uses GCC's instrument-functions feature to trace all +function entry/exit points. These are then recorded in a memory buffer. +The memory buffer can be saved to the host over a network link using +tftpput or by writing to an attached memory device such as MMC. + +On the host, the file is first converted with a tool called 'proftool', +which extracts useful information from it. The resulting trace output +resembles that emitted by Linux's ftrace feature, so can be visually +displayed by pytimechart. + + +Quick-start using Sandbox +------------------------- + +Sandbox is a build of U-Boot that can run under Linux so it is a convenient +way of trying out tracing before you use it on your actual board. To do +this, follow these steps: + +Add the following to include/configs/sandbox.h (if not already there) + +#define CONFIG_TRACE +#define CONFIG_CMD_TRACE +#define CONFIG_TRACE_BUFFER_SIZE (16 << 20) +#define CONFIG_TRACE_EARLY_SIZE (8 << 20) +#define CONFIG_TRACE_EARLY +#define CONFIG_TRACE_EARLY_ADDR 0x00100000 + +Build sandbox U-Boot with tracing enabled: + +$ make FTRACE=1 O=sandbox sandbox_config +$ make FTRACE=1 O=sandbox + +Run sandbox, wait for a bit of trace information to appear, and then capture +a trace: + +$ ./sandbox/u-boot + + +U-Boot 2013.04-rc2-00100-ga72fcef (Apr 17 2013 - 19:25:24) + +DRAM: 128 MiB +trace: enabled +Using default environment + +In: serial +Out: serial +Err: serial +=>trace stats + 671,406 function sites + 69,712 function calls + 0 untracked function calls + 73,373 traced function calls + 16 maximum observed call depth + 15 call depth limit + 66,491 calls not traced due to depth +=>trace stats + 671,406 function sites + 1,279,450 function calls + 0 untracked function calls + 950,490 traced function calls (333217 dropped due to overflow) + 16 maximum observed call depth + 15 call depth limit + 1,275,767 calls not traced due to depth +=>trace calls 0 e00000 +Call list dumped to 00000000, size 0xae0a40 +=>print +baudrate=115200 +profbase=0 +profoffset=ae0a40 +profsize=e00000 +stderr=serial +stdin=serial +stdout=serial + +Environment size: 117/8188 bytes +=>sb save host 0 trace 0 ${profoffset} +11405888 bytes written in 10 ms (1.1 GiB/s) +=>reset + + +Then run proftool to convert the trace information to ftrace format. + +$ ./sandbox/tools/proftool -m sandbox/System.map -p trace dump-ftrace >trace.txt + +Finally run pytimechart to display it: + +$ pytimechart trace.txt + +Using this tool you can zoom and pan across the trace, with the function +calls on the left and little marks representing the start and end of each +function. + + +CONFIG Options +-------------- + +- CONFIG_TRACE + Enables the trace feature in U-Boot. + +- CONFIG_CMD_TRACE + Enables the trace command. + +- CONFIG_TRACE_BUFFER_SIZE + Size of trace buffer to allocate for U-Boot. This buffer is + used after relocation, as a place to put function tracing + information. The address of the buffer is determined by + the relocation code. + +- CONFIG_TRACE_EARLY + Define this to start tracing early, before relocation. + +- CONFIG_TRACE_EARLY_SIZE + Size of 'early' trace buffer. Before U-Boot has relocated + it doesn't have a proper trace buffer. On many boards + you can define an area of memory to use for the trace + buffer until the 'real' trace buffer is available after + relocation. The contents of this buffer are then copied to + the real buffer. + +- CONFIG_TRACE_EARLY_ADDR + Address of early trace buffer + + +Building U-Boot with Tracing Enabled +------------------------------------ + +Pass 'FTRACE=1' to the U-Boot Makefile to actually instrument the code. +This is kept as a separate option so that it is easy to enable/disable +instrumenting from the command line instead of having to change board +config files. + + +Collecting Trace Data +--------------------- + +When you run U-Boot on your board it will collect trace data up to the +limit of the trace buffer size you have specified. Once that is exhausted +no more data will be collected. + +Collecting trace data has an affect on execution time/performance. You +will notice this particularly with trvial functions - the overhead of +recording their execution may even exceed their normal execution time. +In practice this doesn't matter much so long as you are aware of the +effect. Once you have done your optimisations, turn off tracing before +doing end-to-end timing. + +The best time to start tracing is right at the beginning of U-Boot. The +best time to stop tracing is right at the end. In practice it is hard +to achieve these ideals. + +This implementation enables tracing early in board_init_f(). This means +that it captures most of the board init process, missing only the +early architecture-specific init. However, it also misses the entire +SPL stage if there is one. + +U-Boot typically ends with a 'bootm' command which loads and runs an +OS. There is useful trace data in the execution of that bootm +command. Therefore this implementation provides a way to collect trace +data after bootm has finished processing, but just before it jumps to +the OS. In practical terms, U-Boot runs the 'fakegocmd' environment +variable at this point. This variable should have a short script which +collects the trace data and writes it somewhere. + +Trace data collection relies on a microsecond timer, accesed through +timer_get_us(). So the first think you should do is make sure that +this produces sensible results for your board. Suitable sources for +this timer include high resolution timers, PWMs or profile timers if +available. Most modern SOCs have a suitable timer for this. Make sure +that you mark this timer (and anything it calls) with +__attribute__((no_instrument_function)) so that the trace library can +use it without causing an infinite loop. + + +Commands +-------- + +The trace command has variable sub-commands: + +- stats + Display tracing statistics + +- pause + Pause tracing + +- resume + Resume tracing + +- funclist [<addr> <size>] + Dump a list of functions into the buffer + +- calls [<addr> <size>] + Dump function call trace into buffer + +If the address and size are not given, these are obtained from environment +variables (see below). In any case the environment variables are updated +after the command runs. + + +Environment Variables +--------------------- + +The following are used: + +- profbase + Base address of trace output buffer + +- profoffset + Offset of first unwritten byte in trace output buffer + +- profsize + Size of trace output buffer + +All of these are set by the 'trace calls' command. + +These variables keep track of the amount of data written to the trace +output buffer by the 'trace' command. The trace commands which write data +to the output buffer can use these to specify the buffer to write to, and +update profoffset each time. This allows successive commands to append data +to the same buffer, for example: + + trace funclist 10000 e00000 + trace calls + +(the latter command appends more data to the buffer). + + +- fakegocmd + Specifies commands to run just before booting the OS. This + is a useful time to write the trace data to the host for + processing. + + +Writing Out Trace Data +---------------------- + +Once the trace data is in an output buffer in memory there are various ways +to transmit it to the host. Notably you can use tftput to send the data +over a network link: + +fakegocmd=trace pause; usb start; set autoload n; bootp; + trace calls 10000000 1000000; + tftpput ${profbase} ${profoffset} 192.168.1.4:/tftpboot/calls + +This starts up USB (to talk to an attached USB Ethernet dongle), writes +a trace log to address 10000000 and sends it to a host machine using +TFTP. After this, U-Boot will boot the OS normally, albeit a little +later. + + +Converting Trace Output Data +---------------------------- + +The trace output data is kept in a binary format which is not documented +here. To convert it into something useful, you can use proftool. + +This tool must be given the U-Boot map file and the trace data received +from running that U-Boot. It produces a text output file. + +Options + -m <map_file> + Specify U-Boot map file + + -p <trace_file> + Specifiy profile/trace file + +Commands: + +- dump-ftrace + Write a text dump of the file in Linux ftrace format to stdout + + +Viewing the Trace Data +---------------------- + +You can use pytimechart for this (sudo apt-get pytimechart might work on +your Debian-style machine, and use your favourite search engine to obtain +documentation). It expects the file to have a .txt extension. The program +has terse user interface but is very convenient for viewing U-Boot +profile information. + + +Workflow Suggestions +-------------------- + +The following suggestions may be helpful if you are trying to reduce boot +time: + +1. Enable CONFIG_BOOTSTAGE and CONFIG_BOOTSTAGE_REPORT. This should get +you are helpful overall snapshot of the boot time. + +2. Build U-Boot with tracing and run it. Note the difference in boot time +(it is common for tracing to add 10% to the time) + +3. Collect the trace information as descibed above. Use this to find where +all the time is being spent. + +4. Take a look at that code and see if you can optimise it. Perhaps it is +possible to speed up the initialisation of a device, or remove an unused +feature. + +5. Rebuild, run and collect again. Compare your results. + +6. Keep going until you run out of steam, or your boot is fast enough. + + +Configuring Trace +----------------- + +There are a few parameters in the code that you may want to consider. +There is a function call depth limit (set to 15 by default). When the +stack depth goes above this then no tracing information is recorded. +The maximum depth reached is recorded and displayed by the 'trace stats' +command. + + +Future Work +----------- + +Tracing could be a little tidier in some areas, for example providing +run-time configuration options for trace. + +Some other features that might be useful: + +- Trace filter to select which functions are recorded +- Sample-based profiling using a timer interrupt +- Better control over trace depth +- Compression of trace information + + +Simon Glass <sjg@chromium.org> +April 2013 |