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    SubjectRe: [PATCH 1/3] Documention for trace records (trec).
    On Sat, 24 Mar 2007 16:51:37 -0700 Wink Saville wrote:

    > Trec is a light weight tracing mechanism that places
    > trace information into a buffer. The contents of the
    > buffer is dumped when errors occurs or when enabled
    > via SYSRQ commands.
    >
    > Signed-off-by: Wink Saville <wink@saville.com>
    > ---
    > Documentation/trec.txt | 87 ++++++++++++++++++++++++++++++++++++++++++++++++
    > 1 files changed, 87 insertions(+), 0 deletions(-)
    > create mode 100644 Documentation/trec.txt
    >
    > diff --git a/Documentation/trec.txt b/Documentation/trec.txt
    > new file mode 100644
    > index 0000000..2275edd
    > --- /dev/null
    > +++ b/Documentation/trec.txt
    > @@ -0,0 +1,87 @@
    > +Title : Trace Records
    > +Authors : Wink Saville <wink@saville.com>
    > +
    > +CONTENTS
    > +
    > +1. Concepts
    > +2. Architectures Supported
    > +3. Configuring
    > +4. API Reference
    > +5. Overhead
    > +6. TODO
    > +
    > +
    > +1. Concepts
    > +
    > +Trace records are a light weight tracing technique that time stamps

    lightweight ... timestamps

    > +small amounts of information and stores them in a buffer. TREC's are
    > +light enough that they may be sprinkled most anywhere in the kernel
    > +and have very little performance impact.
    > +
    > +For instance they can be placed in the scheduler and ISR's to watch
    > +the interaction between ISR's and the scheduler. They can be placed
    > +in memory handling routines to determine how and when memory is
    > +allocated and freed.
    > +
    > +In the current default configuration the trec's are dumped by calling
    > +trec_print_snapshot when die() or panic() are called as well as when

    is

    > +the kernel itself page faults in do_page_fault.
    > +
    > +If CONFIG_MAGIC_SYSRQ is the 'y' command will execute trec_snapshot
    ^
    enabled,

    > +and the 'z' command will print the current snapshot.
    > +
    > +A general macro TREC allows trec_write to be invoked as a macro and
    > +TRECC allows it to be invoked conditionally. See include/linux/trec.h
    > +for the current set of macros.
    > +
    > +2. Architectures Supported
    > +
    > +Should support all architectures has been tested only on:
    ^
    ;
    > +
    > +- X86_64
    > +
    > +
    > +3. Configuring
    > +
    > +Since trec's are implemented as a device driver they are configured
    > +by enabling support in the "Device Drivers" section of as they could
    ^
    ??

    > +be used early being a module is not supported.
    > +
    > +
    > +4. API Reference
    > +
    > +Trec supports the following API:
    > +
    > +void trec_init(void):
    > +
    > + Initialize the module, this may be called before the driver is loaded
    ^
    ; (or maybe :)

    > + if it is desired to use trec's early.
    > +
    > +void trec_write(unsigned long pc, int pid, unsigned long v1, unsigned long v2);
    > +
    > + This is the routine used to write into the buffer. pc is the program counter

    end above line with ,

    > + pid is the process id and v1 and v2 are two parameters.
    > +
    > +void trec_snapshot(void);
    > +
    > + Calling this function takes a snapshot of the current trec buffer so that it
    > + will not be modified. This is called prior to printing the snapshot via
    > + trec_print_snapshot.
    > +
    > +void trec_print_snapshot(void);
    > +
    > + Print the snapshot.
    > +
    > +5. Overhead
    > +
    > +Measured on a 2.4GHZ Core 2 Duo the readings between two TREC's is
    > +270 tics of the rdtsc or about 0.1us. No attempt has been made to
    > +optimize and less information can be collected if the overhead
    > +is still to high.

    too

    > +
    > +
    > +6. TODO
    > +
    > +a. Add code to dump trec to user space
    > +b. Enhance to allow runtime registration and runtime enable disable.
    > +


    ---
    ~Randy
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