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SubjectRe: [RFC PATCH 1/3] Generic Trace Setup and Control (GTSC) Documentation
* Tom Zanussi (zanussi@us.ibm.com) wrote:
> This is the documentation for the Generic Trace Setup and Control
> patchset, first submitted a couple of weeks ago. See
>
> http://marc.info/?l=linux-kernel&m=118214274912586&w=2
>
> for a more detailed description.
>
> I've updated this patch to incorporate the suggestions made by Alexey
> Dobriyan in that thread.
>

It would be nice, since it claims to be "generic", to support things
brought forward by other tracers like LTTng, such as : multiple channels
(for low, medium and high event rate data, with user-selectable sizes),
hybrid trace mode (combined normal trace channels and flight recorder
channels).

Please see comments below,

> Signed-off-by: Tom Zanussi <zanussi@us.ibm.com>
> Signed-off-by: David Wilder <dwilder@us.ibm.com>
> ---
> gtsc.txt | 247 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 247 insertions(+)
>
> diff --git a/Documentation/gtsc.txt b/Documentation/gtsc.txt
> new file mode 100644
> index 0000000..470d1fc
> --- /dev/null
> +++ b/Documentation/gtsc.txt
> @@ -0,0 +1,247 @@
> +Generic Trace Setup and Control (GTSC)
> +==================================
> +In the kernel, GTSC provides a simple API for starting and managing
> +data channels to user space. GTSC builds on the relay interface. For a
> +complete description of the relay interface, please see:
> +Documentation/filesystems/relay.txt.
> +
> +GTSC provides one layer in a complete tracing application. The idea of
> +the GTSC is to provide a kernel API for the setup and control of tracing
> +channels. User of GTSC must provide a data layer responsible for formatting
> +and writing data into the trace channels.
> +
> +A layered approach to tracing
> +=============================
> +A complete kernel tracing application consists of a data provider and a data
> +consumer. Both provider and consumer contain three layers; each layer works
> +in tandem with the corresponding layer in the opposite side. The layers are
> +represented in the following diagram.
> +
> +Provider Data layer
> + Formats raw trace data and provides data-related service.
> + For example, adding timestamps used by consumer to sort data.
> +
> +Provider Control layer
> + Provided by GTSC. Creates trace channels and informs the data layer
> + and consumer of the current state of the trace channels.
> +
> +Provider Buffering layer
> + Provided by relay. This layer buffers data in the
> + kernel for consumption by the consumer's buffer
> + layer.
> +
> +Provider (in-kernel facility)
> +-----------------------------------------------------------------------------
> +Consumer (user application)
> +
> +
> +Consumer Buffer layer
> + Reads/consumes data from the provider's data buffers.
> +
> +Consumer Control layer
> + Communicates to the provider's control layer to control the state
> + of the trace channels.
> +
> +Consumer Data layer
> + Sorts and formats data as provided by the provider's data layer.
> +
> +The provider is coded as a kernel facility. The consumer is coded as
> +a user application.
> +
> +
> +GTSC - Features
> +==============
> +The GTSC exploits services and features provided by relay. These features are:
> +- The creation and destruction of relay channels.
> +- Buffer management. Overwrite or non-overwrite modes can be selected
> + as well as global or per-CPU buffering.
> +
> +Overwrite mode can be called "flight recorder mode". Flight recorder
> +mode is selected by setting the TRACE_FLIGHT_CHANNEL flag when
> +creating trace channels. In flight mode when a tracing buffer is
> +full, the oldest records in the buffer will be discarded to make room
> +as new records arrive. In the default non-overwrite mode, new records
> +may be written only if the buffer has room. In either case, to
> +prevent data loss, a user space reader must keep the buffers
> +drained. GTSC provides a means to detect the number of records that
> +have been dropped due to a buffer-full condition (non-overwrite mode
> +only).
> +
> +When per-CPU buffers are used, relay creates one debugfs file for each
> +running CPU. The user-space consumer of the data is responsible for
> +reading the per-CPU buffers and collating the records presumably using
> +a time stamp or sequence number included in the trace records. The
> +use of global buffers eliminates this extra work of sequencing
> +records; however the provider's data layer must hold a lock when
> +writing records. The lock prevents writers running on different CPUs
> +from overwriting each other's data. However, buffering may be slower
> +because write to the buffer are serialized. Global buffering is
> +selected by setting the TRACE_GLOBAL_CHANNEL flag when creating trace
> +channels.
> +
> +GTSC User Interface
> +===================
> +When a GTSC channel is created and tracing has been started, the following
> +directories and files are created in the root of the mounted debugfs.
> +
> +/debug (root of the debugfs)
> + /<trace-root-dir>
> + /<trace-name>
> + trace0 ... traceN (Per-CPU trace data, one per CPU)
> + state (Used to start and stop tracing)
> + dropped (number of records dropped due
> + to a full-buffer condition, only)
> + for non-TRACE_FLIGHT_CHANNELs)
> + rewind ('un-consume' channel data i.e.
> + start next read at the beginning
> + again (TRACE_FLIGHT_CHANNELS only)
> + nr_sub (Number of sub-buffers in channel)
> + sub_size (Size of sub-buffers in channnel)
> +
> +Trace data is gathered from the trace[0...N] files using one of the available
> +interfaces provided by relay (see Documentation/filesystems/relay.txt).
> +When using the READ(2) interface, as data is read it is marked as consumed by
> +the relay subsystem. Therefore, subsequent reads will only return unconsumed
> +data.
> +
> +GTSC Kernel API
> +===============
> +An overview of the GTSC Kernel API is now given. More details of the API can
> +be found in linux/gtsc.h.
> +
> +The steps a kernel data provider takes to utilize the GTSC are:
> +1) Set up a gtsc channel - trace_setup()
> +2) Start the GTSC channel - trace_start()
> +3) Write one or more trace records into the channel (using the relay API).
> +4) Optionally stop and start tracing as desired - trace_start()/trace_stop()
> +5) Destroy the GTSC channel and underlying relay channel - trace_cleanup().
> +
> +GTSC Example
> +===========
> +This small sample module creates a GTSC channel. It places a kprobe on the
> +function do_fork(). The kprobe handler will write a timestamp and
> +current->pid to the GTSC channel.
> +
> +You can build the kernel module kprobes_gtsc.ko using the following Makefile:
> +------------------------------------CUT-------------------------------------
> +obj-m := kprobes_gtsc.o
> +KDIR := /lib/modules/$(shell uname -r)/build
> +PWD := $(shell pwd)
> +default:
> + $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
> +clean:
> + rm -f *.mod.c *.ko *.o
> +----------------------------------CUT--------------------------------------
> +/* kprobes_gtsc.c - An example of using GTSC in a kprobes module */
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/kprobes.h>
> +#include <linux/gtsc.h>
> +
> +#define PROBE_POINT "do_fork"
> +
> +static struct kprobe kp;
> +struct trace_info *kprobes_trace;
> +
> +#define GTSC_PRINTF_TMPBUF_SIZE (1024)
> +static char gtsc_printf_tmpbuf[NR_CPUS][GTSC_PRINTF_TMPBUF_SIZE];
> +

Still doing an extra copy hey? :)

> +/* This lock is needed only when using global relay buffers */
> +static DEFINE_SPINLOCK(gtsc_printf_lock);
> +
> +void gtsc_printf(struct trace_info *trace, const char *format, ...)
> +{
> + va_list args;
> + void *buf;
> + int len;
> + char *record;
> +
> + if (!trace)
> + return;
> + if (!trace_running(trace))
> + return;
> +
> + /* get a timestamp */
> + buf = gtsc_printf_tmpbuf[smp_processor_id()];
> + len = sprintf(buf,"[%lld] ", trace_timestamp());
> +
> + /* get the data */
> + va_start(args, format);
> + len += vsnprintf(buf+len, GTSC_PRINTF_TMPBUF_SIZE, format, args);
> + va_end(args);
> +
> + /*
> + * Locking can be eliminated by specifying per-cpu buffers
> + * when calling trace_setup().
> + */
> + spin_lock(&gtsc_printf_lock);
> +

I really hope nobody is willing to call gtsc_printf from an interrupt
handler.. :) Easy dead embrace in perspective.

> + /* Send everything to the relay buffer */
> + if ((record = relay_reserve(trace->rchan, len)))
> + memcpy(record, buf, len);
> +
> + spin_unlock(&gtsc_printf_lock);
> +}
> +
> +
> +int handler_pre(struct kprobe *p, struct pt_regs *regs)
> +{
> + gtsc_printf(kprobes_trace,"%d\n", current->pid);
> + return 0;
> +}
> +
> +int init_module(void)
> +{
> + int ret;
> + /* setup gtsc, use a global relay buffer */
> + kprobes_trace = trace_setup("gtsc_example",PROBE_POINT,
> + 1024,8,
> + TRACE_GLOBAL_CHANNEL | TRACE_FLIGHT_CHANNEL);
> + if (!kprobes_trace)
> + return -1;
> +
> + trace_start(kprobes_trace);
> +
> + /* setup the kprobe */
> + kp.pre_handler = handler_pre;
> + kp.post_handler = NULL;
> + kp.fault_handler = NULL;
> + kp.symbol_name = PROBE_POINT;
> + if ((ret=register_kprobe(&kp)) < 0)
> + gtsc_printf(kprobes_trace,"register_kprobe failed %d\n", ret);
> + return 0;
> +}
> +
> +void cleanup_module(void)
> +{
> + unregister_kprobe(&kp);
> + /* close down the gtsc channel */
> + trace_stop(kprobes_trace);
> + trace_cleanup(kprobes_trace);
> +}
> +
> +MODULE_LICENSE("GPL");
> +-----------------------------CUT--------------------------------------------
> +How to run the example:
> +$ mount -t debugfs /debug
> +$ make
> +$ insmod kprobes_gtsc.ko
> +
> +To view the data produced by the module:
> +$ cat /debug/gtsc_example/do_fork/trace0
> +
> +Remove the module.
> +$ rmmod kprobes_gtsc
> +
> +The function trace_cleanup() is called when the module
> +is removed. This will cause the GTSC channel to be destroyed and the
> +corresponding files to disappear from the debug file system.
> +
> +Credits
> +=======
> +GTSC adapted from blktrace authored by Jens Axboe (axboe@suse.de).
> +
> +Major contributions to GTSC were made by:
> +Tom Zanussi <zanussi@us.ibm.com>
> +Martin Hunt <hunt@redhat.com>
> +David Wilder <dwilder@us.ibm.com>
>
>
>
> -
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--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
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