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Finding spurious relaxes
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========================
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:author: Philippe Gerum
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:email: rpm@xenomai.org
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:categories: Application
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:tags: troubleshooting
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Sources of spurious relaxes
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---------------------------
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In a dual kernel configuration, an application does not want to switch
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to the regular Linux execution mode unexpectedly, aka _secondary
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mode_. When short and bounded response time is a strong requirement,
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the application wants to run in _primary mode_ exclusively, because
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only this mode is deemed to deliver real-time capabilities.
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There are several reasons for an application to be moved out of
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real-time mode automatically by the Xenomai co-kernel. This happens
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when the application is:
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- invoking a regular Linux syscall triggering a processor fault
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- receiving a Linux-originated signal (i.e. *not* a Xenomai-originated
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- one), possibly sent by the Xenomai kernel upon detecting a
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- consistency issue with the application
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At any rate, we want to know *where* in the application code the
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signal was received, which will be a strong hint to find out *why* we
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switched away from real-time mode.
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[TIP]
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The reason Xenomai has to move the application out of real-time mode
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upon such events stems from the way a dual kernel system works. As
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the Cobalt kernel may preempt the regular Linux kernel at any time in
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the course of its execution for dealing with a high priority event,
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including when the latter assumes to be traversing some atomic
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section, special care has to be taken for re-entering a valid Linux
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context before a regular kernel service can be delivered.
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Manually debugging spurious relaxes
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-----------------------------------
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Xenomai provides a mechanism for sending a misbehaving application a
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regular Linux signal immediately when it switches to _secondary mode_
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unexpectedly. When run over GDB, the backtrace of the current thread
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receiving the signal can be examined. This feature is available with
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both Xenomai 2 and Xenomai 3.
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Each time a spurious migration to _secondary mode_ happens, the thread
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is sent a SIGDEBUG signal, along with additional information in the
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+siginfo_t+ data. First of all, the application should trap such
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signal to a handler, like this:
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-------------------------------------------------------------------------------
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#include <signal.h>
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struct sigaction sa;
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sigemptyset(&sa.sa_mask);
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sa.sa_sigaction = sigdebug_handler;
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sa.sa_flags = SA_SIGINFO;
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sigaction(SIGDEBUG, &sa, NULL);
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-------------------------------------------------------------------------------
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The +sigdebug_handler()+ routine would have to retrieve and decode the
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current backtrace, writing out the cause of the SIGDEBUG notification
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and hopefully a meaningful call stack:
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-------------------------------------------------------------------------------
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#include <sys/types.h>
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#include <pthread.h>
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#include <signal.h>
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#include <unistd.h>
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#include <execinfo.h>
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static const char *sigdebug_reasons[] = {
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[SIGDEBUG_UNDEFINED] = "latency: received SIGXCPU for unknown reason",
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[SIGDEBUG_MIGRATE_SIGNAL] = "received signal",
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[SIGDEBUG_MIGRATE_SYSCALL] = "invoked syscall",
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[SIGDEBUG_MIGRATE_FAULT] = "triggered fault",
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[SIGDEBUG_MIGRATE_PRIOINV] = "affected by priority inversion",
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[SIGDEBUG_NOMLOCK] = "Xenomai: process memory not locked "
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"(missing mlockall?)",
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[SIGDEBUG_WATCHDOG] = "Xenomai: watchdog triggered "
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"(period too short?)",
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};
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void sigdebug_handler(int sig, siginfo_t *si, void *context)
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{
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const char fmt[] = "Mode switch (reason: %s), aborting. Backtrace:\n";
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unsigned int reason = sigdebug_reason(si);
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static char buffer[256];
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static void *bt[200];
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unsigned int n;
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if (reason > SIGDEBUG_WATCHDOG)
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reason = SIGDEBUG_UNDEFINED;
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switch(reason) {
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case SIGDEBUG_UNDEFINED:
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case SIGDEBUG_NOMLOCK:
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case SIGDEBUG_WATCHDOG:
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/* These errors are lethal, something went really wrong. */
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n = snprintf(buffer, sizeof(buffer),
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"%s\n", sigdebug_reasons[reason]);
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write(STDERR_FILENO, buffer, n);
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exit(EXIT_FAILURE);
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}
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/* Retrieve the current backtrace, and decode it to stdout. */
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n = snprintf(buffer, sizeof(buffer), fmt, sigdebug_reason[reason]);
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n = write(STDERR_FILENO, buffer, n);
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n = backtrace(bt, sizeof(bt)/sizeof(bt[0]));
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backtrace_symbols_fd(bt, n, STDERR_FILENO);
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signal(sig, SIG_DFL);
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kill(getpid(), sig);
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}
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-------------------------------------------------------------------------------
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Then, you need to enable the _warn_upon_switch_ capability on a
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per-thread basis. For instance, a POSIX-based application would run
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this code from the thread to monitor for spurious relaxes:
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-------------------------------------------------------------------------------
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/* Ask Xenomai to warn us upon switches to secondary mode. */
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pthread_set_mode_np(0, PTHREAD_WARNSW);
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-------------------------------------------------------------------------------
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Alternatively, an application based on the _native_ API would run this
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code instead:
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-------------------------------------------------------------------------------
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/* Ask Xenomai to warn us upon switches to secondary mode. */
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rt_task_set_mode(0, T_WARNSW, NULL);
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-------------------------------------------------------------------------------
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Debugging spurious relaxes with Xenomai 3 and *slackspot*
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----------------------------------------------------------
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Xenomai 3 introduces a new utility called *slackspot*, for pinpointing
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code locations which caused transitions to _secondary mode_.
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*slackspot* combines dedicated kernel support exporting trace
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information about spurious relaxes through the
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+/proc/xenomai/debug/relax+ virtual file, and a userland utility
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called +sbin/slackspot+, which parses the virtual file output to
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display the program backtrace, so that we can locate the offending
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code easily.
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The motivation to centralize tracing information about relaxes
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directly into kernel space is fourfold:
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- it allows to gather all the trace data into a single location and
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keep it safe there, with no external log file involved.
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- enabling the tracing does not impose any requirement on the
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application (aside of being compiled with debug symbols for best
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interpreting that information). We only need a kernel config switch
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for this (i.e. CONFIG_XENO_OPT_DEBUG_TRACE_RELAX).
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- the data is collected and can be made available exactly the same
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way regardless of the application emitting the relax requests, or
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whether it is still alive when the trace data are displayed.
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- the kernel is able to provide accurate and detailed trace
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information, such as the relative offset of instructions causing relax
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requests within dynamic shared objects, without having to guess it
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roughly from /proc/pid/maps, or relying on ldd's --function-relocs
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feature, which both require to run on the target system to get the
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needed information. Instead, we allow a build host to use a
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cross-compilation toolchain later to extract the source location, from
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the raw data the kernel has provided on the target system.
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A typical debug session with *slackspot*
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Here is a typical debugging session, involving a target board running
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the Cobalt-based application, and a host system, providing the
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cross-compilation toolchain used to build that application. We use the
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netcat utility to pull the trace data over the wire from the target,
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and process it locally on our debug host:
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---------------------------------------------------------------------------
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target> netcat -l -p <port> -c < /proc/xenomai/debug/relax
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host> nc <target-ip> <port> | CROSS_COMPILE=ppc_6xx- ./slackspot --path=/opt/rootfs/MPC5200/lib:$HOME/frags/relax --filter thread=Task*
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Thread[828] "Task 2" started by /relax:
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#0 0xfff00000 ???
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#1 0x000001bb ???
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#2 0x00064393 _IO_file_doallocate() in ??:?
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#3 0x00073d6f _IO_doallocbuf() in ??:?
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#4 0x00072d87 _IO_file_overflow() in ??:?
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#5 0x00075f83 __overflow() in ??:?
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#6 0x0006997b putchar() in ??:?
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#7 0x100017db task2_func() in /home/rpm/frags/relax/relax.c:23
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#8 0x000078d7 task_entry() in /home/rpm/git/xenomai-forge/lib/alchemy/task.c:235
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#9 0x00005a6b start_thread() in pthread_create.c:?
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#10 0x000d389f __clone() in ??:?
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Thread[828] "Task 2" started by /relax (4 times):
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#0 0x000c443f write() in ??:?
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#1 0x00072553 _IO_file_write() in ??:?
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#2 0x000721cf _IO_file_seek() in ??:?
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#3 0x000724c7 _IO_do_write() in ??:?
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#4 0x00072c2f _IO_file_sync() in ??:?
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#5 0x00064a4f _IO_fflush() in ??:?
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#6 0x100017eb task2_func() in /home/rpm/frags/relax/relax.c:24
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#7 0x000078d7 task_entry() in /home/rpm/git/xenomai-forge/lib/alchemy/task.c:235
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#8 0x00005a6b start_thread() in pthread_create.c:?
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#9 0x000d389f __clone() in ??:?
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---------------------------------------------------------------------------
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The *slackspot* output here shows that something wrong is happening at
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lines 23 and 24 from the example file, as the application calls
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buffered standard I/O routines from _primary mode_, which in turn
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invoke regular kernel services, causing the thread to relax and leave
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the real-time mode. |
Philippe Gerum
