1. 28 Nov, 2008 2 commits
    • walimis's avatar
      ftrace: improve documentation · c072c249
      walimis authored
      Impact: extend documentation with notice of using wild cards correctly
      
      We know that we can use wild cards to set set_ftrace_filter, but there's
      problem when using them naively such as:
      
         echo h* > /debug/tracing/set_ftrace_filter
      
      If there are files named with "h" prefix in current directory,
      echo "h*" will echo these filenames to set_ftrace_filter, not the
      intended "h*".
      
      For example:
      
        $ cat /debug/tracing/available_filter_functions |grep ^hr |wc -l
        23
        $ ls
        $ touch hraa hrdd
        $ ls
        hraa  hrdd
        $ echo hr* > /debug/tracing/set_ftrace_filter
        $ cat /debug/tracing/set_ftrace_filter
      
      No output in /debug/tracing/set_ftrace_filter!
      
      If we use '' to escape wild cards, it works:
      
        $ ls
        hraa  hrdd
        $ echo "hr*" > /debug/tracing/set_ftrace_filter
        $ cat /debug/tracing/set_ftrace_filter |wc -l
        23
      
      This problem can lead to unexpected result if current directory has a
      lot of files.
      Signed-off-by: default avatarwalimis <walimisdev@gmail.com>
      Acked-by: default avatarSteven Rostedt <srostedt@redhat.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      c072c249
    • Liming Wang's avatar
      ftrace: improve seq_operation of ftrace · 50cdaf08
      Liming Wang authored
      Impact: make ftrace position computing more sane
      
      First remove useless ->pos field. Then we needn't check seq_printf
      in .show like other place.
      Signed-off-by: default avatarLiming Wang <liming.wang@windriver.com>
      Reviewed-by: default avatarBruce Ashfield <bruce.ashfield@windriver.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      50cdaf08
  2. 27 Nov, 2008 1 commit
  3. 26 Nov, 2008 13 commits
    • Ingo Molnar's avatar
      blktrace: port to tracepoints, update · 0bfc2455
      Ingo Molnar authored
      Port to the new tracepoints API: split DEFINE_TRACE() and DECLARE_TRACE()
      sites. Spread them out to the usage sites, as suggested by
      Mathieu Desnoyers.
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      Acked-by: default avatarMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
      0bfc2455
    • Arnaldo Carvalho de Melo's avatar
      blktrace: port to tracepoints · 5f3ea37c
      Arnaldo Carvalho de Melo authored
      This was a forward port of work done by Mathieu Desnoyers, I changed it to
      encode the 'what' parameter on the tracepoint name, so that one can register
      interest in specific events and not on classes of events to then check the
      'what' parameter.
      Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
      Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      5f3ea37c
    • Arjan van de Ven's avatar
      tracing: add "power-tracer": C/P state tracer to help power optimization · f3f47a67
      Arjan van de Ven authored
      Impact: new "power-tracer" ftrace plugin
      
      This patch adds a C/P-state ftrace plugin that will generate
      detailed statistics about the C/P-states that are being used,
      so that we can look at detailed decisions that the C/P-state
      code is making, rather than the too high level "average"
      that we have today.
      
      An example way of using this is:
      
       mount -t debugfs none /sys/kernel/debug
       echo cstate > /sys/kernel/debug/tracing/current_tracer
       echo 1 > /sys/kernel/debug/tracing/tracing_enabled
       sleep 1
       echo 0 > /sys/kernel/debug/tracing/tracing_enabled
       cat /sys/kernel/debug/tracing/trace | perl scripts/trace/cstate.pl > out.svg
      Signed-off-by: default avatarArjan van de Ven <arjan@linux.intel.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      f3f47a67
    • Steven Rostedt's avatar
      ftrace: add cpu annotation for function graph tracer · 437f24fb
      Steven Rostedt authored
      Impact: enhancement for function graph tracer
      
      When run on a SMP box, the function graph tracer is confusing because
      it shows the different CPUS as changes in the trace.
      
      This patch adds the annotation of 'CPU[###]' where ### is a three digit
      number. The output will look similar to this:
      
      CPU[001]     dput() {
      CPU[000] } 726
      CPU[001]     } 487
      CPU[000] do_softirq() {
      CPU[001]   } 2221
      CPU[000]   __do_softirq() {
      CPU[000]     __local_bh_disable() {
      CPU[001]   unroll_tree_refs() {
      CPU[000]     } 569
      CPU[001]   } 501
      CPU[000]     rcu_process_callbacks() {
      CPU[001]   kfree() {
      
      What makes this nice is that now you can grep the file and produce
      readable format for a particular CPU.
      
       # cat /debug/tracing/trace > /tmp/trace
       # grep '^CPU\[000\]' /tmp/trace > /tmp/trace0
       # grep '^CPU\[001\]' /tmp/trace > /tmp/trace1
      
      Will give you:
      
       # head /tmp/trace0
      CPU[000] ------------8<---------- thread sshd-3899 ------------8<----------
      CPU[000]     inotify_dentry_parent_queue_event() {
      CPU[000]     } 2531
      CPU[000]     inotify_inode_queue_event() {
      CPU[000]     } 505
      CPU[000]   } 69626
      CPU[000] } 73089
      CPU[000] audit_syscall_exit() {
      CPU[000]   path_put() {
      CPU[000]     dput() {
      
       # head /tmp/trace1
      CPU[001] ------------8<---------- thread pcscd-3446 ------------8<----------
      CPU[001]               } 4186
      CPU[001]               dput() {
      CPU[001]               } 543
      CPU[001]               vfs_permission() {
      CPU[001]                 inode_permission() {
      CPU[001]                   shmem_permission() {
      CPU[001]                     generic_permission() {
      CPU[001]                     } 501
      CPU[001]                   } 2205
      Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      437f24fb
    • Steven Rostedt's avatar
      ftrace: add thread comm to function graph tracer · 660c7f9b
      Steven Rostedt authored
      Impact: enhancement to function graph tracer
      
      Export the trace_find_cmdline so the function graph tracer can
      use it to print the comms of the threads.
      Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      660c7f9b
    • Steven Rostedt's avatar
      ftrace: let function tracing and function return run together · e53a6319
      Steven Rostedt authored
      Impact: feature
      
      This patch enables function tracing and function return to run together.
      I've tested this by enabling the stack tracer and return tracer, where
      both the function entry and function return are used together with
      dynamic ftrace.
      Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      e53a6319
    • Steven Rostedt's avatar
      ftrace: use code patching for ftrace graph tracer · 5a45cfe1
      Steven Rostedt authored
      Impact: more efficient code for ftrace graph tracer
      
      This patch uses the dynamic patching, when available, to patch
      the function graph code into the kernel.
      
      This patch will ease the way for letting both function tracing
      and function graph tracing run together.
      Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      5a45cfe1
    • Steven Rostedt's avatar
      ftrace: add function tracing to single thread · df4fc315
      Steven Rostedt authored
      Impact: feature to function trace a single thread
      
      This patch adds the ability to function trace a single thread.
      The file:
      
        /debugfs/tracing/set_ftrace_pid
      
      contains the pid to trace. Valid pids are any positive integer.
      Writing any negative number to this file will disable the pid
      tracing and the function tracer will go back to tracing all of
      threads.
      
      This feature works with both static and dynamic function tracing.
      Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      df4fc315
    • Liming Wang's avatar
      ftrace: adding other non-leaving .text sections · d144d5ee
      Liming Wang authored
      Impact: widen the scope of recordmcount.pl
      
      Besides .text section, there are three .text sections that won't
      be freed after kernel booting. They are: .sched.text, .spinlock.text
      and .kprobes.text, which contain functions we can trace. But the last
      section ".kprobes.text" is particular, which has been marked as "notrace",
      we ignore it. Thus we add other two sections.
      Signed-off-by: default avatarLiming Wang <liming.wang@windriver.com>
      Acked-by: default avatarSteven Rostedt <rostedt@goodmis.org>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      d144d5ee
    • Ingo Molnar's avatar
      tracing: function graph tracer, fix · c2324b69
      Ingo Molnar authored
      fix return-tracer => graph-tracer namespace rename fallout.
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      c2324b69
    • Frederic Weisbecker's avatar
      tracing/function-return-tracer: set a more human readable output · 287b6e68
      Frederic Weisbecker authored
      Impact: feature
      
      This patch sets a C-like output for the function graph tracing.
      For this aim, we now call two handler for each function: one on the entry
      and one other on return. This way we can draw a well-ordered call stack.
      
      The pid of the previous trace is loosely stored to be compared against
      the one of the current trace to see if there were a context switch.
      
      Without this little feature, the call tree would seem broken at
      some locations.
      We could use the sched_tracer to capture these sched_events but this
      way of processing is much more simpler.
      
      2 spaces have been chosen for indentation to fit the screen while deep
      calls. The time of execution in nanosecs is printed just after closed
      braces, it seems more easy this way to find the corresponding function.
      If the time was printed as a first column, it would be not so easy to
      find the corresponding function if it is called on a deep depth.
      
      I plan to output the return value but on 32 bits CPU, the return value
      can be 32 or 64, and its difficult to guess on which case we are.
      I don't know what would be the better solution on X86-32: only print
      eax (low-part) or even edx (high-part).
      
      Actually it's thee same problem when a function return a 8 bits value, the
      high part of eax could contain junk values...
      
      Here is an example of trace:
      
      sys_read() {
        fget_light() {
        } 526
        vfs_read() {
          rw_verify_area() {
            security_file_permission() {
              cap_file_permission() {
              } 519
            } 1564
          } 2640
          do_sync_read() {
            pipe_read() {
              __might_sleep() {
              } 511
              pipe_wait() {
                prepare_to_wait() {
                } 760
                deactivate_task() {
                  dequeue_task() {
                    dequeue_task_fair() {
                      dequeue_entity() {
                        update_curr() {
                          update_min_vruntime() {
                          } 504
                        } 1587
                        clear_buddies() {
                        } 512
                        add_cfs_task_weight() {
                        } 519
                        update_min_vruntime() {
                        } 511
                      } 5602
                      dequeue_entity() {
                        update_curr() {
                          update_min_vruntime() {
                          } 496
                        } 1631
                        clear_buddies() {
                        } 496
                        update_min_vruntime() {
                        } 527
                      } 4580
                      hrtick_update() {
                        hrtick_start_fair() {
                        } 488
                      } 1489
                    } 13700
                  } 14949
                } 16016
                msecs_to_jiffies() {
                } 496
                put_prev_task_fair() {
                } 504
                pick_next_task_fair() {
                } 489
                pick_next_task_rt() {
                } 496
                pick_next_task_fair() {
                } 489
                pick_next_task_idle() {
                } 489
      
      ------------8<---------- thread 4 ------------8<----------
      
      finish_task_switch() {
      } 1203
      do_softirq() {
        __do_softirq() {
          __local_bh_disable() {
          } 669
          rcu_process_callbacks() {
            __rcu_process_callbacks() {
              cpu_quiet() {
                rcu_start_batch() {
                } 503
              } 1647
            } 3128
            __rcu_process_callbacks() {
            } 542
          } 5362
          _local_bh_enable() {
          } 587
        } 8880
      } 9986
      kthread_should_stop() {
      } 669
      deactivate_task() {
        dequeue_task() {
          dequeue_task_fair() {
            dequeue_entity() {
              update_curr() {
                calc_delta_mine() {
                } 511
                update_min_vruntime() {
                } 511
              } 2813
      Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
      Acked-by: default avatarSteven Rostedt <rostedt@goodmis.org>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      287b6e68
    • Frederic Weisbecker's avatar
      tracing/function-return-tracer: change the name into function-graph-tracer · fb52607a
      Frederic Weisbecker authored
      Impact: cleanup
      
      This patch changes the name of the "return function tracer" into
      function-graph-tracer which is a more suitable name for a tracing
      which makes one able to retrieve the ordered call stack during
      the code flow.
      Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
      Acked-by: default avatarSteven Rostedt <rostedt@goodmis.org>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      fb52607a
    • Ingo Molnar's avatar
  4. 25 Nov, 2008 12 commits
  5. 24 Nov, 2008 1 commit
  6. 23 Nov, 2008 11 commits
    • Török Edwin's avatar
      vfs, seqfile: fix comment style on mangle_path · 958086d1
      Török Edwin authored
      Impact: use standard docbook tags
      Reported-by: default avatarRandy Dunlap <randy.dunlap@oracle.com>
      Signed-off-by: default avatarTörök Edwin <edwintorok@gmail.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      958086d1
    • Frederic Weisbecker's avatar
      tracing/function-return-tracer: free the return stack on free_task() · 65afa5e6
      Frederic Weisbecker authored
      Impact: avoid losing some traces when a task is freed
      
      do_exit() is not the last function called when a task finishes.
      There are still some functions which are to be called such as
      ree_task().  So we delay the freeing of the return stack to the
      last moment.
      Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      65afa5e6
    • Pekka Paalanen's avatar
      tracing, doc: update mmiotrace documentation · fb91ee6c
      Pekka Paalanen authored
      Impact: update documentation
      
      Update to reflect the current state of the tracing framework:
      
       - "none" tracer has been replaced by "nop" tracer
       - tracing_enabled must be toggled when changing buffer size
      Signed-off-by: default avatarPekka Paalanen <pq@iki.fi>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      fb91ee6c
    • Pekka Paalanen's avatar
      x86, mmiotrace: fix buffer overrun detection · 7ee1768d
      Pekka Paalanen authored
      Impact: fix mmiotrace overrun tracing
      
      When ftrace framework moved to use the ring buffer facility, the buffer
      overrun detection was broken after 2.6.27 by commit
      
      | commit 3928a8a2
      | Author: Steven Rostedt <rostedt@goodmis.org>
      | Date:   Mon Sep 29 23:02:41 2008 -0400
      |
      |     ftrace: make work with new ring buffer
      |
      |     This patch ports ftrace over to the new ring buffer.
      
      The detection is now fixed by using the ring buffer API.
      
      When mmiotrace detects a buffer overrun, it will report the number of
      lost events. People reading an mmiotrace log must know if something was
      missed, otherwise the data may not make sense.
      Signed-off-by: default avatarPekka Paalanen <pq@iki.fi>
      Acked-by: default avatarSteven Rostedt <rostedt@goodmis.org>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      7ee1768d
    • Frederic Weisbecker's avatar
      tracing/function-return-tracer: don't trace kfree while it frees the return stack · eae849ca
      Frederic Weisbecker authored
      Impact: fix a crash
      
      While I killed the cat process, I got sometimes the following (but rare)
      crash:
      
      [   65.689027] Pid: 2969, comm: cat Not tainted (2.6.28-rc6-tip #83) AMILO Li 2727
      [   65.689027] EIP: 0060:[<00000000>] EFLAGS: 00010082 CPU: 1
      [   65.689027] EIP is at 0x0
      [   65.689027] EAX: 00000000 EBX: f66cd780 ECX: c019a64a EDX: f66cd780
      [   65.689027] ESI: 00000286 EDI: f66cd780 EBP: f630be2c ESP: f630be24
      [   65.689027]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
      [   65.689027] Process cat (pid: 2969, ti=f630a000 task=f66cd780 task.ti=f630a000)
      [   65.689027] Stack:
      [   65.689027]  00000012 f630bd54 f630be7c c012c853 00000000 c0133cc9 f66cda54 f630be5c
      [   65.689027]  f630be68 f66cda54 f66cd88c f66cd878 f7070000 00000001 f630be90 c0135dbc
      [   65.689027]  f614a614 f630be68 f630be68 f65ba200 00000002 f630bf10 f630be90 c012cad6
      [   65.689027] Call Trace:
      [   65.689027]  [<c012c853>] ? do_exit+0x603/0x850
      [   65.689027]  [<c0133cc9>] ? next_signal+0x9/0x40
      [   65.689027]  [<c0135dbc>] ? dequeue_signal+0x8c/0x180
      [   65.689027]  [<c012cad6>] ? do_group_exit+0x36/0x90
      [   65.689027]  [<c013709c>] ? get_signal_to_deliver+0x20c/0x390
      [   65.689027]  [<c0102b69>] ? do_notify_resume+0x99/0x8b0
      [   65.689027]  [<c02e6d1a>] ? tty_ldisc_deref+0x5a/0x80
      [   65.689027]  [<c014db9b>] ? trace_hardirqs_on+0xb/0x10
      [   65.689027]  [<c02e6d1a>] ? tty_ldisc_deref+0x5a/0x80
      [   65.689027]  [<c02e39b0>] ? n_tty_write+0x0/0x340
      [   65.689027]  [<c02e1812>] ? redirected_tty_write+0x82/0x90
      [   65.689027]  [<c019ee99>] ? vfs_write+0x99/0xd0
      [   65.689027]  [<c02e1790>] ? redirected_tty_write+0x0/0x90
      [   65.689027]  [<c019f342>] ? sys_write+0x42/0x70
      [   65.689027]  [<c01035ca>] ? work_notifysig+0x13/0x19
      [   65.689027] Code:  Bad EIP value.
      [   65.689027] EIP: [<00000000>] 0x0 SS:ESP 0068:f630be24
      
      This is because on do_exit(), kfree is called to free the return addresses stack
      but kfree is traced and stored its return address in this stack.
      This patch fixes it.
      Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      eae849ca
    • Ingo Molnar's avatar
      Merge branch 'ppc/ftrace' of... · 813b8520
      Ingo Molnar authored
      Merge branch 'ppc/ftrace' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace into tracing/powerpc
      813b8520
    • Ian Campbell's avatar
      xen: pin correct PGD on suspend · 86bbc2c2
      Ian Campbell authored
      Impact: fix Xen guest boot failure
      
      commit eefb47f6 ("xen: use
      spin_lock_nest_lock when pinning a pagetable") changed xen_pgd_walk to
      walk over mm->pgd rather than taking pgd as an argument.
      
      This breaks xen_mm_(un)pin_all() because it makes init_mm.pgd readonly
      instead of the pgd we are interested in and therefore the pin subsequently
      fails.
      
      (XEN) mm.c:2280:d15 Bad type (saw 00000000e8000001 != exp 0000000060000000) for mfn bc464 (pfn 21ca7)
      (XEN) mm.c:2665:d15 Error while pinning mfn bc464
      
      [   14.586913] 1 multicall(s) failed: cpu 0
      [   14.586926] Pid: 14, comm: kstop/0 Not tainted 2.6.28-rc5-x86_32p-xenU-00172-gee2f6cc7 #200
      [   14.586940] Call Trace:
      [   14.586955]  [<c030c17a>] ? printk+0x18/0x1e
      [   14.586972]  [<c0103df3>] xen_mc_flush+0x163/0x1d0
      [   14.586986]  [<c0104bc1>] __xen_pgd_pin+0xa1/0x110
      [   14.587000]  [<c015a330>] ? stop_cpu+0x0/0xf0
      [   14.587015]  [<c0104d7b>] xen_mm_pin_all+0x4b/0x70
      [   14.587029]  [<c022bcb9>] xen_suspend+0x39/0xe0
      [   14.587042]  [<c015a330>] ? stop_cpu+0x0/0xf0
      [   14.587054]  [<c015a3cd>] stop_cpu+0x9d/0xf0
      [   14.587067]  [<c01417cd>] run_workqueue+0x8d/0x150
      [   14.587080]  [<c030e4b3>] ? _spin_unlock_irqrestore+0x23/0x40
      [   14.587094]  [<c014558a>] ? prepare_to_wait+0x3a/0x70
      [   14.587107]  [<c0141918>] worker_thread+0x88/0xf0
      [   14.587120]  [<c01453c0>] ? autoremove_wake_function+0x0/0x50
      [   14.587133]  [<c0141890>] ? worker_thread+0x0/0xf0
      [   14.587146]  [<c014509c>] kthread+0x3c/0x70
      [   14.587157]  [<c0145060>] ? kthread+0x0/0x70
      [   14.587170]  [<c0109d1b>] kernel_thread_helper+0x7/0x10
      [   14.587181]   call  1/3: op=14 arg=[c0415000] result=0
      [   14.587192]   call  2/3: op=14 arg=[e1ca2000] result=0
      [   14.587204]   call  3/3: op=26 arg=[c1808860] result=-22
      Signed-off-by: default avatarIan Campbell <ian.campbell@citrix.com>
      Acked-by: default avatarJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      86bbc2c2
    • Török Edwin's avatar
      tracing/stack-tracer: avoid races accessing file · e38da592
      Török Edwin authored
      Impact: fix race
      
      vma->vm_file reference is only stable while holding the mmap_sem,
      so move usage of it to within the critical section.
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      e38da592
    • Ingo Molnar's avatar
      Merge branch 'oprofile-for-tip' of... · 3d994e10
      Ingo Molnar authored
      Merge branch 'oprofile-for-tip' of git://git.kernel.org/pub/scm/linux/kernel/git/rric/oprofile into x86/urgent
      3d994e10
    • Thomas Gleixner's avatar
      x86: revert irq number limitation · a1967d64
      Thomas Gleixner authored
      Impact: fix MSIx not enough irq numbers available regression
      
      The manual revert of the sparse_irq patches missed to bring the number
      of possible irqs back to the .27 status. This resulted in a regression
      when two multichannel network cards were placed in a system with only
      one IO_APIC - causing the networking driver to not have the right
      IRQ and the device not coming up.
      
      Remove the dynamic allocation logic leftovers and simply return
      NR_IRQS in probe_nr_irqs() for now.
      
         Fixes: http://lkml.org/lkml/2008/11/19/354Reported-by: default avatarJesper Dangaard Brouer <hawk@diku.dk>
      Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
      Tested-by: default avatarJesper Dangaard Brouer <hawk@diku.dk>
      Acked-by: default avatarYinghai Lu <yinghai@kernel.org>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      a1967d64
    • Török Edwin's avatar
      tracing/stack-tracer: introduce CONFIG_USER_STACKTRACE_SUPPORT · 8d26487f
      Török Edwin authored
      Impact: cleanup
      
      User stack tracing is just implemented for x86, but it is not x86 specific.
      
      Introduce a generic config flag, that is currently enabled only for x86.
      When other arches implement it, they will have to
      SELECT USER_STACKTRACE_SUPPORT.
      Signed-off-by: default avatarTörök Edwin <edwintorok@gmail.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      8d26487f