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Daniel Bristot de Oliveirarostedt
Daniel Bristot de Oliveira
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Documentation/rtla: Add timerlat-top auto-analysis options
Add the new options to the man page, as well as updating the example to include the new output. Link: https://lkml.kernel.org/r/a3f5fb49432a55c3323b18725fc6e702f16ccc79.1675179318.git.bristot@kernel.org Cc: Daniel Bristot de Oliveira <[email protected]> Cc: Jonathan Corbet <[email protected]> Signed-off-by: Daniel Bristot de Oliveira <[email protected]> Signed-off-by: Steven Rostedt (Google) <[email protected]>
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‎Documentation/tools/rtla/common_timerlat_aa.rst

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**--dump-tasks**
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prints the task running on all CPUs if stop conditions are met (depends on !--no-aa)
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**--no-aa**
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disable auto-analysis, reducing rtla timerlat cpu usage

‎Documentation/tools/rtla/rtla-timerlat-top.rst

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.. include:: common_options.rst
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.. include:: common_timerlat_aa.rst
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EXAMPLE
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=======
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In the example below, the *timerlat* tracer is set to capture the stack trace at
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the IRQ handler, printing it to the buffer if the *Thread* timer latency is
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higher than *30 us*. It is also set to stop the session if a *Thread* timer
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latency higher than *30 us* is hit. Finally, it is set to save the trace
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buffer if the stop condition is hit::
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In the example below, the timerlat tracer is dispatched in cpus *1-23* in the
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automatic trace mode, instructing the tracer to stop if a *40 us* latency or
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higher is found::
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[root@alien ~]# rtla timerlat top -s 30 -T 30 -t
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Timer Latency
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0 00:00:59 | IRQ Timer Latency (us) | Thread Timer Latency (us)
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# timerlat -a 40 -c 1-23 -q
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Timer Latency
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0 00:00:12 | IRQ Timer Latency (us) | Thread Timer Latency (us)
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CPU COUNT | cur min avg max | cur min avg max
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0 #58634 | 1 0 1 10 | 11 2 10 23
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1 #58634 | 1 0 1 9 | 12 2 9 23
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2 #58634 | 0 0 1 11 | 10 2 9 23
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3 #58634 | 1 0 1 11 | 11 2 9 24
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4 #58634 | 1 0 1 10 | 11 2 9 26
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5 #58634 | 1 0 1 8 | 10 2 9 25
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6 #58634 | 12 0 1 12 | 30 2 10 30 <--- CPU with spike
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7 #58634 | 1 0 1 9 | 11 2 9 23
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8 #58633 | 1 0 1 9 | 11 2 9 26
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9 #58633 | 1 0 1 9 | 10 2 9 26
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10 #58633 | 1 0 1 13 | 11 2 9 28
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11 #58633 | 1 0 1 13 | 12 2 9 24
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12 #58633 | 1 0 1 8 | 10 2 9 23
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13 #58633 | 1 0 1 10 | 10 2 9 22
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14 #58633 | 1 0 1 18 | 12 2 9 27
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15 #58633 | 1 0 1 10 | 11 2 9 28
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16 #58633 | 0 0 1 11 | 7 2 9 26
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17 #58633 | 1 0 1 13 | 10 2 9 24
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18 #58633 | 1 0 1 9 | 13 2 9 22
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19 #58633 | 1 0 1 10 | 11 2 9 23
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20 #58633 | 1 0 1 12 | 11 2 9 28
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21 #58633 | 1 0 1 14 | 11 2 9 24
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22 #58633 | 1 0 1 8 | 11 2 9 22
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23 #58633 | 1 0 1 10 | 11 2 9 27
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timerlat hit stop tracing
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saving trace to timerlat_trace.txt
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[root@alien bristot]# tail -60 timerlat_trace.txt
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[...]
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timerlat/5-79755 [005] ....... 426.271226: #58634 context thread timer_latency 10823 ns
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sh-109404 [006] dnLh213 426.271247: #58634 context irq timer_latency 12505 ns
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sh-109404 [006] dNLh313 426.271258: irq_noise: local_timer:236 start 426.271245463 duration 12553 ns
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sh-109404 [006] d...313 426.271263: thread_noise: sh:109404 start 426.271245853 duration 4769 ns
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timerlat/6-79756 [006] ....... 426.271264: #58634 context thread timer_latency 30328 ns
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timerlat/6-79756 [006] ....1.. 426.271265: <stack trace>
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=> timerlat_irq
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=> __hrtimer_run_queues
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=> hrtimer_interrupt
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=> __sysvec_apic_timer_interrupt
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=> sysvec_apic_timer_interrupt
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=> asm_sysvec_apic_timer_interrupt
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=> _raw_spin_unlock_irqrestore <---- spinlock that disabled interrupt.
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=> try_to_wake_up
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=> autoremove_wake_function
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=> __wake_up_common
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=> __wake_up_common_lock
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=> ep_poll_callback
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=> __wake_up_common
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=> __wake_up_common_lock
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=> fsnotify_add_event
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=> inotify_handle_inode_event
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=> fsnotify
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=> __fsnotify_parent
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=> __fput
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=> task_work_run
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=> exit_to_user_mode_prepare
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=> syscall_exit_to_user_mode
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=> do_syscall_64
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=> entry_SYSCALL_64_after_hwframe
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=> 0x7265000001378c
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=> 0x10000cea7
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=> 0x25a00000204a
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=> 0x12e302d00000000
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=> 0x19b51010901b6
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=> 0x283ce00726500
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=> 0x61ea308872
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=> 0x00000fe3
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bash-109109 [007] d..h... 426.271265: #58634 context irq timer_latency 1211 ns
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timerlat/6-79756 [006] ....... 426.271267: timerlat_main: stop tracing hit on cpu 6
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In the trace, it is possible the notice that the *IRQ* timer latency was
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already high, accounting *12505 ns*. The IRQ delay was caused by the
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*bash-109109* process that disabled IRQs in the wake-up path
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(*_try_to_wake_up()* function). The duration of the IRQ handler that woke
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up the timerlat thread, informed with the **osnoise:irq_noise** event, was
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also high and added more *12553 ns* to the Thread latency. Finally, the
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**osnoise:thread_noise** added by the currently running thread (including
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the scheduling overhead) added more *4769 ns*. Summing up these values,
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the *Thread* timer latency accounted for *30328 ns*.
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The primary reason for this high value is the wake-up path that was hit
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twice during this case: when the *bash-109109* was waking up a thread
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and then when the *timerlat* thread was awakened. This information can
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then be used as the starting point of a more fine-grained analysis.
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1 #12322 | 0 0 1 15 | 10 3 9 31
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2 #12322 | 3 0 1 12 | 10 3 9 23
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3 #12322 | 1 0 1 21 | 8 2 8 34
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4 #12322 | 1 0 1 17 | 10 2 11 33
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5 #12322 | 0 0 1 12 | 8 3 8 25
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6 #12322 | 1 0 1 14 | 16 3 11 35
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7 #12322 | 0 0 1 14 | 9 2 8 29
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8 #12322 | 1 0 1 22 | 9 3 9 34
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9 #12322 | 0 0 1 14 | 8 2 8 24
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10 #12322 | 1 0 0 12 | 9 3 8 24
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11 #12322 | 0 0 0 15 | 6 2 7 29
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12 #12321 | 1 0 0 13 | 5 3 8 23
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13 #12319 | 0 0 1 14 | 9 3 9 26
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14 #12321 | 1 0 0 13 | 6 2 8 24
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15 #12321 | 1 0 1 15 | 12 3 11 27
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16 #12318 | 0 0 1 13 | 7 3 10 24
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17 #12319 | 0 0 1 13 | 11 3 9 25
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18 #12318 | 0 0 0 12 | 8 2 8 20
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19 #12319 | 0 0 1 18 | 10 2 9 28
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20 #12317 | 0 0 0 20 | 9 3 8 34
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21 #12318 | 0 0 0 13 | 8 3 8 28
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22 #12319 | 0 0 1 11 | 8 3 10 22
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23 #12320 | 28 0 1 28 | 41 3 11 41
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rtla timerlat hit stop tracing
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## CPU 23 hit stop tracing, analyzing it ##
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IRQ handler delay: 27.49 us (65.52 %)
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IRQ latency: 28.13 us
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Timerlat IRQ duration: 9.59 us (22.85 %)
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Blocking thread: 3.79 us (9.03 %)
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objtool:49256 3.79 us
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Blocking thread stacktrace
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-> timerlat_irq
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-> __hrtimer_run_queues
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-> hrtimer_interrupt
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-> __sysvec_apic_timer_interrupt
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-> sysvec_apic_timer_interrupt
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-> asm_sysvec_apic_timer_interrupt
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-> _raw_spin_unlock_irqrestore
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-> cgroup_rstat_flush_locked
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-> cgroup_rstat_flush_irqsafe
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-> mem_cgroup_flush_stats
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-> mem_cgroup_wb_stats
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-> balance_dirty_pages
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-> balance_dirty_pages_ratelimited_flags
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-> btrfs_buffered_write
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-> btrfs_do_write_iter
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-> vfs_write
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-> __x64_sys_pwrite64
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-> do_syscall_64
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-> entry_SYSCALL_64_after_hwframe
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------------------------------------------------------------------------
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Thread latency: 41.96 us (100%)
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The system has exit from idle latency!
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Max timerlat IRQ latency from idle: 17.48 us in cpu 4
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Saving trace to timerlat_trace.txt
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In this case, the major factor was the delay suffered by the *IRQ handler*
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that handles **timerlat** wakeup: *65.52%*. This can be caused by the
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current thread masking interrupts, which can be seen in the blocking
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thread stacktrace: the current thread (*objtool:49256*) disabled interrupts
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via *raw spin lock* operations inside mem cgroup, while doing write
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syscall in a btrfs file system.
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The raw trace is saved in the **timerlat_trace.txt** file for further analysis.
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Note that **rtla timerlat** was dispatched without changing *timerlat* tracer
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threads' priority. That is generally not needed because these threads hava

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