tracing: make a snapshot feature available from userspace
Subsystems (4)
cover tracing: make a snapshot feature available from userspace
Hi, Steven,
This patch series make a snapshot feature available from userspace via debugfs.
(But I know that you are working for multi-buffer. If these patches collide with your work much, I will resubmit my patches after that. What would you think?)
If we set CONFIG_TRACER_SNAPSHOT, this snapshot feature becomes available to all non latency tracers. (Latency tracers which record max latency, such as "irqsoff" or "wakeup", can't use this feature, since those are already using the snapshot mechanism internally.)
Snapshot preserves a trace buffer at a particular point in time without stopping tracing. Ftrace swaps the current buffer with a spare buffer, and tracing continues in the (previous) spare buffer.
The following debugfs files in "tracing" are related to this feature:
snapshot:
This is used to take a snapshot and to read the output of the snapshot. Echo 1 into this file to allocate a spare buffer and to take a snapshot, then read the snapshot from the file in the same format as "trace". Both reads snapshot and tracing are executable in parallel. Echoing 0 erases the snapshot contents.
snapshot_allocate:
This is used to pre-allocate or free a spare buffer. Echo 1 into this file to pre-allocate a spare buffer if you don't want to fail in the next snapshot due to memory allocation failure, or if you don't want to lose older trace data while allocating buffer. Echo 0 to free the spare buffer when the snapshot becomes unnecessary. If you take the next snapshot again, you can reuse the buffer, then just erase the snapshot contents by echoing 1 into the "snapshot" file, instead of freeing the buffer.
Reads from this file display whether the spare buffer is allocated. When current_tracer is changed, the allocated spare buffer is freed. If the next tracer is one of the latency tracers, this value turns into 1 and can't be changed, or else the value starts with 0.
Here is an example of using the snapshot feature.
# echo 1 > snapshot_allocate (if you want to pre-allocate the spare buffer)
# echo 1 > events/sched/enable
# echo 1 > snapshot
# cat snapshot
# tracer: nop
#
# entries-in-buffer/entries-written: 71/71 #P:8
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
<idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
[...]
<idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120
# cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 77/77 #P:8
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
<idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
[...]---
Hiraku Toyooka (4):
tracing: add description of snapshot to Documentation/trace/ftrace.txt
tracing: make a snapshot feature available from userspace
tracing: add a resize function for making one buffer equivalent to the other buffer
tracing: change tracer's integer flags to bool
Documentation/trace/ftrace.txt | 97 +++++++++++++++ include/linux/ftrace_event.h | 3 kernel/trace/Kconfig | 11 ++ kernel/trace/trace.c | 247 +++++++++++++++++++++++++++++++------ kernel/trace/trace.h | 5 - kernel/trace/trace_irqsoff.c | 12 +- kernel/trace/trace_sched_wakeup.c | 8 + 7 files changed, 332 insertions(+), 51 deletions(-)
Signature · 4 lines
Hiraku TOYOOKA Linux Technology Center Yokohama Research Laboratory Hitachi Ltd.
PATCH 1/4 tracing: change tracer's integer flags to bool
print_max and use_max_tr in struct tracer are "int" variables and used like flags. This is wasteful, so change the type to "bool".
---
kernel/trace/trace.h | 4 ++-- kernel/trace/trace_irqsoff.c | 12 ++++++------ kernel/trace/trace_sched_wakeup.c | 8 ++++---- 3 files changed, 12 insertions(+), 12 deletions(-)
kernel/trace/trace.h +2 -2
@@ -285,8 +285,8 @@ struct tracer {int (*set_flag)(u32 old_flags,u32 bit,int set);struct tracer *next;struct tracer_flags *flags; -int print_max; -int use_max_tr; +bool print_max; +bool use_max_tr; };
kernel/trace/trace_irqsoff.c +6 -6
@@ -604,7 +604,7 @@ static struct tracer irqsoff_tracer __read_mostly = .reset = irqsoff_tracer_reset, .start = irqsoff_tracer_start, .stop = irqsoff_tracer_stop, - .print_max = 1, + .print_max = true, .print_header = irqsoff_print_header, .print_line = irqsoff_print_line, .flags = &tracer_flags,
@@ -614,7 +614,7 @@ static struct tracer irqsoff_tracer __read_mostly =#endif .open = irqsoff_trace_open, .close = irqsoff_trace_close, - .use_max_tr = 1, + .use_max_tr = true, };# define register_irqsoff (trace) register_tracer(&trace)#else
@@ -637,7 +637,7 @@ static struct tracer preemptoff_tracer __read_mostly = .reset = irqsoff_tracer_reset, .start = irqsoff_tracer_start, .stop = irqsoff_tracer_stop, - .print_max = 1, + .print_max = true, .print_header = irqsoff_print_header, .print_line = irqsoff_print_line, .flags = &tracer_flags,
@@ -647,7 +647,7 @@ static struct tracer preemptoff_tracer __read_mostly =#endif .open = irqsoff_trace_open, .close = irqsoff_trace_close, - .use_max_tr = 1, + .use_max_tr = true, };# define register_preemptoff (trace) register_tracer(&trace)#else
@@ -672,7 +672,7 @@ static struct tracer preemptirqsoff_tracer __read_mostly = .reset = irqsoff_tracer_reset, .start = irqsoff_tracer_start, .stop = irqsoff_tracer_stop, - .print_max = 1, + .print_max = true, .print_header = irqsoff_print_header, .print_line = irqsoff_print_line, .flags = &tracer_flags,
@@ -682,7 +682,7 @@ static struct tracer preemptirqsoff_tracer __read_mostly =#endif .open = irqsoff_trace_open, .close = irqsoff_trace_close, - .use_max_tr = 1, + .use_max_tr = true, };# define register_preemptirqsoff (trace) register_tracer(&trace)
kernel/trace/trace_sched_wakeup.c +4 -4
@@ -589,7 +589,7 @@ static struct tracer wakeup_tracer __read_mostly = .reset = wakeup_tracer_reset, .start = wakeup_tracer_start, .stop = wakeup_tracer_stop, - .print_max = 1, + .print_max = true, .print_header = wakeup_print_header, .print_line = wakeup_print_line, .flags = &tracer_flags,
@@ -599,7 +599,7 @@ static struct tracer wakeup_tracer __read_mostly =#endif .open = wakeup_trace_open, .close = wakeup_trace_close, - .use_max_tr = 1, + .use_max_tr = true, }; static struct tracer wakeup_rt_tracer __read_mostly =
@@ -610,7 +610,7 @@ static struct tracer wakeup_rt_tracer __read_mostly = .start = wakeup_tracer_start, .stop = wakeup_tracer_stop, .wait_pipe = poll_wait_pipe, - .print_max = 1, + .print_max = true, .print_header = wakeup_print_header, .print_line = wakeup_print_line, .flags = &tracer_flags,
@@ -620,7 +620,7 @@ static struct tracer wakeup_rt_tracer __read_mostly =#endif .open = wakeup_trace_open, .close = wakeup_trace_close, - .use_max_tr = 1, + .use_max_tr = true, };__init static int init_wakeup_tracer (void )
[tip:perf/core] tracing: Change tracer's integer flags to bool
Commit-ID: f43c738bfa8608424610e4fc1aef4d4644e2ce11 Gitweb: http://git.kernel.org/tip/f43c738bfa8608424610e4fc1aef4d4644e2ce11 Author: Hiraku Toyooka <[email protected]> AuthorDate: Tue, 2 Oct 2012 17:27:10 +0900 Committer: Steven Rostedt <[email protected]> CommitDate: Wed, 31 Oct 2012 16:45:25 -0400
tracing: Change tracer's integer flags to bool
print_max and use_max_tr in struct tracer are "int" variables and used like flags. This is wasteful, so change the type to "bool".
--- kernel/trace/trace.h | 4 ++-- kernel/trace/trace_irqsoff.c | 12 ++++++------ kernel/trace/trace_sched_wakeup.c | 8 ++++---- 3 files changed, 12 insertions(+), 12 deletions(-)
kernel/trace/trace.h +2 -2
@@ -285,8 +285,8 @@ struct tracer {int (*set_flag)(u32 old_flags,u32 bit,int set);struct tracer *next;struct tracer_flags *flags; -int print_max; -int use_max_tr; +bool print_max; +bool use_max_tr; };
kernel/trace/trace_irqsoff.c +6 -6
@@ -604,7 +604,7 @@ static struct tracer irqsoff_tracer __read_mostly = .reset = irqsoff_tracer_reset, .start = irqsoff_tracer_start, .stop = irqsoff_tracer_stop, - .print_max = 1, + .print_max = true, .print_header = irqsoff_print_header, .print_line = irqsoff_print_line, .flags = &tracer_flags,
@@ -614,7 +614,7 @@ static struct tracer irqsoff_tracer __read_mostly =#endif .open = irqsoff_trace_open, .close = irqsoff_trace_close, - .use_max_tr = 1, + .use_max_tr = true, };# define register_irqsoff (trace) register_tracer(&trace)#else
@@ -637,7 +637,7 @@ static struct tracer preemptoff_tracer __read_mostly = .reset = irqsoff_tracer_reset, .start = irqsoff_tracer_start, .stop = irqsoff_tracer_stop, - .print_max = 1, + .print_max = true, .print_header = irqsoff_print_header, .print_line = irqsoff_print_line, .flags = &tracer_flags,
@@ -647,7 +647,7 @@ static struct tracer preemptoff_tracer __read_mostly =#endif .open = irqsoff_trace_open, .close = irqsoff_trace_close, - .use_max_tr = 1, + .use_max_tr = true, };# define register_preemptoff (trace) register_tracer(&trace)#else
@@ -672,7 +672,7 @@ static struct tracer preemptirqsoff_tracer __read_mostly = .reset = irqsoff_tracer_reset, .start = irqsoff_tracer_start, .stop = irqsoff_tracer_stop, - .print_max = 1, + .print_max = true, .print_header = irqsoff_print_header, .print_line = irqsoff_print_line, .flags = &tracer_flags,
@@ -682,7 +682,7 @@ static struct tracer preemptirqsoff_tracer __read_mostly =#endif .open = irqsoff_trace_open, .close = irqsoff_trace_close, - .use_max_tr = 1, + .use_max_tr = true, };# define register_preemptirqsoff (trace) register_tracer(&trace)
kernel/trace/trace_sched_wakeup.c +4 -4
@@ -589,7 +589,7 @@ static struct tracer wakeup_tracer __read_mostly = .reset = wakeup_tracer_reset, .start = wakeup_tracer_start, .stop = wakeup_tracer_stop, - .print_max = 1, + .print_max = true, .print_header = wakeup_print_header, .print_line = wakeup_print_line, .flags = &tracer_flags,
@@ -599,7 +599,7 @@ static struct tracer wakeup_tracer __read_mostly =#endif .open = wakeup_trace_open, .close = wakeup_trace_close, - .use_max_tr = 1, + .use_max_tr = true, }; static struct tracer wakeup_rt_tracer __read_mostly =
@@ -610,7 +610,7 @@ static struct tracer wakeup_rt_tracer __read_mostly = .start = wakeup_tracer_start, .stop = wakeup_tracer_stop, .wait_pipe = poll_wait_pipe, - .print_max = 1, + .print_max = true, .print_header = wakeup_print_header, .print_line = wakeup_print_line, .flags = &tracer_flags,
@@ -620,7 +620,7 @@ static struct tracer wakeup_rt_tracer __read_mostly =#endif .open = wakeup_trace_open, .close = wakeup_trace_close, - .use_max_tr = 1, + .use_max_tr = true, };__init static int init_wakeup_tracer (void )
PATCH 2/4 tracing: add a resize function for making one buffer equivalent to the other buffer
In reply to Hiraku ToyookaTrace buffer size is now per-cpu, so that there are following two patterns in resize of the buffers.
(1) resize per-cpu buffers to same given size
(2) resize per-cpu buffers to the other trace_array's buffer size for each CPU (such as preparing the max_tr which is equivalent to the global_trace's size)
__tracing_resize_ring_buffer() can be used for (1), and had implemented (2) inside it for resetting the global_trace to the original size.
(2) was also implemented in other place. So this patch assembles them in a new function - resize_buffer_even().
---
kernel/trace/trace.c | 57 ++++++++++++++++++++++++++------------------------ 1 files changed, 30 insertions(+), 27 deletions(-)
kernel/trace/trace.c +30 -27
@@ -3017,6 +3017,31 @@ static void set_buffer_entries(struct trace_array *tr, unsigned long val) tr->data[cpu]->entries = val; } +/* resize @tr's buffer to the size of @size_tr's entries */ +static int resize_buffer_even (struct trace_array *tr, +struct trace_array *size_tr,int cpu_id) +{ +int cpu, ret =0 ; + +if (cpu_id ==RING_BUFFER_ALL_CPUS ) { +for_each_tracing_cpu (cpu) { + ret =ring_buffer_resize (tr->buffer, + size_tr->data[cpu]->entries, cpu); +if (ret <0 ) +break ; + tr->data[cpu]->entries = size_tr->data[cpu]->entries; + } + }else { + ret =ring_buffer_resize (tr->buffer, + size_tr->data[cpu_id]->entries, cpu_id); +if (ret ==0 ) + tr->data[cpu_id]->entries = + size_tr->data[cpu_id]->entries; + } + +return ret; +} +static int __tracing_resize_ring_buffer (unsigned long size,int cpu) {int ret;
@@ -3037,23 +3062,7 @@ static int __tracing_resize_ring_buffer(unsigned long size, int cpu) ret =ring_buffer_resize (max_tr.buffer, size, cpu);if (ret <0 ) { -int r =0 ; - -if (cpu ==RING_BUFFER_ALL_CPUS ) { -int i; -for_each_tracing_cpu (i) { - r =ring_buffer_resize (global_trace.buffer, - global_trace.data[i]->entries, - i); -if (r <0 ) -break ; - } - }else { - r =ring_buffer_resize (global_trace.buffer, - global_trace.data[cpu]->entries, - cpu); - } - +int r =resize_buffer_even (&global_trace, &global_trace, cpu);if (r <0 ) {/* * AARGH! We are left with different
@@ -3191,17 +3200,11 @@ static int tracing_set_tracer(const char *buf) topts =create_trace_option_files (t);if (t->use_max_tr) { -int cpu;/* we need to make per cpu buffer sizes equivalent */ -for_each_tracing_cpu (cpu) { - ret =ring_buffer_resize (max_tr.buffer, - global_trace.data[cpu]->entries, - cpu); -if (ret <0 ) - goto out; - max_tr.data[cpu]->entries = - global_trace.data[cpu]->entries; - } + ret =resize_buffer_even (&max_tr, &global_trace, +RING_BUFFER_ALL_CPUS ); +if (ret <0 ) + goto out; }if (t->init) {
Steven Rostedt
maintainerRe: [PATCH -tip 2/4] tracing: add a resize function for making one buffer equivalent to the other buffer
Quoted 30 lines · Hiraku Toyooka
Trace buffer size is now per-cpu, so that there are following two patterns in resize of the buffers.
(1) resize per-cpu buffers to same given size
(2) resize per-cpu buffers to the other trace_array's buffer size for each CPU (such as preparing themax_trwhich is equivalent to the global_trace's size)
__tracing_resize_ring_buffer()can be used for (1), and had implemented (2) inside it for resetting theglobal_traceto the original size.(2) was also implemented in other place. So this patch assembles them in a new function -
resize_buffer_even().Signed-off-by Hiraku Toyooka <[email protected]>Cc Steven Rostedt <[email protected]>Cc Frederic Weisbecker <[email protected]>Cc Ingo Molnar <[email protected]>---
kernel/trace/trace.c | 57 ++++++++++++++++++++++++++------------------------ 1 files changed, 30 insertions(+), 27 deletions(-)kernel/trace/trace.c +0
-0
kernel/trace/trace.c static void set_buffer_entries(struct trace_array *tr, unsigned long val) · line 3021
Quoted 5 lines
kernel/trace/trace.c +2
-0@@ -3017,6 +3017,31 @@ static void set_buffer_entries(struct trace_array *tr, unsigned long val) tr->data[cpu]->entries = val; } +/* resize @tr's buffer to the size of @size_tr's entries */ +static int resize_buffer_even(struct trace_array *tr,
I don't mind this patch, but I just hate the name "resize_buffer_even". What about "resize_buffer_duplicate_size"?
-- Steve
Re: [PATCH -tip 2/4] tracing: add a resize function for making one buffer equivalent to the other buffer
Steven,
I'm sorry for my late reply.
(I was outside the office for business trip.)
Quoted 6 lines · Steven Rostedt
kernel/trace/trace.c static void set_buffer_entries(struct trace_array *tr, unsigned long val) · line 3021
Quoted 2 lines
kernel/trace/trace.c +2
-0+/* resize @tr's buffer to the size of @size_tr's entries */ +static int resize_buffer_even(struct trace_array *tr,I don't mind this patch, but I just hate the name "
resize_buffer_even". What about "resize_buffer_duplicate_size"?
O.K.
I'll send modified version of this patch later.
Regards,
Hiraku Toyooka
Steven Rostedt
maintainerRe: [PATCH -tip 2/4] tracing: add a resize function for making one buffer equivalent to the other buffer
Quoted 5 lines · Hiraku Toyooka
Steven,
I'm sorry for my late reply.
(I was outside the office for business trip.)
No problem. I'm currently on a business trip now :-)
-- Steve
PATCH 3/4 tracing: make a snapshot feature available from userspace
In reply to Hiraku ToyookaFtrace has a snapshot feature available from kernel space and latency tracers (e.g. irqsoff) are using it. This patch enables user applictions to take a snapshot via debugfs.
Add following two debugfs files in "tracing" directory.
snapshot:
This is used to take a snapshot and to read the output of the snapshot.
# echo 1 > snapshot
# cat snapshot
snapshot_allocate:
Echoing 1 to the "snapshot" allocates the max_tr buffer if it is not allocated. So taking a snapshot may delay or fail beacuse of memory allocation. To avoid that, this file provides a mean to pre-allocate (or free) the max_tr buffer.
# echo 1 > snapshot_allocate
[...]
# echo 1 > snapshot
---
include/linux/ftrace_event.h | 3 + kernel/trace/Kconfig | 11 ++ kernel/trace/trace.c | 190 +++++++++++++++++++++++++++++++++++++++--- kernel/trace/trace.h | 1 4 files changed, 193 insertions(+), 12 deletions(-)
include/linux/ftrace_event.h +3 -0
@@ -84,6 +84,9 @@ struct trace_iterator {long idx;cpumask_var_t started; + +/* it's true when current open file is snapshot */ +bool snapshot; };
kernel/trace/Kconfig +11 -0
@@ -102,6 +102,17 @@ config RING_BUFFER_ALLOW_SWAP Allow the use of ring_buffer_swap_cpu. Adds a very slight overhead to tracing when enabled. +config TRACER_SNAPSHOT + bool + default y + select TRACER_MAX_TRACE + help + Allow tracing users to take snapshot of the current buffer using the + ftrace interface, e.g.: + + echo 1 > /sys/kernel/debug/tracing/snapshot + cat snapshot + # All tracer options should select GENERIC_TRACER. For those options that are # enabled by all tracers (context switch and event tracer) they select TRACING. # This allows those options to appear when no other tracer is selected. But the
kernel/trace/trace.c +178 -12
283 lines not on this page · raw message
kernel/trace/trace.h +1 -0
@@ -287,6 +287,7 @@ struct tracer {struct tracer_flags *flags;bool print_max;bool use_max_tr; +bool allocated_snapshot; };
PATCH 4/4 tracing: add description of snapshot to Documentation/trace/ftrace.txt
In reply to Hiraku ToyookaThis patch adds snapshot description in ftrace documentation. This description includes what the snapshot is and how to use it.
---
Documentation/trace/ftrace.txt | 97 ++++++++++++++++++++++++++++++++++++++++ 1 files changed, 97 insertions(+), 0 deletions(-)
Documentation/trace/ftrace.txt +97 -0
@@ -1842,6 +1842,103 @@ an error. # cat buffer_size_kb 85 +Snapshot +-------- +CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature +available to all non latency tracers. (Latency tracers which +record max latency, such as "irqsoff" or "wakeup", can't use +this feature, since those are already using the snapshot +mechanism internally.) + +Snapshot preserves a trace buffer at a particular point in time +without stopping tracing. Ftrace swaps the current buffer with a +spare buffer, and tracing continues in the (previous) spare +buffer. + +The following debugfs files in "tracing" are related to this +feature: + + snapshot: + + This is used to take a snapshot and to read the output + of the snapshot. Echo 1 into this file to allocate a + spare buffer and to take a snapshot, then read the + snapshot from the file in the same format as "trace" + (described above in the section "The File System"). Both + reads snapshot and tracing are executable in parallel. + Echoing 0 erases the snapshot contents. + + snapshot_allocate: + + This is used to pre-allocate or free a spare buffer. + Echo 1 into this file to pre-allocate a spare buffer if + you don't want to fail in the next snapshot due to + memory allocation failure, or if you don't want to lose + older trace data while allocating buffer. Echo 0 to free + the spare buffer when the snapshot becomes unnecessary. + If you take the next snapshot again, you can reuse the + buffer, then just erase the snapshot contents by echoing + 1 into the "snapshot" file, instead of freeing the + buffer. + + Reads from this file display whether the spare buffer is + allocated. When current_tracer is changed, the allocated + spare buffer is freed. If the next tracer is one of the + latency tracers, this value turns into 1 and can't be + changed, or else the value starts with 0. + + +Here is an example of using the snapshot feature. + + # echo 1 > snapshot_allocate (if you want to pre-allocate the spare buffer) + # echo 1 > events/sched/enable + # echo 1 > snapshot + # cat snapshot +# tracer: nop +# +# entries-in-buffer/entries-written: 71/71 #P:8 +# +# _-----=> irqs-off +# / _----=> need-resched +# | / _---=> hardirq/softirq +# || / _--=> preempt-depth +# ||| / delay +# TASK-PID CPU# |||| TIMESTAMP FUNCTION +# | | | |||| | | + <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120 + sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120 +[...] + <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120 + # cat trace +# tracer: nop +# +# entries-in-buffer/entries-written: 77/77 #P:8 +# +# _-----=> irqs-off +# / _----=> need-resched +# | / _---=> hardirq/softirq +# || / _--=> preempt-depth +# ||| / delay +# TASK-PID CPU# |||| TIMESTAMP FUNCTION +# | | | |||| | | + <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120 + snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120 +[...] + + +If you try to use this snapshot feature when current tracer is +one of the latency tracers, you will get the following results. + + # echo wakeup > current_tracer + # cat snapshot_allocate +1 + # echo 1 > snapshot_allocate +bash: echo: write error: Device or resource busy + # echo 1 > snapshot +bash: echo: write error: Device or resource busy + # cat snapshot +cat: snapshot: Device or resource busy + ----------- More details can be found in the source code, in the