Hello,
This is the first version of off-cpu profiling support. Together with
(PMU-based) cpu profiling, it can show holistic view of the performance
characteristics of your application or system.
With BPF, it can aggregate scheduling stats for interested tasks
and/or states and convert the data into a form of perf sample records.
I chose the bpf-output event which is a software event supposed to be
consumed by BPF programs and renamed it as "offcpu-time". So it
requires no change on the perf report side except for setting sample
types of bpf-output event.
Basically it collects userspace callstack for tasks as it's what users
want mostly. Maybe we can add support for the kernel stacks but I'm
afraid that it'd cause more overhead. So the offcpu-time event will
always have callchains regardless of the command line option, and it
enables the children mode in perf report by default.
It adds --off-cpu option to perf record like below:
$ sudo perf record -a --off-cpu -- perf bench sched messaging -l 1000
# Running 'sched/messaging' benchmark:
# 20 sender and receiver processes per group
# 10 groups == 400 processes run
Total time: 1.518 [sec]
[ perf record: Woken up 9 times to write data ]
[ perf record: Captured and wrote 5.313 MB perf.data (53341 samples) ]
Then we can run perf report as usual. The below is just to skip less
important parts.
$ sudo perf report --stdio --call-graph=no --percent-limit=2
# To display the perf.data header info, please use --header/--header-only options.
#
#
# Total Lost Samples: 0
#
# Samples: 52K of event 'cycles'
# Event count (approx.): 42522453276
#
# Children Self Command Shared Object Symbol
# ........ ........ ............... ................ ..................................
#
9.58% 9.58% sched-messaging [kernel.vmlinux] [k] audit_filter_rules.constprop.0
8.46% 8.46% sched-messaging [kernel.vmlinux] [k] audit_filter_syscall
4.54% 4.54% sched-messaging [kernel.vmlinux] [k] copy_user_enhanced_fast_string
2.94% 2.94% sched-messaging [kernel.vmlinux] [k] unix_stream_read_generic
2.45% 2.45% sched-messaging [kernel.vmlinux] [k] memcg_slab_free_hook
# Samples: 983 of event 'offcpu-time'
# Event count (approx.): 684538813464
#
# Children Self Command Shared Object Symbol
# ........ ........ ............... .................... ..........................
#
83.86% 0.00% sched-messaging libc-2.33.so [.] __libc_start_main
83.86% 0.00% sched-messaging perf [.] cmd_bench
83.86% 0.00% sched-messaging perf [.] main
83.86% 0.00% sched-messaging perf [.] run_builtin
83.64% 0.00% sched-messaging perf [.] bench_sched_messaging
41.35% 41.35% sched-messaging libpthread-2.33.so [.] __read
38.88% 38.88% sched-messaging libpthread-2.33.so [.] __write
3.41% 3.41% sched-messaging libc-2.33.so [.] __poll
The perf bench sched messaging created 400 processes to send/receive
messages through unix sockets. It spent a large portion of cpu cycles
for audit filter and read/copy the messages while most of the
offcpu-time was in read and write calls.
You can get the code from 'perf/offcpu-v1' branch in my tree at
git://git.kernel.org/pub/scm/linux/kernel/git/namhyung/linux-perf.git
Enjoy! :)
Thanks,
Namhyung
Namhyung Kim (4):
perf report: Do not extend sample type of bpf-output event
perf record: Enable off-cpu analysis with BPF
perf record: Implement basic filtering for off-cpu
perf record: Handle argument change in sched_switch
tools/perf/Makefile.perf | 1 +
tools/perf/builtin-record.c | 21 ++
tools/perf/util/Build | 1 +
tools/perf/util/bpf_off_cpu.c | 301 +++++++++++++++++++++++++
tools/perf/util/bpf_skel/off_cpu.bpf.c | 214 ++++++++++++++++++
tools/perf/util/evsel.c | 4 +-
6 files changed, 540 insertions(+), 2 deletions(-)
create mode 100644 tools/perf/util/bpf_off_cpu.c
create mode 100644 tools/perf/util/bpf_skel/off_cpu.bpf.c
base-commit: 41204da4c16071be9090940b18f566832d46becc
--
2.36.0.rc2.479.g8af0fa9b8e-goog
Recently sched_switch tracepoint added a new argument for prev_state,
but it's hard to handle the change in a BPF program. Instead, we can
check the function prototype in BTF before loading the program.
Thus I make two copies of the tracepoint handler and select one based
on the BTF info.
Signed-off-by: Namhyung Kim <[email protected]>
---
tools/perf/util/bpf_off_cpu.c | 32 +++++++++++++++
tools/perf/util/bpf_skel/off_cpu.bpf.c | 55 ++++++++++++++++++++------
2 files changed, 76 insertions(+), 11 deletions(-)
diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c
index 89f36229041d..38aeb13d3d25 100644
--- a/tools/perf/util/bpf_off_cpu.c
+++ b/tools/perf/util/bpf_off_cpu.c
@@ -86,6 +86,37 @@ static void off_cpu_finish(void *arg __maybe_unused)
off_cpu_bpf__destroy(skel);
}
+/* recent kernel added prev_state arg, so it needs to call the proper function */
+static void check_sched_switch_args(void)
+{
+ const struct btf *btf = bpf_object__btf(skel->obj);
+ const struct btf_type *t1, *t2, *t3;
+ u32 type_id;
+
+ type_id = btf__find_by_name_kind(btf, "bpf_trace_sched_switch",
+ BTF_KIND_TYPEDEF);
+ if ((s32)type_id < 0)
+ goto old_format;
+
+ t1 = btf__type_by_id(btf, type_id);
+ if (t1 == NULL)
+ goto old_format;
+
+ t2 = btf__type_by_id(btf, t1->type);
+ if (t2 == NULL || !btf_is_ptr(t2))
+ goto old_format;
+
+ t3 = btf__type_by_id(btf, t2->type);
+ if (t3 && btf_is_func_proto(t3) && btf_vlen(t3) == 4) {
+ /* new format: disable old functions */
+ bpf_program__set_autoload(skel->progs.on_switch3, false);
+ return;
+ }
+
+old_format:
+ bpf_program__set_autoload(skel->progs.on_switch4, false);
+}
+
int off_cpu_prepare(struct evlist *evlist, struct target *target)
{
int err, fd, i;
@@ -114,6 +145,7 @@ int off_cpu_prepare(struct evlist *evlist, struct target *target)
}
set_max_rlimit();
+ check_sched_switch_args();
err = off_cpu_bpf__load(skel);
if (err) {
diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
index 27425fe361e2..e11e198af86f 100644
--- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
+++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
@@ -121,22 +121,13 @@ static inline int can_record(struct task_struct *t, int state)
return 1;
}
-SEC("tp_btf/sched_switch")
-int on_switch(u64 *ctx)
+static int on_switch(u64 *ctx, struct task_struct *prev,
+ struct task_struct *next, int state)
{
__u64 ts;
- int state;
__u32 pid, stack_id;
- struct task_struct *prev, *next;
struct tstamp_data elem, *pelem;
- if (!enabled)
- return 0;
-
- prev = (struct task_struct *)ctx[1];
- next = (struct task_struct *)ctx[2];
- state = get_task_state(prev);
-
ts = bpf_ktime_get_ns();
if (!can_record(prev, state))
@@ -178,4 +169,46 @@ int on_switch(u64 *ctx)
return 0;
}
+SEC("tp_btf/sched_switch")
+int on_switch3(u64 *ctx)
+{
+ struct task_struct *prev, *next;
+ int state;
+
+ if (!enabled)
+ return 0;
+
+ /*
+ * TP_PROTO(bool preempt, struct task_struct *prev,
+ * struct task_struct *next)
+ */
+ prev = (struct task_struct *)ctx[1];
+ next = (struct task_struct *)ctx[2];
+
+ state = get_task_state(prev);
+
+ return on_switch(ctx, prev, next, state);
+}
+
+SEC("tp_btf/sched_switch")
+int on_switch4(u64 *ctx)
+{
+ struct task_struct *prev, *next;
+ int prev_state;
+
+ if (!enabled)
+ return 0;
+
+ /*
+ * TP_PROTO(bool preempt, int prev_state,
+ * struct task_struct *prev,
+ * struct task_struct *next)
+ */
+ prev = (struct task_struct *)ctx[2];
+ next = (struct task_struct *)ctx[3];
+ prev_state = (int)ctx[1];
+
+ return on_switch(ctx, prev, next, prev_state);
+}
+
char LICENSE[] SEC("license") = "Dual BSD/GPL";
--
2.36.0.rc2.479.g8af0fa9b8e-goog
Hi Jiri,
On Fri, Apr 22, 2022 at 3:11 AM Jiri Olsa <[email protected]> wrote:
>
> On Thu, Apr 21, 2022 at 10:33:57PM -0700, Namhyung Kim wrote:
>
> SNIP
>
> > The perf bench sched messaging created 400 processes to send/receive
> > messages through unix sockets. It spent a large portion of cpu cycles
> > for audit filter and read/copy the messages while most of the
> > offcpu-time was in read and write calls.
> >
> > You can get the code from 'perf/offcpu-v1' branch in my tree at
> >
> > git://git.kernel.org/pub/scm/linux/kernel/git/namhyung/linux-perf.git
> >
> > Enjoy! :)
>
> CC builtin-record.o
> builtin-record.c:52:10: fatal error: util/off_cpu.h: No such file or directory
> 52 | #include "util/off_cpu.h"
>
> forgot to add util/off_cpu.h ?
Oops, you're right. Will resend soon.
Thanks,
Namhyung
Currently evsel__new_idx() sets more sample_type bits when it finds a
BPF-output event. But it should honor what's recorded in the perf
data file rather than blindly sets the bits. Otherwise it could lead
to a parse error when it recorded with a modified sample_type.
Signed-off-by: Namhyung Kim <[email protected]>
---
tools/perf/util/evsel.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 2a1729e7aee4..5f947adc16cb 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -269,8 +269,8 @@ struct evsel *evsel__new_idx(struct perf_event_attr *attr, int idx)
return NULL;
evsel__init(evsel, attr, idx);
- if (evsel__is_bpf_output(evsel)) {
- evsel->core.attr.sample_type |= (PERF_SAMPLE_RAW | PERF_SAMPLE_TIME |
+ if (evsel__is_bpf_output(evsel) && !attr->sample_type) {
+ evsel->core.attr.sample_type = (PERF_SAMPLE_RAW | PERF_SAMPLE_TIME |
PERF_SAMPLE_CPU | PERF_SAMPLE_PERIOD),
evsel->core.attr.sample_period = 1;
}
--
2.36.0.rc2.479.g8af0fa9b8e-goog
It should honor cpu and task filtering with -a, -C or -p, -t options.
Signed-off-by: Namhyung Kim <[email protected]>
---
tools/perf/builtin-record.c | 2 +-
tools/perf/util/bpf_off_cpu.c | 78 +++++++++++++++++++++++---
tools/perf/util/bpf_skel/off_cpu.bpf.c | 52 +++++++++++++++--
3 files changed, 119 insertions(+), 13 deletions(-)
diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index 3d24d528ba8e..592384e058c3 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -907,7 +907,7 @@ static int record__config_text_poke(struct evlist *evlist)
static int record__config_off_cpu(struct record *rec)
{
- return off_cpu_prepare(rec->evlist);
+ return off_cpu_prepare(rec->evlist, &rec->opts.target);
}
static bool record__kcore_readable(struct machine *machine)
diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c
index 1f87d2a9b86d..89f36229041d 100644
--- a/tools/perf/util/bpf_off_cpu.c
+++ b/tools/perf/util/bpf_off_cpu.c
@@ -6,6 +6,9 @@
#include "util/off_cpu.h"
#include "util/perf-hooks.h"
#include "util/session.h"
+#include "util/target.h"
+#include "util/cpumap.h"
+#include "util/thread_map.h"
#include <bpf/bpf.h>
#include "bpf_skel/off_cpu.skel.h"
@@ -57,8 +60,23 @@ static int off_cpu_config(struct evlist *evlist)
return 0;
}
-static void off_cpu_start(void *arg __maybe_unused)
+static void off_cpu_start(void *arg)
{
+ struct evlist *evlist = arg;
+
+ /* update task filter for the given workload */
+ if (!skel->bss->has_cpu && !skel->bss->has_task &&
+ perf_thread_map__pid(evlist->core.threads, 0) != -1) {
+ int fd;
+ u32 pid;
+ u8 val = 1;
+
+ skel->bss->has_task = 1;
+ fd = bpf_map__fd(skel->maps.task_filter);
+ pid = perf_thread_map__pid(evlist->core.threads, 0);
+ bpf_map_update_elem(fd, &pid, &val, BPF_ANY);
+ }
+
skel->bss->enabled = 1;
}
@@ -68,31 +86,75 @@ static void off_cpu_finish(void *arg __maybe_unused)
off_cpu_bpf__destroy(skel);
}
-int off_cpu_prepare(struct evlist *evlist)
+int off_cpu_prepare(struct evlist *evlist, struct target *target)
{
- int err;
+ int err, fd, i;
+ int ncpus = 1, ntasks = 1;
if (off_cpu_config(evlist) < 0) {
pr_err("Failed to config off-cpu BPF event\n");
return -1;
}
- set_max_rlimit();
-
- skel = off_cpu_bpf__open_and_load();
+ skel = off_cpu_bpf__open();
if (!skel) {
pr_err("Failed to open off-cpu skeleton\n");
return -1;
}
+ /* don't need to set cpu filter for system-wide mode */
+ if (target->cpu_list) {
+ ncpus = perf_cpu_map__nr(evlist->core.user_requested_cpus);
+ bpf_map__set_max_entries(skel->maps.cpu_filter, ncpus);
+ }
+
+ if (target__has_task(target)) {
+ ncpus = perf_thread_map__nr(evlist->core.threads);
+ bpf_map__set_max_entries(skel->maps.task_filter, ntasks);
+ }
+
+ set_max_rlimit();
+
+ err = off_cpu_bpf__load(skel);
+ if (err) {
+ pr_err("Failed to load off-cpu skeleton\n");
+ goto out;
+ }
+
+ if (target->cpu_list) {
+ u32 cpu;
+ u8 val = 1;
+
+ skel->bss->has_cpu = 1;
+ fd = bpf_map__fd(skel->maps.cpu_filter);
+
+ for (i = 0; i < ncpus; i++) {
+ cpu = perf_cpu_map__cpu(evlist->core.user_requested_cpus, i).cpu;
+ bpf_map_update_elem(fd, &cpu, &val, BPF_ANY);
+ }
+ }
+
+ if (target__has_task(target)) {
+ u32 pid;
+ u8 val = 1;
+
+ skel->bss->has_task = 1;
+ fd = bpf_map__fd(skel->maps.task_filter);
+
+ for (i = 0; i < ntasks; i++) {
+ pid = perf_thread_map__pid(evlist->core.threads, i);
+ bpf_map_update_elem(fd, &pid, &val, BPF_ANY);
+ }
+ }
+
err = off_cpu_bpf__attach(skel);
if (err) {
pr_err("Failed to attach off-cpu skeleton\n");
goto out;
}
- if (perf_hooks__set_hook("record_start", off_cpu_start, NULL) ||
- perf_hooks__set_hook("record_end", off_cpu_finish, NULL)) {
+ if (perf_hooks__set_hook("record_start", off_cpu_start, evlist) ||
+ perf_hooks__set_hook("record_end", off_cpu_finish, evlist)) {
pr_err("Failed to attach off-cpu skeleton\n");
goto out;
}
diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
index 2bc6f7cc59ea..27425fe361e2 100644
--- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
+++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
@@ -49,12 +49,28 @@ struct {
__uint(max_entries, MAX_ENTRIES);
} off_cpu SEC(".maps");
+struct {
+ __uint(type, BPF_MAP_TYPE_HASH);
+ __uint(key_size, sizeof(__u32));
+ __uint(value_size, sizeof(__u8));
+ __uint(max_entries, 1);
+} cpu_filter SEC(".maps");
+
+struct {
+ __uint(type, BPF_MAP_TYPE_HASH);
+ __uint(key_size, sizeof(__u32));
+ __uint(value_size, sizeof(__u8));
+ __uint(max_entries, 1);
+} task_filter SEC(".maps");
+
/* old kernel task_struct definition */
struct task_struct___old {
long state;
} __attribute__((preserve_access_index));
int enabled = 0;
+int has_cpu = 0;
+int has_task = 0;
/*
* recently task_struct->state renamed to __state so it made an incompatible
@@ -74,6 +90,37 @@ static inline int get_task_state(struct task_struct *t)
return BPF_CORE_READ(t_old, state);
}
+static inline int can_record(struct task_struct *t, int state)
+{
+ if (has_cpu) {
+ __u32 cpu = bpf_get_smp_processor_id();
+ __u8 *ok;
+
+ ok = bpf_map_lookup_elem(&cpu_filter, &cpu);
+ if (!ok)
+ return 0;
+ }
+
+ if (has_task) {
+ __u8 *ok;
+ __u32 pid = t->pid;
+
+ ok = bpf_map_lookup_elem(&task_filter, &pid);
+ if (!ok)
+ return 0;
+ }
+
+ /* kernel threads don't have user stack */
+ if (t->flags & PF_KTHREAD)
+ return 0;
+
+ if (state != TASK_INTERRUPTIBLE &&
+ state != TASK_UNINTERRUPTIBLE)
+ return 0;
+
+ return 1;
+}
+
SEC("tp_btf/sched_switch")
int on_switch(u64 *ctx)
{
@@ -92,10 +139,7 @@ int on_switch(u64 *ctx)
ts = bpf_ktime_get_ns();
- if (prev->flags & PF_KTHREAD)
- goto next;
- if (state != TASK_INTERRUPTIBLE &&
- state != TASK_UNINTERRUPTIBLE)
+ if (!can_record(prev, state))
goto next;
stack_id = bpf_get_stackid(ctx, &stacks,
--
2.36.0.rc2.479.g8af0fa9b8e-goog
On Thu, Apr 21, 2022 at 10:33:57PM -0700, Namhyung Kim wrote:
SNIP
> The perf bench sched messaging created 400 processes to send/receive
> messages through unix sockets. It spent a large portion of cpu cycles
> for audit filter and read/copy the messages while most of the
> offcpu-time was in read and write calls.
>
> You can get the code from 'perf/offcpu-v1' branch in my tree at
>
> git://git.kernel.org/pub/scm/linux/kernel/git/namhyung/linux-perf.git
>
> Enjoy! :)
CC builtin-record.o
builtin-record.c:52:10: fatal error: util/off_cpu.h: No such file or directory
52 | #include "util/off_cpu.h"
forgot to add util/off_cpu.h ?
jirka
>
> Thanks,
> Namhyung
>
>
> Namhyung Kim (4):
> perf report: Do not extend sample type of bpf-output event
> perf record: Enable off-cpu analysis with BPF
> perf record: Implement basic filtering for off-cpu
> perf record: Handle argument change in sched_switch
>
> tools/perf/Makefile.perf | 1 +
> tools/perf/builtin-record.c | 21 ++
> tools/perf/util/Build | 1 +
> tools/perf/util/bpf_off_cpu.c | 301 +++++++++++++++++++++++++
> tools/perf/util/bpf_skel/off_cpu.bpf.c | 214 ++++++++++++++++++
> tools/perf/util/evsel.c | 4 +-
> 6 files changed, 540 insertions(+), 2 deletions(-)
> create mode 100644 tools/perf/util/bpf_off_cpu.c
> create mode 100644 tools/perf/util/bpf_skel/off_cpu.bpf.c
>
>
> base-commit: 41204da4c16071be9090940b18f566832d46becc
> --
> 2.36.0.rc2.479.g8af0fa9b8e-goog
>
On Freitag, 22. April 2022 07:33:57 CEST Namhyung Kim wrote:
> Hello,
>
> This is the first version of off-cpu profiling support. Together with
> (PMU-based) cpu profiling, it can show holistic view of the performance
> characteristics of your application or system.
Hey Namhyung,
this is awesome news! In hotspot, I've long done off-cpu profiling manually by
looking at the time between --switch-events. The downside is that we also need
to track the sched:sched_switch event to get a call stack. But this approach
also works with dwarf based unwinding, and also includes kernel stacks.
> With BPF, it can aggregate scheduling stats for interested tasks
> and/or states and convert the data into a form of perf sample records.
> I chose the bpf-output event which is a software event supposed to be
> consumed by BPF programs and renamed it as "offcpu-time". So it
> requires no change on the perf report side except for setting sample
> types of bpf-output event.
>
> Basically it collects userspace callstack for tasks as it's what users
> want mostly. Maybe we can add support for the kernel stacks but I'm
> afraid that it'd cause more overhead. So the offcpu-time event will
> always have callchains regardless of the command line option, and it
> enables the children mode in perf report by default.
Has anything changed wrt perf/bpf and user applications not compiled with `-
fno-omit-frame-pointer`? I.e. does this new utility only work for specially
compiled applications, or do we also get backtraces for "normal" binaries that
we can install through package managers?
Thanks
--
Milian Wolff | [email protected] | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts
On Mon, Apr 25, 2022 at 5:42 AM Milian Wolff <[email protected]> wrote:
>
> On Freitag, 22. April 2022 17:01:15 CEST Namhyung Kim wrote:
> > Hi Milian,
> >
> > On Fri, Apr 22, 2022 at 3:21 AM Milian Wolff <[email protected]> wrote:
> > > On Freitag, 22. April 2022 07:33:57 CEST Namhyung Kim wrote:
> > > > Hello,
> > > >
> > > > This is the first version of off-cpu profiling support. Together with
> > > > (PMU-based) cpu profiling, it can show holistic view of the performance
> > > > characteristics of your application or system.
> > >
> > > Hey Namhyung,
> > >
> > > this is awesome news! In hotspot, I've long done off-cpu profiling
> > > manually by looking at the time between --switch-events. The downside is
> > > that we also need to track the sched:sched_switch event to get a call
> > > stack. But this approach also works with dwarf based unwinding, and also
> > > includes kernel stacks.
> >
> > Thanks, I've also briefly thought about the switch event based off-cpu
> > profiling as it doesn't require root. But collecting call stacks is hard
> > and I'd like to do it in kernel/bpf to reduce the overhead.
>
> I'm all for reducing the overhead, I just wonder about the practicality. At
> the very least, please make sure to note this limitation explicitly to end
> users. As a preacher for perf, I have come across lots of people stumbling
> over `perf record -g` not producing any sensible output because they are
> simply not aware that this requires frame pointers which are basically non
> existing on most "normal" distributions. Nowadays `man perf record` tries to
> educate people, please do the same for the new `--off-cpu` switch.
I think documenting that off-cpu has a dependency on frame pointers
makes sense. There has been work to make LBR work:
https://lore.kernel.org/bpf/[email protected]/
DWARF unwinding is problematic and is probably something best kept in
user land. There is also Intel's CET that may provide an alternate
backtraces.
More recent Intel and AMD cpus have techniques to turn memory
locations into registers, an approach generally called memory
renaming. There is some description here:
https://www.agner.org/forum/viewtopic.php?t=41
In LLVM there is a pass to promote memory locations into registers
called mem2reg. Having the frame pointer as an extra register will
help this pass as there will be 1 more register to replace something
from memory. The memory renaming optimization is similar to mem2reg
except done in the CPU's front-end. It would be interesting to see
benchmark results on modern CPUs with and without omit-frame-pointer.
My expectation is that the performance wins aren't as great, if any,
as they used to be (cc-ed Michael Larabel as I Iove phoronix and it'd
be awesome if someone could do an omit-frame-pointer shoot-out).
> > > > With BPF, it can aggregate scheduling stats for interested tasks
> > > > and/or states and convert the data into a form of perf sample records.
> > > > I chose the bpf-output event which is a software event supposed to be
> > > > consumed by BPF programs and renamed it as "offcpu-time". So it
> > > > requires no change on the perf report side except for setting sample
> > > > types of bpf-output event.
> > > >
> > > > Basically it collects userspace callstack for tasks as it's what users
> > > > want mostly. Maybe we can add support for the kernel stacks but I'm
> > > > afraid that it'd cause more overhead. So the offcpu-time event will
> > > > always have callchains regardless of the command line option, and it
> > > > enables the children mode in perf report by default.
> > >
> > > Has anything changed wrt perf/bpf and user applications not compiled with
> > > `- fno-omit-frame-pointer`? I.e. does this new utility only work for
> > > specially compiled applications, or do we also get backtraces for
> > > "normal" binaries that we can install through package managers?
> >
> > I am not aware of such changes, it still needs a frame pointer to get
> > backtraces.
>
> May I ask what kind of setup you are using this on? Do you use something like
> Gentoo or yocto where you compile your whole system with `-fno-omit-frame-
> pointer`? Because otherwise, any kind of off-cpu time in system libraries will
> not be resolved properly, no?
I agree with your point. Often in cloud environments binaries are
static blobs linking in all their dependencies. This can aid
deployment, bug compatibility, etc. Fwiw, all backtraces gathered in
Google's profiling are frame pointer based. A large motivation for
this is the security aspect of having a privileged application able to
snapshot other threads stacks that happens with dwarf based unwinding.
In summary, your point is that frame pointer based unwinding is
largely broken on all major distributions today limiting the utility
of off-CPU as it is here. I agree, memory renaming in hardware could
hopefully mean that this isn't the case in distributions in the
future. Even if it isn't there are alternate backtraces from sources
like LBR and CET that mean we can fix this other ways.
Thanks,
Ian
> Thanks
> --
> Milian Wolff | [email protected] | Senior Software Engineer
> KDAB (Deutschland) GmbH, a KDAB Group company
> Tel: +49-30-521325470
> KDAB - The Qt, C++ and OpenGL Experts