Re: [PATCH 0/3] tracing: Add dynamic strings for synthetic events

From: Axel Rasmussen
Date: Tue Sep 29 2020 - 18:20:05 EST

On Tue, Sep 29, 2020 at 1:33 PM Tom Zanussi <zanussi@xxxxxxxxxx> wrote:
>
> Hi,
>
> This patchset adds support for dynamic strings for synthetic events,
> as requested by Axel Rasmussen.
>
> Actually, the first two patches should be applied in any case - the
> first just changes the current max string length and the second fixes
> a bug I found while testing.
>
> It works fine for my simple test cases, but I still need to do a lot
> more testing, especially of the in-kernel API parts, which don't
> affect Axel's use case.
>
> Anyway, Axel, please try it out and send me your Tested-by: if it
> works for you, and your broken testcase(s) if it doesn't. ;-)

Thanks for working on this!

Capturing a histogram with a histogram trigger on the synthetic
event[1] seems to work as expected for me.

However, I tried creating the synthetic event, and then capturing a
histogram from it using bpftrace [2], and this doesn't seem to work.
It's completely possible I'm just doing something wrong, or the
bpftrace I'm using is buggy (getting it to work with __data_loc fields
at all requires a super bleeding edge bpftrace [3]). bpftrace reports
that it fails to attach to the synthetic event, and I hit some
WARN_ONs [4].

Does it seem possible this issue is related? If not, feel free to
disregard. :) I'll spend some more time digging into this tomorrow.



[1]:
echo 'mmap_lock_latency u64 time; char memcg_path[], bool write' > \
/sys/kernel/tracing/synthetic_events

echo 'hist:keys=memcg_path,write:ts0=common_timestamp.usecs' > \
/sys/kernel/tracing/events/mmap_lock/mmap_lock_start_locking/trigger

echo 'hist:keys=memcg_path,write:latency=common_timestamp.usecs-$ts0:onmatch(mmap_lock.mmap_lock_start_locking).mmap_lock_latency($latency,memcg_path,write)'
> \
/sys/kernel/tracing/events/mmap_lock/mmap_lock_acquire_returned/trigger

echo 'hist:keys=memcg_path,write:values=time' > \
/sys/kernel/tracing/events/synthetic/mmap_lock_latency/trigger

[2]:
(Same setup of the synthetic event as [1], except adding the histogram
trigger...)

cat <<EOF > /root/bpftypes.h
#include <stdbool.h>
#include <stdint.h>
typedef uint64_t u64;
typedef int64_t s64;
EOF

cat <<EOF > /root/mmap-lock-per-memcg.bpf
tracepoint:synthetic:mmap_lock_latency {
@latency[args->memcg_path] = hist(args->time);
}
EOF

/home/axelrasmussen/bpftrace/build/src/bpftrace \
-v \
-I "/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include" \
--include "/root/bpftypes.h" \
/root/mmap-lock-per-memcg.bpf

[3]: https://github.com/iovisor/bpftrace/pull/1542

[4]:
[ 31.361814] kauditd_printk_skb: 6 callbacks suppressed
[ 31.361815] audit: type=1334 audit(1601414765.236:34): prog-id=11 op=LOAD

>From `WARN_ON(!tp_func->func)`:

[ 31.365310] ------------[ cut here ]------------
[ 31.365316] WARNING: CPU: 0 PID: 235 at kernel/tracepoint.c:136
tracepoint_add_func+0x2ad/0x300
[ 31.366594] Modules linked in: xhci_pci xhci_hcd virtio_net
net_failover failover virtio_crypto crypto_engine virtio_console
virtio_blk virtio_balloon uhci_hcd ohci_pci ohci_ht
[ 31.366603] CPU: 0 PID: 235 Comm: bpftrace Not tainted 5.9.0-smp-DEV #1
[ 31.366604] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.13.0-1 04/01/2014
[ 31.366606] RIP: 0010:tracepoint_add_func+0x2ad/0x300
[ 31.366607] Code: 17 ff ff ff 4c 89 ff e8 e1 0c 0a 00 e9 22 ff ff
ff 41 ff 57 18 85 c0 4c 8b 04 24 8b 54 24 08 0f 89 9c fd ff ff e9 31
ff ff ff <0f> 0b b8 ea ff ff ff e9 f9 fd5
[ 31.366607] RSP: 0018:ffff911554fdbc50 EFLAGS: 00010246
[ 31.366608] RAX: 0000000000000000 RBX: ffff9115583c1200 RCX: 0000000000000000
[ 31.366609] RDX: 000000000000000a RSI: 0000000000000000 RDI: ffff9115583c1200
[ 31.366609] RBP: ffff911554fdbc90 R08: ffff911554fdbca0 R09: ffffcaabffdb5018
[ 31.366610] R10: 0000000000000080 R11: 0000000000000080 R12: ffff911558013c90
[ 31.366610] R13: 0000000000000000 R14: ffff911554f84d10 R15: ffff9115583c1200
[ 31.366611] FS: 00007f9bdf8f0c00(0000) GS:ffff91155fc00000(0000)
knlGS:0000000000000000
[ 31.366612] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 31.366614] CR2: 00007f9bdefbc000 CR3: 0000000814fe0006 CR4: 0000000000370ef0
[ 31.366615] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 31.366615] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 31.366616] Call Trace:
[ 31.366620] tracepoint_probe_register+0x58/0x90
[ 31.366625] trace_event_reg+0x56/0xa0
[ 31.366626] perf_trace_event_reg.isra.2+0xba/0x130
[ 31.366627] perf_trace_event_init+0x32/0x70
[ 31.366628] perf_trace_init+0x6e/0xa0
[ 31.366631] perf_tp_event_init+0x28/0x40
[ 31.366632] perf_try_init_event+0xe7/0x130
[ 31.366633] perf_event_alloc+0x4ba/0xdc0
[ 31.366635] ? __alloc_fd+0x40/0x160
[ 31.366637] __do_sys_perf_event_open+0x2bb/0xe90
[ 31.366638] __x64_sys_perf_event_open+0x20/0x30
[ 31.366641] do_syscall_64+0x31/0x40
[ 31.366644] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 31.366645] RIP: 0033:0x7f9be4886d5d
[ 31.366646] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e
fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24
08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 488
[ 31.366646] RSP: 002b:00007ffdcf6e4d68 EFLAGS: 00000246 ORIG_RAX:
000000000000012a
[ 31.366647] RAX: ffffffffffffffda RBX: 00007ffdcf6e5e6c RCX: 00007f9be4886d5d
[ 31.366647] RDX: 0000000000000000 RSI: 00000000ffffffff RDI: 00007ffdcf6e4d80
[ 31.366648] RBP: 00007ffdcf6e4d80 R08: 0000000000000008 R09: 000056095c148578
[ 31.366651] R10: 00000000ffffffff R11: 0000000000000246 R12: 000000000000000b
[ 31.366652] R13: 00000000ffffffff R14: 00007ffdcf6e5e70 R15: 00007ffdcf6e4e00
[ 31.366653] ---[ end trace 25e0e681c1442c93 ]---

>From `WARN_ON_ONCE(PTR_ERR(old) != -ENOMEM);`:

[ 31.367345] ------------[ cut here ]------------
[ 31.367347] WARNING: CPU: 0 PID: 235 at kernel/tracepoint.c:243
tracepoint_add_func+0xb2/0x300
[ 31.368612] Modules linked in: xhci_pci xhci_hcd virtio_net
net_failover failover virtio_crypto crypto_engine virtio_console
virtio_blk virtio_balloon uhci_hcd ohci_pci ohci_ht
[ 31.368615] CPU: 0 PID: 235 Comm: bpftrace Tainted: G W
5.9.0-smp-DEV #1
[ 31.368615] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.13.0-1 04/01/2014
[ 31.368617] RIP: 0010:tracepoint_add_func+0xb2/0x300
[ 31.368617] Code: c6 48 8b 01 48 85 c0 74 28 41 83 fd ff 75 df 3b
51 10 44 0f 4f eb 48 39 c6 75 d8 49 8b 40 08 48 39 41 08 75 ce b8 ef
ff ff ff <0f> 0b e9 25 01 00 00 8d 43 02d
[ 31.368618] RSP: 0018:ffff911554fdbc50 EFLAGS: 00010246
[ 31.368618] RAX: 00000000ffffffea RBX: ffff9115583c1200 RCX: 0000000000000000
[ 31.368619] RDX: 000000000000000a RSI: 0000000000000000 RDI: ffff9115583c1200
[ 31.368619] RBP: ffff911554fdbc90 R08: ffff911554fdbca0 R09: ffffcaabffdb5018
[ 31.368620] R10: 0000000000000080 R11: 0000000000000080 R12: ffff911558013c90
[ 31.368620] R13: 0000000000000000 R14: ffff911554f84d10 R15: ffff9115583c1200
[ 31.368621] FS: 00007f9bdf8f0c00(0000) GS:ffff91155fc00000(0000)
knlGS:0000000000000000
[ 31.368621] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 31.368623] CR2: 00007f9bdefbc000 CR3: 0000000814fe0006 CR4: 0000000000370ef0
[ 31.368624] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 31.368624] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 31.368625] Call Trace:
[ 31.368626] tracepoint_probe_register+0x58/0x90
[ 31.368627] trace_event_reg+0x56/0xa0
[ 31.368628] perf_trace_event_reg.isra.2+0xba/0x130
[ 31.368629] perf_trace_event_init+0x32/0x70
[ 31.368630] perf_trace_init+0x6e/0xa0
[ 31.368631] perf_tp_event_init+0x28/0x40
[ 31.368632] perf_try_init_event+0xe7/0x130
[ 31.368632] perf_event_alloc+0x4ba/0xdc0
[ 31.368634] ? __alloc_fd+0x40/0x160
[ 31.368635] __do_sys_perf_event_open+0x2bb/0xe90
[ 31.368636] __x64_sys_perf_event_open+0x20/0x30
[ 31.368637] do_syscall_64+0x31/0x40
[ 31.368638] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 31.368639] RIP: 0033:0x7f9be4886d5d
[ 31.368639] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e
fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24
08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 488
[ 31.368640] RSP: 002b:00007ffdcf6e4d68 EFLAGS: 00000246 ORIG_RAX:
000000000000012a
[ 31.368640] RAX: ffffffffffffffda RBX: 00007ffdcf6e5e6c RCX: 00007f9be4886d5d
[ 31.368641] RDX: 0000000000000000 RSI: 00000000ffffffff RDI: 00007ffdcf6e4d80
[ 31.368641] RBP: 00007ffdcf6e4d80 R08: 0000000000000008 R09: 000056095c148578
[ 31.368642] R10: 00000000ffffffff R11: 0000000000000246 R12: 000000000000000b
[ 31.368642] R13: 00000000ffffffff R14: 00007ffdcf6e5e70 R15: 00007ffdcf6e4e00
[ 31.368643] ---[ end trace 25e0e681c1442c94 ]---
[ 31.377271] audit: type=1334 audit(1601414765.252:35): prog-id=11 op=UNLOAD

>
> Thanks,
>
> Tom
>
> The following changes since commit 106c076d23cca67c959a6fd1ccadb5b3ef01ddc9:
>
> mmap_lock: add tracepoints around lock acquisition (2020-09-23 08:48:08 -0500)
>
> are available in the Git repository at:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/zanussi/linux-trace.git ftrace/synth-dynstring-v0
>
> Tom Zanussi (3):
> tracing: Change STR_VAR_MAX_LEN
> tracing: Fix parse_synth_field() error handling
> tracing: Add support for dynamic strings to synthetic events
>
> Documentation/trace/events.rst | 15 ++-
> Documentation/trace/histogram.rst | 18 ++++
> kernel/trace/trace_events_hist.c | 9 ++
> kernel/trace/trace_events_synth.c | 163 ++++++++++++++++++++++++++----
> kernel/trace/trace_synth.h | 6 +-
> 5 files changed, 186 insertions(+), 25 deletions(-)
>
> --
> 2.17.1
>