Re: [PATCH v4 0/2]: perf: reduce data loss when profiling highly parallel CPU bound workloads

[Alexey Budankov]

Hi Kim,

On 28.08.2018 22:43, Kim Phillips wrote:
> On Tue, 28 Aug 2018 18:44:57 +0300
> Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> wrote:
> 
>> Experiment with profiling matrix multiplication code executing 128 
>> threads on Intel Xeon Phi (KNM) with 272 cores, like below,
>> demonstrates data loss metrics value of 98%:
> 
> So I took these two patches for a quick test-drive on intel-pt.

Thanks for testing that out in this scenario! It hasn't been tested yet.

> 
> BEFORE (acme's today's perf/core branch):
> 
> $ sudo perf version
> perf version 4.18.rc7.g55fc647
> $ sudo perf record -e intel_pt//  dd if=/dev/zero of=/dev/null count=100000
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB, 49 MiB) copied, 0.0868081 s, 590 MB/s
> [ perf record: Woken up 21 times to write data ]
> [ perf record: Captured and wrote 41.302 MB perf.data ]
> $ 
> 
> AFTER (== BEFORE + these two patches):
> 
> $ sudo ./perf version
> perf version 4.18.rc7.gbc1c99
> $ sudo perf record -e intel_pt//  dd if=/dev/zero of=/dev/null count=100000
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB, 49 MiB) copied, 0.0931142 s, 550 MB/s
> 
> ...and it's still running, minutes afterwards.  Before I kill it,
> here's some strace output:
> 
> nanosleep({tv_sec=0, tv_nsec=500000}, NULL) = 0
> lseek(3, 332556518, SEEK_SET)           = 332556518
> write(3, "D\0\0\0\0\0\10\0", 8)         = 8
> lseek(3, 0, SEEK_CUR)                   = 332556526
> futex(0x7f221e7252c8, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252cc, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252c8, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252cc, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252c8, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252cc, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252c8, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> nanosleep({tv_sec=0, tv_nsec=500000}, NULL) = 0
> lseek(3, 332578462, SEEK_SET)           = 332578462
> write(3, "D\0\0\0\0\0\10\0", 8)         = 8
> lseek(3, 0, SEEK_CUR)                   = 332578470
> futex(0x7f221e7252cc, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> nanosleep({tv_sec=0, tv_nsec=500000}, NULL) = 0
> lseek(3, 332598822, SEEK_SET)           = 332598822
> write(3, "D\0\0\0\0\0\10\0", 8)         = 8
> lseek(3, 0, SEEK_CUR)                   = 332598830
> futex(0x7f221e7252c8, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252cc, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252c8, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252cc, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252c8, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e725200, FUTEX_WAKE_PRIVATE, 1) = 1
> futex(0x7f221e7252cc, FUTEX_WAKE_PRIVATE, 1) = 1
> ^Cstrace: Process 3597 detached
> 
> I can't prove that it's these two patches that create the hang, but
> this does look like a livelock situation...hm, hitting ^C doesn't stop
> it...had to kill -9 it...erm, does 'perf record -e intel_pt// dd...'
> work for you on a more standard machine?:
> 
> $ dmesg | grep Perf
> [    0.044226] Performance Events: PEBS fmt3+, Skylake events, 32-deep LBR, full-width counters, Intel PMU driver.

Strace patterns look similar to the ones implemented in the patches. 
Let me reproduce and investigate the hang locally.

Thanks,
Alexey

> 
> Thanks,
> 
> Kim
>