[pgpool-hackers: 1976] Re: Problem with log_line_prefix()
Tatsuo Ishii
ishii at sraoss.co.jp
Wed Jan 11 09:44:11 JST 2017
> On Tue, Jan 10, 2017 at 8:08 AM, Tatsuo Ishii <ishii at sraoss.co.jp> wrote:
>
>> Hi Usama,
>>
>> Problem is, there are some places where standard library functions
>> that calls malloc() internally.
>> (For example, asprintf(), getaddrinfo()).
>>
>> You are right, this problem makes this solution not feasible.
>
>
>
>> Also blocking signals/unblocking signals every time malloc() called
>> might hurt performance (I have not done an actual measurement. So this
>> just a guess).
>>
>
> Yes the benchmark can revel the actual overhead, but I think the impact of
> this change should be
> very small, since Memory manager allocates the memory in chunks and
> malloc is not issued for every palloc call,
> But again its hard to guess the actual impact before doing the measurements.
>
> So I think we can proceed with the simple solution for now and remove the
> ereports from the signal handlers.
I have pushed changes in the direction.
Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp
> Kind regards
> Muhammad Usama
>
>
>
>> Best regards,
>> --
>> Tatsuo Ishii
>> SRA OSS, Inc. Japan
>> English: http://www.sraoss.co.jp/index_en.php
>> Japanese:http://www.sraoss.co.jp
>>
>> > Hi Ishii-San
>> >
>> > I have been thinking about this and I think there is one solution we can
>> > adopt.
>> >
>> > We can block signals before doing malloc() in, AllocSetAlloc() and
>> > AllocSetContextCreate() function, And since pgpool-II only uses palloc
>> and
>> > friends to allocate memory and there are only few places where the
>> malloc()
>> > are issued by MemoryManager API, So if we block signals before calling
>> the
>> > malloc(), we can safely use elog/ereport from signal handler routines.
>> > What are your thoughts on this. I am attaching the sample patch for the
>> > reference.
>> >
>> > Kind regards
>> > Muhammad Usama
>> >
>> >
>> > On Fri, Jan 6, 2017 at 12:03 PM, Muhammad Usama <m.usama at gmail.com>
>> wrote:
>> >
>> >> Hi Ishii-San
>> >>
>> >>
>> >> On Fri, Jan 6, 2017 at 11:22 AM, Tatsuo Ishii <ishii at sraoss.co.jp>
>> wrote:
>> >>
>> >>> After some investigations, I found PostgreSQL does not block all
>> >>> signals: SIGTERM etc. still can interrupt.
>> >>>
>> >>> Probably we should take out elog/ereport call from child.c:die().
>> >>>
>> >>
>> >> Yes, this seems to be a good fix for now, But I am looking for some more
>> >> future proof fix. Give me a couple of more days to get to the solution,
>> >> otherwise we will go with removing the ereport from child.c:die.
>> >>
>> >> Thanks
>> >> Best regards
>> >> Muhammad Usama
>> >>
>> >>
>> >>> Best regards,
>> >>> --
>> >>> Tatsuo Ishii
>> >>> SRA OSS, Inc. Japan
>> >>> English: http://www.sraoss.co.jp/index_en.php
>> >>> Japanese:http://www.sraoss.co.jp
>> >>>
>> >>> > I have asked this how PostgreSQL solves similar problems like this.
>> >>> > It turned out that PostgreSQL mostly blocks all signals:
>> >>> >
>> >>> > https://www.postgresql.org/message-id/18405.1482794285%
>> 40sss.pgh.pa.us
>> >>> >
>> >>> > Probably we should do the same way...
>> >>> >
>> >>> > Best regards,
>> >>> > --
>> >>> > Tatsuo Ishii
>> >>> > SRA OSS, Inc. Japan
>> >>> > English: http://www.sraoss.co.jp/index_en.php
>> >>> > Japanese:http://www.sraoss.co.jp
>> >>> >
>> >>> >> Usama,
>> >>> >>
>> >>> >> If I hit CTRL-C key while executing the regression test, sometimes a
>> >>> >> orphan pgpool child remains. Here's a stack trace of the process:
>> >>> >>
>> >>> >> t-ishii at localhost: gdb temp/installed/bin/pgpool 3794
>> >>> >> GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
>> >>> >> [snip]
>> >>> >> Loaded symbols for /lib/x86_64-linux-gnu/libnss_files.so.2
>> >>> >> __lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linu
>> >>> x/x86_64/lowlevellock.S:95
>> >>> >> 95 ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:
>> >>> そのようなファイルやディレクトリはありません.
>> >>> >> (gdb) bt
>> >>> >> #0 __lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linu
>> >>> x/x86_64/lowlevellock.S:95
>> >>> >> #1 0x00007f67fe20ccba in _L_lock_12808 () from
>> >>> /lib/x86_64-linux-gnu/libc.so.6
>> >>> >> #2 0x00007f67fe20a6b5 in __GI___libc_malloc (bytes=15) at
>> >>> malloc.c:2887
>> >>> >> #3 0x00007f67fe21072a in __GI___strdup (s=0x7f67fe305dd8
>> >>> "/etc/localtime") at strdup.c:42
>> >>> >> #4 0x00007f67fe239f51 in tzset_internal (always=<optimized out>,
>> >>> explicit=explicit at entry=1)
>> >>> >> at tzset.c:444
>> >>> >> #5 0x00007f67fe23a913 in __tz_convert (timer=timer at entry
>> =0x7ffce1c1b
>> >>> 7f8,
>> >>> >> use_localtime=use_localtime at entry=1, tp=tp at entry=0x7f67fe54bde0
>> >>> <_tmbuf>) at tzset.c:632
>> >>> >> #6 0x00007f67fe2387d1 in __GI_localtime (t=t at entry=0x7ffce1c1b7f8)
>> >>> at localtime.c:42
>> >>> >> #7 0x000000000045627b in log_line_prefix (buf=buf at entry
>> =0x7ffce1c1b8d0,
>> >>> line_prefix=<optimized out>,
>> >>> >> edata=<optimized out>) at ../../src/utils/error/elog.c:2059
>> >>> >> #8 0x000000000045894d in send_message_to_server_log (edata=0x753320
>> >>> <errordata>)
>> >>> >> at ../../src/utils/error/elog.c:2084
>> >>> >> #9 EmitErrorReport () at ../../src/utils/error/elog.c:1129
>> >>> >> #10 0x0000000000456d8e in errfinish (dummy=<optimized out>) at
>> >>> ../../src/utils/error/elog.c:434
>> >>> >> #11 0x0000000000421f57 in die (sig=2) at protocol/child.c:925
>> >>> >> #12 <signal handler called>
>> >>> >> #13 _int_malloc (av=0x7f67fe546760 <main_arena>, bytes=4176) at
>> >>> malloc.c:3302
>> >>> >> #14 0x00007f67fe20a6c0 in __GI___libc_malloc (bytes=4176) at
>> >>> malloc.c:2891
>> >>> >> #15 0x000000000045561d in AllocSetAlloc (context=0xf90000,
>> size=4128)
>> >>> at ../../src/utils/mmgr/aset.c:674
>> >>> >> #16 0x000000000042cd1e in pool_init_cp () at
>> >>> protocol/pool_connection_pool.c:70
>> >>> >> #17 0x0000000000423024 in do_child (fds=fds at entry=0xfa35c0) at
>> >>> protocol/child.c:194
>> >>> >> #18 0x00000000004086f5 in fork_a_child (fds=0xfa35c0, id=11) at
>> >>> main/pgpool_main.c:723
>> >>> >> #19 0x00000000004090cc in reaper () at main/pgpool_main.c:2427
>> >>> >> #20 0x000000000040d869 in PgpoolMain (discard_status=discard_
>> status at entry=1
>> >>> '\001',
>> >>> >> clear_memcache_oidmaps=clear_memcache_oidmaps at entry=0 '\000')
>> at
>> >>> main/pgpool_main.c:457
>> >>> >> #21 0x0000000000406eb3 in main (argc=<optimized out>,
>> argv=<optimized
>> >>> out>) at main/main.c:305
>> >>> >>
>> >>> >> As you can see, the process was stuck while calling malloc().
>> >>> >>
>> >>> >> pool_init_cp() was interrupted by SIGINT (because I hit the key),
>> and
>> >>> >> the signal handler die() was called. In die(), elog() was
>> >>> >> called. elog() calls log_line_prefix(), which calls
>> >>> >> localtime(). localtime() calls malloc(), which seems to try to
>> obtain
>> >>> >> a lock, then it causes a dead lock since pool_init_cp() was
>> >>> >> interrupted while calling malloc().
>> >>> >>
>> >>> >> log_line_prefix() was copied from PostgreSQL but I see subtle
>> >>> >> differences from the PostgreSQL's original log_line_prefix().:
>> >>> >>
>> >>> >> pg_strftime(strfbuf, sizeof(strfbuf),
>> >>> >> "%Y-%m-%d %H:%M:%S %Z",
>> >>> >> pg_localtime(&stamp_time, log_timezone));
>> >>> >>
>> >>> >> Ours:
>> >>> >> strftime(strbuf, 128, "%Y-%m-%d %H:%M:%S", localtime(&now));
>> >>> >>
>> >>> >> Apparently the call stuck shows that the hang happened inside
>> >>> >> localtime() because it calls malloc(). On the other hand,
>> >>> >> pg_localtime() seems to avoid calling localtime() by using their
>> home
>> >>> >> made version of localtime(), which does *not* call malloc(). I'm not
>> >>> >> sure if it's a intention but it seems it'd better to follow
>> >>> >> PostgreSQL's way to avoid the hang problem.
>> >>> >>
>> >>> >> What do you think, Usama?
>> >>> >>
>> >>> >> Best regards,
>> >>> >> --
>> >>> >> Tatsuo Ishii
>> >>> >> SRA OSS, Inc. Japan
>> >>> >> English: http://www.sraoss.co.jp/index_en.php
>> >>> >> Japanese:http://www.sraoss.co.jp
>> >>> >> _______________________________________________
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>> >>
>> >>
>>
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