Horton

Re: per backend WAL statistics

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

View thread View on postgr.es
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Alexander Lakhin <exclusion@gmail.com>, Andres Freund <andres@anarazel.de>, Xuneng Zhou <xunengzhou@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-03-31T07:42:19Z
Lists: pgsql-hackers

Attachments

Hi,

On Sat, Mar 29, 2025 at 07:14:16AM +0900, Michael Paquier wrote:
> On Fri, Mar 28, 2025 at 09:00:00PM +0200, Alexander Lakhin wrote:
> > Please try the following query:
> > BEGIN;
> > SET LOCAL stats_fetch_consistency = snapshot;
> > SELECT * FROM pg_stat_get_backend_wal(pg_backend_pid());

Thanks for the report! I'm able to reproduce it on my side. The issue can
also be triggered with pg_stat_get_backend_io().

The issue is that in pgstat_fetch_stat_backend_by_pid() (and with
stats_fetch_consistency set to snapshot) a call to
pgstat_clear_backend_activity_snapshot() is done:

#0  __memset_evex_unaligned_erms () at ../sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S:250
#1  0x0000000001833bf2 in wipe_mem (ptr=0x632000018800, size=80800) at ../../../../src/include/utils/memdebug.h:42
#2  0x0000000001834c51 in AllocSetReset (context=0x619000003c80) at aset.c:586
#3  0x000000000184f32d in MemoryContextResetOnly (context=0x619000003c80) at mcxt.c:419
#4  0x0000000001834ede in AllocSetDelete (context=0x619000003c80) at aset.c:636
#5  0x000000000184f79b in MemoryContextDeleteOnly (context=0x619000003c80) at mcxt.c:528
#6  0x000000000184f5a9 in MemoryContextDelete (context=0x619000003c80) at mcxt.c:482
#7  0x0000000001361e84 in pgstat_clear_backend_activity_snapshot () at backend_status.c:541
#8  0x0000000001367f08 in pgstat_clear_snapshot () at pgstat.c:943
#9  0x0000000001368ac3 in pgstat_prep_snapshot () at pgstat.c:1121
#10 0x00000000013680b9 in pgstat_fetch_entry (kind=6, dboid=0, objid=0) at pgstat.c:961
#11 0x000000000136dd05 in pgstat_fetch_stat_backend (procNumber=0) at pgstat_backend.c:94
#12 0x000000000136de7d in pgstat_fetch_stat_backend_by_pid (pid=3294022, bktype=0x0) at pgstat_backend.c:136

*before* we check for "beentry->st_procpid != pid".

I think we can simply move the pgstat_fetch_stat_backend() call at the end
of pgstat_fetch_stat_backend_by_pid(), like in the attached. With this in place
the issue is fixed on my side.

Thoughts?

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

Commits

  1. Fix use-after-free in pgstat_fetch_stat_backend_by_pid()

  2. Remove initialization from PendingBackendStats

  3. Add WAL data to backend statistics

  4. Improve check for detection of pending data in backend statistics

  5. Fix some gaps in pg_stat_io with WAL receiver and WAL summarizer

  6. Handle auxiliary processes in SQL functions of backend statistics

  7. Invent pgstat_fetch_stat_backend_by_pid()

  8. Refactor code of pg_stat_get_wal() building result tuple

  9. Adding new PgStat_WalCounters structure in pgstat.h

  10. Remove pgstat_flush_wal()

  11. Refactor some code related to backend statistics

  12. Add backend-level statistics to pgstats