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From: Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: Wed, 15 Jan 2025 11:03:54 +0300
Message-ID: <CAN55FZ0ZQTf-D+5Rup4H6zp+n3E2EuonYuzTiPkp=1GhesZU_A@mail.gmail.com>
Subject: Re: per backend I/O statistics
To: Michael Paquier <michael@paquier.xyz>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, 
	Kyotaro Horiguchi <horikyota.ntt@gmail.com>, pgsql-hackers@lists.postgresql.org
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Hi,

On Thu, 19 Dec 2024 at 07:22, Michael Paquier <michael@paquier.xyz> wrote:
>
> Fixed that, bumped the two version counters, and done.

I encountered a problem while trying to add WAL stats to pg_stat_io
and I wanted to hear your thoughts.

Right now, pgstat_prep_backend_pending() is called in both
pgstat_count_io_op() and pgstat_count_io_op_time() to create a local
PgStat_BackendPending entry. In that process,
pgstat_prep_pending_entry() -> MemoryContextAllocZero() is called. The
problem is that MemoryContextAllocZero() can not be called in the
critical sections.

For example, here is what happens in the walsender backend:

'''
... ->
exec_replication_command() ->
SendBaseBackup() ->
...  ->
XLogInsertRecord() ->
START_CRIT_SECTION() /* Now we are in the critical section */ ->
... ->
XLogWrite() ->
pgstat_count_io_op_time() for the pg_pwrite() IO ->
pgstat_prep_backend_pending() ->
pgstat_prep_pending_entry() ->
MemoryContextAllocZero() ->
Failed at Assert("CritSectionCount == 0 || (context)->allowInCritSection")
'''

With this commit it may not be possible to count IOs in the critical
sections. I think the problem happens only if the local
PgStat_BackendPending entry is being created for the first time for
this backend in the critical section.

--
Regards,
Nazir Bilal Yavuz
Microsoft