Re: per backend I/O statistics

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