Subject: Re: MultiXact\SLRU buffers configuration

From: Alvaro Herrera <[email-masked]>
Date: Wed, 21 Aug 2024 13:55:06 -0400

On 2024-Aug-21, Michael Paquier wrote:

> From fd8ab7b6845a2c56aa2c8d9c60f404f6b3407338 Mon Sep 17 00:00:00 2001
> From: Michael Paquier <[email-masked]>
> Date: Wed, 21 Aug 2024 15:16:06 +0900
> Subject: [PATCH 2/3] injection_point: Add injection_points.stats

> This GUC controls if statistics should be used or not in the module.
> Custom statistics require the module to be loaded with
> shared_preload_libraries, hence this GUC is made PGC_POSTMASTER.  By
> default, stats are disabled.
> 
> This will be used by an upcoming change in a test where stats should not
> be used, as the test has a dependency on a critical section.

I find it's strange that the information that stats cannot be used with
injection points that have dependency on critical sections (?), is only
in the commit message and not in the code.

Also, maybe it'd make sense for stats to be globally enabled, and that
only the tests that require it would disable them?  (It's probably easy
enough to have a value "injection_points.stats=auto" which means, if the
module is loaded in shared_preload_libraries them set stats on,
otherwise turn them off.)  TBH I don't understand why the issue that
stats require shared_preload_libraries only comes up now ...  Maybe
another approach is to say that if an injection point is loaded via
_LOAD() rather than the normal way, then stats are disabled for that one
rather than globally?  Or give the _LOAD() macro a boolean argument to
indicate whether to collect stats for that injection point or not.

Lastly, it's not clear to me what does it mean that the test has a
"dependency" on a critical section.  Do you mean to say that the
injection point runs inside a critical section?

> From e5329d080b9d8436af8f65aac118745cf1f81ca2 Mon Sep 17 00:00:00 2001
> From: Michael Paquier <[email-masked]>
> Date: Wed, 21 Aug 2024 15:09:06 +0900
> Subject: [PATCH 3/3] Rework new SLRU test with injection points
> 
> Rather than the SQL injection_points_load, this commit makes the test
> rely on the two macros to load and run an injection point from the
> cache, acting as an example of how to use them.

No issues with this, feel free to go ahead.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Cada quien es cada cual y baja las escaleras como quiere" (JMSerrat)

---

From: Michael Paquier <[email-masked]>
Date: Thu, 22 Aug 2024 09:29:30 +0900

> in the commit message and not in the code.

A comment close to where inj_stats_enabled is declared in
injection_points.c may be adapted for that, say:
"This GUC is useful to control if statistics should be enabled or not
during a test with injection points, like for example if a test relies
on a callback run in a critical section where no allocation should
happen."

> otherwise turn them off.)

I'm not sure that we need to get down to that until somebody has a
case where they want to rely on stats of injection points for their
stuff.  At this stage, I only want the stats to be enabled to provide
automated checks for the custom pgstats APIs, so disabling it by
default and enabling it only in the stats test of the module
injection_points sounds kind of enough to me for now.  The module
could always be tweaked to do that in the future, if there's a case.

> TBH I don't understand why the issue that
> stats require shared_preload_libraries only comes up now ...

Because there was no need to, simply.  It is the first test that
relies on a critical section, and we need allocations if we want to
use a wait condition. 

> Maybe another approach is to say that if an injection point is loaded via
> rather than globally?

One trick would be to force the GUC to be false for the duration of
the callback based on a check of CritSectionCount, a second one would
be to just skip the stats if are under CritSectionCount.  A third
option, that I find actually interesting, would be to call
MemoryContextAllowInCriticalSection in some strategic code paths of
the test module injection_points because we're OK to live with this
restriction in the module.

> Or give the _LOAD() macro a boolean argument to

Sticking some knowledge about the stats in the backend part of
injection points does not sound like a good idea to me.

> injection point runs inside a critical section?

Yes.

> No issues with this, feel free to go ahead.

Cool, thanks.
--
Michael