linux-kernel - RE: [PATCH v2 2/2] x86/resctrl: Don't workqueue local event counter reads

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Message-ID: <SJ1PR11MB6083D9983DFD226A1FE826ABFC5C2@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date: Thu, 7 Nov 2024 23:30:06 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: "Chatre, Reinette" <reinette.chatre@...el.com>, Peter Newman
	<peternewman@...gle.com>
CC: "Yu, Fenghua" <fenghua.yu@...el.com>, "babu.moger@....com"
	<babu.moger@....com>, "bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "Eranian,
 Stephane" <eranian@...gle.com>, "hpa@...or.com" <hpa@...or.com>,
	"james.morse@....com" <james.morse@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "mingo@...hat.com" <mingo@...hat.com>,
	"nert.pinx@...il.com" <nert.pinx@...il.com>, "tan.shaopeng@...itsu.com"
	<tan.shaopeng@...itsu.com>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>
Subject: RE: [PATCH v2 2/2] x86/resctrl: Don't workqueue local event counter
 reads

> > E.g. read a counter:
> >
> > $ cat mbm_local_bytes
> > 123456789
> >
> > H/w counter for this event/group assigned elsewhere.
> >
> > H/w counter assigned back to this event/group
> >
> > $ cat mbm_local_bytes
> > 23456
> >
> > Bandwidth calculation sees traffic amount:
> >      (23456 - 123456789) = -123433333
> > Oops. Negative!
>
> As I understand this is already an issue today on AMD systems without assignable counters
> that may run out of counters. On these systems, any RMID that is no longer being tracked will
> be reset to zero. [1]

My understanding too.

> The support for assignable counters give user space control over this unexpected reset of
> counters.
>
> The scenario you present seem to demonstrate how two independent user space systems
> can trample on each other when interacting with the same resources. Is this something you expect
> resctrl should protect against? I would expect that there would be a single user space system
> doing something like above and it would reset history after unassigning a counter.

As we are discussing adding a new interface, I thought it worth considering adding
a way for user space to be aware of the re-assignment of counters. IMHO it would be
a nice to have feature. Not required if all users of resctrl are aware of each other's
actions.

> This does indeed highlight that if resctrl does start to dynamically assign counters (which
> has only been speculated in this thread and is not part of current [1] design) then it may cause
> problems on user space side.

Agreed. Dynamic assignment would break "the user knows what is happening" assumption.
Seems like a bad idea.

> Reinette
>
> [1] https://lore.kernel.org/all/cover.1730244116.git.babu.moger@amd.com/