linux-kernel - Re: [PATCH v3] sched/topology: Enable topology_span

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Message-ID: <0f51023c-7413-4503-95d4-42113fa3e2e4@amd.com>
Date: Tue, 11 Feb 2025 16:26:40 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, Naman Jain
	<namjain@...ux.microsoft.com>
CC: <stable@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Steve Wahl
	<steve.wahl@....com>, Saurabh Singh Sengar <ssengar@...ux.microsoft.com>,
	<srivatsa@...il.mit.edu>, Michael Kelley <mhklinux@...look.com>, Ingo Molnar
	<mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli
	<juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
	<rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH v3] sched/topology: Enable topology_span_sane check only
 for debug builds

Hello Shrikanth,

On 2/11/2025 11:22 AM, Shrikanth Hegde wrote:
> 
> [..snip..]
>>
>> What I'm getting to is that the arch specific topology parsing code
>> can set a "SDTL_ARCH_VERIFIED" flag indicating that the arch specific
>> bits have verified that the cpumasks are either equal or disjoint and
>> since sched_debug() is "false" by default, topology_span_sane() can
>> bail out if:
>>
>>      if (!sched_debug() && (tl->flags & SDTL_ARCH_VERIFIED))
>>          return;
>>
> 
> it would simpler to use sched_debug(). no?
> 
> Since it can be enabled at runtime by "echo Y > verbose", if one one needs to enable even after boot. Wouldn't that suffice to run topology_span_sane by doing a hotplug?

Ack! It was a suggestion in case folks felt apprehensive about guarding
the check behind sched_debug() ...

> 
>> In case arch specific parsing was wrong, "sched_verbose" can always
>> be used to double check the topology and for the arch that require
>> this sanity check, Steve's optimized version of
>> topology_span_sane() can be run to be sure of the sanity.
>>
>> All this justification is in case folks want to keep
>> topology_span_sane() around but if no one cares, Naman and Saurabh's
>> approach works as intended.

... which is why I ended that long explanation with this :)

Valentin seems to be on board with the current approach from Naman and
Saurabh and it works as intended.

>>
> 

-- 
Thanks and Regards,
Prateek