linux-kernel - Re: [PATCH v2 3/3] lib/group_cpus.c: honor housekeeping config when grouping CPUs

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Date: Mon, 1 Jul 2024 10:37:46 +0200
From: Hannes Reinecke <hare@...e.de>
To: Ming Lei <ming.lei@...hat.com>
Cc: Daniel Wagner <dwagner@...e.de>, Jens Axboe <axboe@...nel.dk>,
 Keith Busch <kbusch@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
 Thomas Gleixner <tglx@...utronix.de>, Christoph Hellwig <hch@....de>,
 Frederic Weisbecker <fweisbecker@...e.com>, Mel Gorman <mgorman@...e.de>,
 Sridhar Balaraman <sbalaraman@...allelwireless.com>,
 "brookxu.cn" <brookxu.cn@...il.com>, linux-kernel@...r.kernel.org,
 linux-block@...r.kernel.org, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH v2 3/3] lib/group_cpus.c: honor housekeeping config when
 grouping CPUs

On 7/1/24 09:10, Ming Lei wrote:
> On Mon, Jul 01, 2024 at 08:43:34AM +0200, Hannes Reinecke wrote:
>> On 7/1/24 04:09, Ming Lei wrote:
[ .. ]
>>>
>>> Since this patch, some isolated CPUs may not be covered in
>>> blk-mq queue mapping.
>>>
>>> Meantime people still may submit IO workload from isolated CPUs
>>> such as by 'taskset -c', blk-mq may not work well for this situation,
>>> for example, IO hang may be caused during cpu hotplug.
>>>
>>> I did see this kind of usage in some RH Openshift workloads.
>>>
>>> If blk-mq problem can be solved, I am fine with this kind of
>>> change.
>>>
>> That was kinda the idea of this patchset; when 'isolcpus' is active any
>> in-kernel driver can only run on the housekeeping CPUs, and I/O from the
>> isolcpus is impossible.
>> (Otherwise they won't be isolated anymore, and the whole concepts becomes
>> ever so shaky.).
> 
> Userspace may still force to run IO workload from isolated CPUs when they do
> not care CPU isolation, and kernel still should complete IO from isolated CPUs,
> and can't run into hang or panic meantime.
> 
> And we do support this kind of usage now, then regression is caused by
> this patch.
> 
Hmm. Guess we need to modify the grouping algorithm to group across all 
cpus, but ensure that each group consists either of all housekeeping 
CPUs or all isolated cpus.
Daniel?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich