linux-kernel - Re: [PATCH v4 8/9] blk-mq: use hk cpus only when isolcpus=managed

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Message-ID: <cc5e44dd-e1dc-4f24-88d9-ce45a8b0794f@flourine.local>
Date: Thu, 19 Dec 2024 16:38:43 +0100
From: Daniel Wagner <dwagner@...e.de>
To: Ming Lei <ming.lei@...hat.com>
Cc: Daniel Wagner <wagi@...nel.org>, Jens Axboe <axboe@...nel.dk>, 
	Keith Busch <kbusch@...nel.org>, Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>, 
	Kashyap Desai <kashyap.desai@...adcom.com>, Sumit Saxena <sumit.saxena@...adcom.com>, 
	Shivasharan S <shivasharan.srikanteshwara@...adcom.com>, Chandrakanth patil <chandrakanth.patil@...adcom.com>, 
	"Martin K. Petersen" <martin.petersen@...cle.com>, Nilesh Javali <njavali@...vell.com>, 
	GR-QLogic-Storage-Upstream@...vell.com, Don Brace <don.brace@...rochip.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>, 
	Eugenio Pérez <eperezma@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Costa Shulyupin <costa.shul@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
	Valentin Schneider <vschneid@...hat.com>, Waiman Long <llong@...hat.com>, 
	Michal Koutný <mkoutny@...e.com>, Frederic Weisbecker <frederic@...nel.org>, 
	Mel Gorman <mgorman@...e.de>, Hannes Reinecke <hare@...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, 
	megaraidlinux.pdl@...adcom.com, linux-scsi@...r.kernel.org, storagedev@...rochip.com, 
	virtualization@...ts.linux.dev
Subject: Re: [PATCH v4 8/9] blk-mq: use hk cpus only when
 isolcpus=managed_irq is enabled

On Thu, Dec 19, 2024 at 05:20:44PM +0800, Ming Lei wrote:
> > +	cpumask_andnot(isol_mask,
> > +		       cpu_possible_mask,
> > +		       housekeeping_cpumask(HK_TYPE_MANAGED_IRQ));
> > +
> > +	for_each_cpu(cpu, isol_mask) {
> > +		qmap->mq_map[cpu] = qmap->queue_offset + queue;
> > +		queue = (queue + 1) % qmap->nr_queues;
> > +	}
> 
> Looks the IO hang issue in V3 isn't addressed yet, is it?
> 
> https://lore.kernel.org/linux-block/ZrtX4pzqwVUEgIPS@fedora/

I've added an explanation in the cover letter why this is not
addressed. From the cover letter:

I've experimented for a while and all solutions I came up were horrible
hacks (the hotpath needs to be touched) and I don't want to slow down all
other users (which are almost everyone). IMO, it's just not worth trying
to fix this corner case. If the user is using isolcpus and does CPU
hotplug, we can expect that the user can also first offline the isolated
CPUs. I've discussed this topic during ALPSS and the room came to the
same conclusion. Thus I just added a patch which issues a warning that
IOs are likely to hang.