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Date: Tue, 23 Jan 2024 09:37:41 -0700
From: Keith Busch <kbusch@kernel.org>
To: Sagi Grimberg <sagi@grimberg.me>
Cc: Stuart Hayes <stuart.w.hayes@gmail.com>, linux-kernel@vger.kernel.org,
	Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
	linux-nvme@lists.infradead.org
Subject: Re: [PATCH v2] nvme_core: scan namespaces asynchronously
Message-ID: <Za_rVS-0MEr2U731@kbusch-mbp.dhcp.thefacebook.com>
References: <20240118210303.10484-1-stuart.w.hayes@gmail.com>
 <189cde89-9750-476f-8fbb-1c95dc056efb@grimberg.me>
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In-Reply-To: <189cde89-9750-476f-8fbb-1c95dc056efb@grimberg.me>

On Mon, Jan 22, 2024 at 11:13:15AM +0200, Sagi Grimberg wrote:
> On 1/18/24 23:03, Stuart Hayes wrote:
> > @@ -3901,19 +3932,25 @@ static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
> >   			goto free;
> >   		}
> > +		/*
> > +		 * scan list starting at list offset 0
> > +		 */
> > +		atomic_set(&scan_state.count, 0);
> >   		for (i = 0; i < nr_entries; i++) {
> >   			u32 nsid = le32_to_cpu(ns_list[i]);
> >   			if (!nsid)	/* end of the list? */
> >   				goto out;
> > -			nvme_scan_ns(ctrl, nsid);
> > +			async_schedule_domain(nvme_scan_ns, &scan_state, &domain);
> >   			while (++prev < nsid)
> >   				nvme_ns_remove_by_nsid(ctrl, prev);
> >   		}
> > +		async_synchronize_full_domain(&domain);

You mentioned async scanning was an improvement if you have 1000
namespaces, but wouldn't this be worse if you have very few namespaces?
IOW, the decision to use the async schedule should be based on
nr_entries, right?