Re: [PATCH 2/2] nvme-pci: poll IO after batch submission for multi-mapping queue

From: Ming Lei <ming.lei@redhat.com>
To: Long Li <longli@microsoft.com>
Cc: Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
	Christoph Hellwig <hch@lst.de>,
	"linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
	Sagi Grimberg <sagi@grimberg.me>
Subject: Re: [PATCH 2/2] nvme-pci: poll IO after batch submission for multi-mapping queue
Date: Tue, 12 Nov 2019 10:39:20 +0800	[thread overview]
Message-ID: <20191112023920.GD15079@ming.t460p> (raw)
In-Reply-To: <CY4PR21MB0741004E62F9C50B8EF7DA9ECE770@CY4PR21MB0741.namprd21.prod.outlook.com>

On Tue, Nov 12, 2019 at 12:33:50AM +0000, Long Li wrote:
> >From: Christoph Hellwig <hch@lst.de>
> >Sent: Monday, November 11, 2019 12:45 PM
> >To: Ming Lei <ming.lei@redhat.com>
> >Cc: linux-nvme@lists.infradead.org; Keith Busch <kbusch@kernel.org>; Jens
> >Axboe <axboe@fb.com>; Christoph Hellwig <hch@lst.de>; Sagi Grimberg
> ><sagi@grimberg.me>; Long Li <longli@microsoft.com>
> >Subject: Re: [PATCH 2/2] nvme-pci: poll IO after batch submission for multi-
> >mapping queue
> >
> >On Fri, Nov 08, 2019 at 11:55:08AM +0800, Ming Lei wrote:
> >> f9dde187fa92("nvme-pci: remove cq check after submission") removes cq
> >> check after submission, this change actually causes performance
> >> regression on some NVMe drive in which single nvmeq handles requests
> >> originated from more than one blk-mq sw queues(call it multi-mapping
> >> queue).
> >
> >> Follows test result done on Azure L80sv2 guest with NVMe drive(
> >> Microsoft Corporation Device b111). This guest has 80 CPUs and 10 numa
> >> nodes, and each NVMe drive supports 8 hw queues.
> >
> >Have you actually seen this on a real nvme drive as well?
> >
> >Note that it is kinda silly to limit queues like that in VMs, so I really don't think
> >we should optimize the driver for this particular case.
> 
> I tested on an Azure L80s_v2 VM with newer Samsung P983 NVMe SSD (with 32 hardware queues). Tests also showed soft lockup when 32 queues are shared by 80 CPUs. 
> 

BTW, do you see if this simple change makes a difference?

> The issue will likely show up if the number of NVMe hardware queues is less than the number of CPUs. I think this may be a likely configuration on a very large system. (e.g. the largest VM on Azure has 416 cores)
> 

'the number of NVMe hardware queues' above should be the number of single NVMe drive.
I believe 32 hw queues is common, also poll queues may take several from the total 32.
When interrupt handling on single CPU core can't catch up with NVMe's IO handling,
soft lockup could be triggered. Of course, there are lot kinds of supported processors
by Linux.

Also when (nr_nvme_drives * nr_nvme_hw_queues) > nr_cpu_cores, one same CPU
can be assigned to handle more than 1 nvme IO queue interrupt from different
NVMe drive, the situation becomes worse.

Thanks,
Ming

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