Re: [PATCH v7 21/23] block: Avoid that flushing triggers a lockdep complaint

[Ming Lei]

On Tue, Feb 26, 2019 at 07:08:02PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 15, 2019 at 10:26:59AM +0800, Ming Lei wrote:
> > There might be lots of blk_flush_queue instance which is allocated
> > for each hctx, then lots of class key slot may be wasted.
> 
> What is 'lots' ? for someone who doesn't really know all that much about
> the block layer.

Each hw queue has one instance of blk_flush_queue, and one device may
has lots of hw queues(may be > all possible cpus, such as nvme), and there
may be lots of block devices in one system.

Suppose one system has 10 NVMe hosts, 8 disks attached to each host, and
256 CPU cores in the system, there can be 10 * 8 * 256 = 20K instances of
blk_flush_queue.

Not mention there are other block devices(loop, nbd, scsi, ...) in the system.

That is why I suggest to use one single lock class for addressing this
nvme loop specific issue:

https://marc.info/?l=linux-kernel&m=155019765724564&w=2

Thanks,
Ming