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

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Date:   Wed, 27 Feb 2019 15:24:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Bart Van Assche <bvanassche@....org>, mingo@...hat.com,
        will.deacon@....com, tj@...nel.org, longman@...hat.com,
        johannes.berg@...el.com, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH v7 21/23] block: Avoid that flushing triggers a lockdep
 complaint

On Wed, Feb 27, 2019 at 09:35:56AM +0800, Ming Lei wrote:
> 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

Right; that is rather a lot. But what causes the recursion, and thus how
is it specific to NVME ?