Re: [PATCH] nvme: Drop WQ_MEM_RECLAIM flag from core workqueues

From: Daniel Wagner <dwagner@suse.de>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org,
	Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
	Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
	Steve Wise <swise@opengridcomputing.com>,
	Leon Romanovsky <leon@kernel.org>,
	Potnuri Bharat Teja <bharat@chelsio.com>
Subject: Re: [PATCH] nvme: Drop WQ_MEM_RECLAIM flag from core workqueues
Date: Tue, 13 Apr 2021 10:54:04 +0200	[thread overview]
Message-ID: <20210413085404.tzam5lprtspwcek4@beryllium.lan> (raw)
In-Reply-To: <20210412130402.GF227011@ziepe.ca>

On Mon, Apr 12, 2021 at 10:04:02AM -0300, Jason Gunthorpe wrote:
> Basically the allocation of importance in the workqueue is assigning a
> worker, so pre-allocating a worker ensures the work can continue to
> progress without becoming dependent on allocations.

Ah okay, got it. I didn't really understood this part. So the
WQ_MEM_RECLAIM is 'just' avoiding a new worker creation.

> This is why work under the WQ_MEM_RECLAIM cannot recurse back into the
> allocator as it would get a rescurer thread stuck at a point when all
> other threads are already stuck.
>
> To remove WQ_MEM_RECLAIM you have to make assertions about the calling
> contexts and blocking contexts of the workqueue, not what the work
> itself is doing.

Hmm, I am struggling with your last statement. If a worker does an
allocation it might block. I understand this is something which a worker
in a WQ_MEM_RECLAIM context is not allowed to do.

My aim is still to get rid of the warning triggered by the rdma
code.

Anyway, thanks for explaining.