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

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Message-ID: <20210413085404.tzam5lprtspwcek4@beryllium.lan>
Date:   Tue, 13 Apr 2021 10:54:04 +0200
From:   Daniel Wagner <dwagner@...e.de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Steve Wise <swise@...ngridcomputing.com>,
        Leon Romanovsky <leon@...nel.org>,
        Potnuri Bharat Teja <bharat@...lsio.com>
Subject: Re: [PATCH] nvme: Drop WQ_MEM_RECLAIM flag from core workqueues

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.