linux-ext4 - Re: [PATCH 5/8] nowait aio: return on congested block device

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Date:   Wed, 8 Mar 2017 09:17:04 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Sagi Grimberg <sagi@...mberg.me>, jack@...e.com
Cc:     hch@...radead.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 5/8] nowait aio: return on congested block device

On 03/08/2017 08:00 AM, Goldwyn Rodrigues wrote:
> 
> 
> On 03/08/2017 01:03 AM, Sagi Grimberg wrote:
>>
>>> -        if (likely(blk_queue_enter(q, false) == 0)) {
>>> +        if (likely(blk_queue_enter(q, bio_flagged(bio, BIO_NOWAIT))
>>> == 0)) {
>>>              ret = q->make_request_fn(q, bio);
>>
>> I think that for ->make_request to not block we'd need to set
>> BLK_MQ_REQ_NOWAIT in blk_mq_alloc_data to avoid blocking on a tag
>> allocation.
>>
>> Something like the untested addition below:
> 
> I did that in the first series, but there are too many reasons to block
> in blk-mq [1]. I dropped blk-mq work in v2.

That's complete nonsense, there are no more places in blk-mq that will
block that in the legacy path. Most of the examples from your URL:

> [1] https://patchwork.kernel.org/patch/9571051/

are not blk-mq, but writeback throttling, and drivers that explicitly
hook into ->make_request_fn.

As others have mentioned, it's a total non-starter to focus on the
deprecated IO path and just ignore the new one. Back to the drawing
board.

-- 
Jens Axboe