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

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Date:   Wed, 8 Mar 2017 20:18:54 -0600
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
To:     Jens Axboe <axboe@...nel.dk>, 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 10:17 AM, Jens Axboe wrote:
> 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.
> 

Thanks, I understood what I was doing wrong. I will include blk-mq in
the patch.

-- 
Goldwyn