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

From: Goldwyn Rodrigues <rgoldwyn@suse.de>
To: Jens Axboe <axboe@kernel.dk>, Sagi Grimberg <sagi@grimberg.me>,
	jack@suse.com
Cc: hch@infradead.org, linux-fsdevel@vger.kernel.org,
	linux-block@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
	Goldwyn Rodrigues <rgoldwyn@suse.com>
Subject: Re: [PATCH 5/8] nowait aio: return on congested block device
Date: Wed, 8 Mar 2017 20:18:54 -0600	[thread overview]
Message-ID: <310c6725-3263-2790-a445-d58e464fd3e4@suse.de> (raw)
In-Reply-To: <a661cdc7-4859-4a39-3037-8036cc8c4712@kernel.dk>

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