From: Jens Axboe Subject: Re: [PATCH 5/8] nowait aio: return on congested block device Date: Wed, 8 Mar 2017 09:17:04 -0700 Message-ID: References: <20170228233610.25456-1-rgoldwyn@suse.de> <20170228233610.25456-6-rgoldwyn@suse.de> <0d1ba678-69c0-16ce-6c3e-475cd8da203c@grimberg.me> <44cc02b1-f7ac-cbc6-6f09-726ae8225358@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit 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 To: Goldwyn Rodrigues , Sagi Grimberg , jack@suse.com Return-path: In-Reply-To: <44cc02b1-f7ac-cbc6-6f09-726ae8225358@suse.de> Sender: linux-btrfs-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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