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([2601:647:4000:d7:2484:4c29:8774:98e4]) by smtp.gmail.com with ESMTPSA id i11sm3184732pjg.0.2020.02.07.07.26.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 07 Feb 2020 07:26:50 -0800 (PST) Subject: Re: [PATCH] block: Limit number of items taken from the I/O scheduler in one go To: Salman Qazi , Jens Axboe , Ming Lei , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Jesse Barnes , Gwendal Grignou , Hannes Reinecke , Christoph Hellwig References: <20200206101833.GA20943@ming.t460p> <20200206211222.83170-1-sqazi@google.com> From: Bart Van Assche Autocrypt: addr=bvanassche@acm.org; prefer-encrypt=mutual; keydata= mQENBFSOu4oBCADcRWxVUvkkvRmmwTwIjIJvZOu6wNm+dz5AF4z0FHW2KNZL3oheO3P8UZWr LQOrCfRcK8e/sIs2Y2D3Lg/SL7qqbMehGEYcJptu6mKkywBfoYbtBkVoJ/jQsi2H0vBiiCOy fmxMHIPcYxaJdXxrOG2UO4B60Y/BzE6OrPDT44w4cZA9DH5xialliWU447Bts8TJNa3lZKS1 AvW1ZklbvJfAJJAwzDih35LxU2fcWbmhPa7EO2DCv/LM1B10GBB/oQB5kvlq4aA2PSIWkqz4 3SI5kCPSsygD6wKnbRsvNn2mIACva6VHdm62A7xel5dJRfpQjXj2snd1F/YNoNc66UUTABEB AAG0JEJhcnQgVmFuIEFzc2NoZSA8YnZhbmFzc2NoZUBhY20ub3JnPokBOQQTAQIAIwUCVI67 igIbAwcLCQgHAwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJEHFcPTXFzhAJ8QkH/1AdXblKL65M Y1Zk1bYKnkAb4a98LxCPm/pJBilvci6boefwlBDZ2NZuuYWYgyrehMB5H+q+Kq4P0IBbTqTa jTPAANn62A6jwJ0FnCn6YaM9TZQjM1F7LoDX3v+oAkaoXuq0dQ4hnxQNu792bi6QyVdZUvKc macVFVgfK9n04mL7RzjO3f+X4midKt/s+G+IPr4DGlrq+WH27eDbpUR3aYRk8EgbgGKvQFdD CEBFJi+5ZKOArmJVBSk21RHDpqyz6Vit3rjep7c1SN8s7NhVi9cjkKmMDM7KYhXkWc10lKx2 RTkFI30rkDm4U+JpdAd2+tP3tjGf9AyGGinpzE2XY1K5AQ0EVI67igEIAKiSyd0nECrgz+H5 PcFDGYQpGDMTl8MOPCKw/F3diXPuj2eql4xSbAdbUCJzk2ETif5s3twT2ER8cUTEVOaCEUY3 eOiaFgQ+nGLx4BXqqGewikPJCe+UBjFnH1m2/IFn4T9jPZkV8xlkKmDUqMK5EV9n3eQLkn5g lco+FepTtmbkSCCjd91EfThVbNYpVQ5ZjdBCXN66CKyJDMJ85HVr5rmXG/nqriTh6cv1l1Js T7AFvvPjUPknS6d+BETMhTkbGzoyS+sywEsQAgA+BMCxBH4LvUmHYhpS+W6CiZ3ZMxjO8Hgc ++w1mLeRUvda3i4/U8wDT3SWuHcB3DWlcppECLkAEQEAAYkBHwQYAQIACQUCVI67igIbDAAK CRBxXD01xc4QCZ4dB/0QrnEasxjM0PGeXK5hcZMT9Eo998alUfn5XU0RQDYdwp6/kMEXMdmT oH0F0xB3SQ8WVSXA9rrc4EBvZruWQ+5/zjVrhhfUAx12CzL4oQ9Ro2k45daYaonKTANYG22y //x8dLe2Fv1By4SKGhmzwH87uXxbTJAUxiWIi1np0z3/RDnoVyfmfbbL1DY7zf2hYXLLzsJR mSsED/1nlJ9Oq5fALdNEPgDyPUerqHxcmIub+pF0AzJoYHK5punqpqfGmqPbjxrJLPJfHVKy goMj5DlBMoYqEgpbwdUYkH6QdizJJCur4icy8GUNbisFYABeoJ91pnD4IGei3MTdvINSZI5e Message-ID: <5707b17f-e5d7-c274-de6a-694098c4e9a2@acm.org> Date: Fri, 7 Feb 2020 07:26:49 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200206211222.83170-1-sqazi@google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 2020-02-06 13:12, Salman Qazi wrote: > + * > + * Returns true if hctx->dispatch was found non-empty and > + * run_work has to be run again. Please elaborate this comment and explain why this is necessary (to avoid that flush processing is postponed forever). > + * Returns true if hctx->dispatch was found non-empty and > + * run_work has to be run again. Same comment here. > +again: > + run_again = false; > + > /* > * If we have previous entries on our dispatch list, grab them first for > * more fair dispatch. > @@ -208,19 +234,28 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx) > blk_mq_sched_mark_restart_hctx(hctx); > if (blk_mq_dispatch_rq_list(q, &rq_list, false)) { > if (has_sched_dispatch) > - blk_mq_do_dispatch_sched(hctx); > + run_again = blk_mq_do_dispatch_sched(hctx); > else > - blk_mq_do_dispatch_ctx(hctx); > + run_again = blk_mq_do_dispatch_ctx(hctx); > } > } else if (has_sched_dispatch) { > - blk_mq_do_dispatch_sched(hctx); > + run_again = blk_mq_do_dispatch_sched(hctx); > } else if (hctx->dispatch_busy) { > /* dequeue request one by one from sw queue if queue is busy */ > - blk_mq_do_dispatch_ctx(hctx); > + run_again = blk_mq_do_dispatch_ctx(hctx); > } else { > blk_mq_flush_busy_ctxs(hctx, &rq_list); > blk_mq_dispatch_rq_list(q, &rq_list, false); > } > + > + if (run_again) { > + if (!restarted) { > + restarted = true; > + goto again; > + } > + > + blk_mq_run_hw_queue(hctx, true); > + } So this patch changes blk_mq_sched_dispatch_requests() such that it iterates at most two times? How about implementing that loop with an explicit for-loop? I think that will make blk_mq_sched_dispatch_requests() easier to read. As you may know forward goto's are accepted in kernel code but backward goto's are frowned upon. Thanks, Bart.