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[66.29.188.166]) by smtp.gmail.com with ESMTPSA id o4-v6sm23172594pfi.172.2018.11.13.05.44.47 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 13 Nov 2018 05:44:47 -0800 (PST) Subject: Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly To: Jianchao Wang Cc: ming.lei@redhat.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <1542103016-21037-1-git-send-email-jianchao.w.wang@oracle.com> <1542103016-21037-4-git-send-email-jianchao.w.wang@oracle.com> From: Jens Axboe Message-ID: Date: Tue, 13 Nov 2018 06:44:46 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <1542103016-21037-4-git-send-email-jianchao.w.wang@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/13/18 2:56 AM, Jianchao Wang wrote: > When issue request directly and the task is migrated out of the > original cpu where it allocates request, hctx could be ran on > the cpu where it is not mapped. > To fix this, > - insert the request forcibly if BLK_MQ_F_BLOCKING is set. > - check whether the current is mapped to the hctx, if not, insert > forcibly. > - invoke __blk_mq_issue_directly under preemption disabled. I'm not too crazy about this one, adding a get/put_cpu() in the hot path, and a cpumask test. The fact is that most/no drivers care about strict placement. We always try to do so, if convenient, since it's faster, but this seems to be doing the opposite. I'd be more inclined to have a driver flag if it needs guaranteed placement, using one an ops BLK_MQ_F_STRICT_CPU flag or similar. What do you think? -- Jens Axboe