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Wed, 20 Jun 2018 01:45:08 +0000 Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by aserv0022.oracle.com (8.14.4/8.14.4) with ESMTP id w5K1j8XR007421 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 20 Jun 2018 01:45:08 GMT Received: from abhmp0013.oracle.com (abhmp0013.oracle.com [141.146.116.19]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id w5K1j7xg023420; Wed, 20 Jun 2018 01:45:07 GMT Received: from [10.182.69.179] (/10.182.69.179) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 19 Jun 2018 18:45:07 -0700 Subject: Re: [PATCH] blk-mq: use blk_mq_timeout_work to limit the max timeout To: Bart Van Assche , "axboe@kernel.dk" Cc: "linux-kernel@vger.kernel.org" , "linux-block@vger.kernel.org" References: <1529391610-1654-1-git-send-email-jianchao.w.wang@oracle.com> <884a5cbb5622ca5c890585a8d5dff0e3a1d60cfe.camel@wdc.com> <08b5a3e94f2265815a98abae7a8dde0894512d4e.camel@wdc.com> From: "jianchao.wang" Message-ID: Date: Wed, 20 Jun 2018 09:45:26 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=8929 signatures=668702 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1805220000 definitions=main-1806200017 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/20/2018 09:37 AM, jianchao.wang wrote: > > > On 06/20/2018 09:35 AM, Bart Van Assche wrote: >> On Wed, 2018-06-20 at 09:28 +0800, jianchao.wang wrote: >>> Hi Bart >>> >>> Thanks for your kindly response. >>> >>> On 06/19/2018 11:18 PM, Bart Van Assche wrote: >>>> On Tue, 2018-06-19 at 15:00 +0800, Jianchao Wang wrote: >>>>> blk_rq_timeout is needed to limit the max timeout value, otherwise, >>>>> a idle hctx cannot be deactivated timely in shared-tag case. >>>>> >>>>> Fixes: 12f5b931 (blk-mq: Remove generation seqeunce) >>>>> Signed-off-by: Jianchao Wang >>>>> --- >>>>> block/blk-mq.c | 2 +- >>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>> >>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c >>>>> index 70c65bb..ccebe7b 100644 >>>>> --- a/block/blk-mq.c >>>>> +++ b/block/blk-mq.c >>>>> @@ -868,7 +868,7 @@ static void blk_mq_timeout_work(struct work_struct *work) >>>>> blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next); >>>>> >>>>> if (next != 0) { >>>>> - mod_timer(&q->timeout, next); >>>>> + mod_timer(&q->timeout, blk_rq_timeout(round_jiffies_up(next))); >>>>> } else { >>>>> /* >>>>> * Request timeouts are handled as a forward rolling timer. If >>>> >>>> Hello Jianchao, >>>> >>>> What makes you think that it would be necessary to call blk_rq_timeout() from >>>> blk_mq_timeout_work()? Have you noticed that blk_add_timer() already calls that >>>> function? I think it is not necessary to call blk_rq_timeout() from >>>> blk_mq_timeout_work() because it is guaranteed in that function that the next >>>> timeout is less than BLK_MAX_TIMEOUT jiffies in the future. >>>> >>> >>> blk_add_timer will not re-arm the timer if the timer's expire value is before the new rq's expire value. >>> >>> Let's look at the following scenario. >>> >>> 0 +30s >>>> __________________|___| >>> >>> T0 T1 T2 >>> >>> T1 = T2 - 1 jiffies >>> >>> T0: rq_a is issued and q->timer is armed and will expire at T2 >>> then rq_a is completed. >>> T1: rq_b is issued and q->timer is not re-armed, because its next expire time is T2 < (T1 + 30s) >>> >>> T2: if rq_b have not been completed when timer expires at T2, timer would be re-armed based on the rq_b >>> If we don't have blk_rq_timeout here, the next expire time is about T2 + 30s. >> >> Hello Jianchao, >> >> I disagree with the last sentence above. I think for your example blk_mq_req_expired() >> will set next to T1 + 30s instead of T2 + 30s. >> > > Would you please explain the reason ? > Oops, yes, it is T1. I thought you were saying T0. :) In this scenario, I have said, the T1 = T2 - 1 jiifies, it is very closed to T2. So I said "the next expire time is about T2 + 30s" It's my bad description. The next time is T1 + 30s, but it is also not a good value > Thanks > Jianchao > >> Bart. >> >> >