From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5DE87C56202 for ; Thu, 26 Nov 2020 02:59:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D75DE21734 for ; Thu, 26 Nov 2020 02:59:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="GcUWVrv/" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730660AbgKZC7D (ORCPT ); Wed, 25 Nov 2020 21:59:03 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:20112 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729679AbgKZC7D (ORCPT ); Wed, 25 Nov 2020 21:59:03 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1606359541; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=1O3biWysUvvAR2mvflPSVw+B4WljGNc5/mukonHUSoQ=; b=GcUWVrv/LffGv4SDGsoQhDCfJkfrO3g3T7/n/yxxWqY527jHnCyV0fkWRcf9hIs/tZqr5y E4cAbJoqKd3JnWYVR1EFe1F+eFx4L8bRUJxOq9w9y5MgETPmgceF0zhZddMOMFS1BtX6BI WAnWRGOiwBvnlL8yLf22QyYcHU/XtCw= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-29-4dtTisQFMSC-CWRxXz24Kg-1; Wed, 25 Nov 2020 21:58:59 -0500 X-MC-Unique: 4dtTisQFMSC-CWRxXz24Kg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 795441074640; Thu, 26 Nov 2020 02:58:58 +0000 (UTC) Received: from T590 (ovpn-13-94.pek2.redhat.com [10.72.13.94]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 27CA119C46; Thu, 26 Nov 2020 02:58:51 +0000 (UTC) Date: Thu, 26 Nov 2020 10:58:47 +0800 From: Ming Lei To: Jeffle Xu Cc: axboe@kernel.dk, linux-block@vger.kernel.org, joseph.qi@linux.alibaba.com, hch@infradead.org Subject: Re: [PATCH v8] block: disable iopoll for split bio Message-ID: <20201126025847.GB42718@T590> References: <20201126022837.15545-1-jefflexu@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201126022837.15545-1-jefflexu@linux.alibaba.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, Nov 26, 2020 at 10:28:37AM +0800, Jeffle Xu wrote: > iopoll is initially for small size, latency sensitive IO. It doesn't > work well for big IO, especially when it needs to be split to multiple > bios. In this case, the returned cookie of __submit_bio_noacct_mq() is > indeed the cookie of the last split bio. The completion of *this* last > split bio done by iopoll doesn't mean the whole original bio has > completed. Callers of iopoll still need to wait for completion of other > split bios. > > Besides bio splitting may cause more trouble for iopoll which isn't > supposed to be used in case of big IO. > > iopoll for split bio may cause potential race if CPU migration happens > during bio submission. Since the returned cookie is that of the last > split bio, polling on the corresponding hardware queue doesn't help > complete other split bios, if these split bios are enqueued into > different hardware queues. Since interrupts are disabled for polling > queues, the completion of these other split bios depends on timeout > mechanism, thus causing a potential hang. > > iopoll for split bio may also cause hang for sync polling. Currently > both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling > in direct IO routine. These routines will submit bio without REQ_NOWAIT > flag set, and then start sync polling in current process context. The > process may hang in blk_mq_get_tag() if the submitted bio has to be > split into multiple bios and can rapidly exhaust the queue depth. The > process are waiting for the completion of the previously allocated > requests, which should be reaped by the following polling, and thus > causing a deadlock. > > To avoid these subtle trouble described above, just disable iopoll for > split bio and return BLK_QC_T_NONE in this case. The side effect is that > non-HIPRI IO also returns BLK_QC_T_NONE now. It should be acceptable > since the returned cookie is never used for non-HIPRI IO. > > Suggested-by: Ming Lei > Signed-off-by: Jeffle Xu > --- > block/blk-merge.c | 8 ++++++++ > block/blk-mq.c | 5 +++++ > 2 files changed, 13 insertions(+) > > diff --git a/block/blk-merge.c b/block/blk-merge.c > index bcf5e4580603..8a2f1fb7bb16 100644 > --- a/block/blk-merge.c > +++ b/block/blk-merge.c > @@ -279,6 +279,14 @@ static struct bio *blk_bio_segment_split(struct request_queue *q, > return NULL; > split: > *segs = nsegs; > + > + /* > + * Bio splitting may cause subtle trouble such as hang when doing sync > + * iopoll in direct IO routine. Given performance gain of iopoll for > + * big IO can be trival, disable iopoll when split needed. > + */ > + bio->bi_opf &= ~REQ_HIPRI; > + > return bio_split(bio, sectors, GFP_NOIO, bs); > } > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 55bcee5dc032..580fd570be23 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -2157,6 +2157,7 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio) > unsigned int nr_segs; > blk_qc_t cookie; > blk_status_t ret; > + bool hipri; > > blk_queue_bounce(q, &bio); > __blk_queue_split(&bio, &nr_segs); > @@ -2173,6 +2174,8 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio) > > rq_qos_throttle(q, bio); > > + hipri = bio->bi_opf & REQ_HIPRI; > + > data.cmd_flags = bio->bi_opf; > rq = __blk_mq_alloc_request(&data); > if (unlikely(!rq)) { > @@ -2265,7 +2268,9 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio) > blk_mq_sched_insert_request(rq, false, true, true); > } > > + cookie = hipri ? cookie : BLK_QC_T_NONE; > return cookie; > + > queue_exit: > blk_queue_exit(q); > return BLK_QC_T_NONE; > -- > 2.27.0 > Reviewed-by: Ming Lei Thanks, Ming