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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7B0A5C433F5 for ; Tue, 29 Mar 2022 15:55:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239042AbiC2P4n (ORCPT ); Tue, 29 Mar 2022 11:56:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49202 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237925AbiC2P4n (ORCPT ); Tue, 29 Mar 2022 11:56:43 -0400 Received: from mail-pj1-x1029.google.com (mail-pj1-x1029.google.com [IPv6:2607:f8b0:4864:20::1029]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 183E633E02 for ; Tue, 29 Mar 2022 08:55:00 -0700 (PDT) Received: by mail-pj1-x1029.google.com with SMTP id jx9so17902252pjb.5 for ; Tue, 29 Mar 2022 08:55:00 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=Cuhk1XdERu69A8XvjiWHEvjoeMu5OdiIwkUINeMnFcc=; b=WuSO8BgjKAhJz2zqNOvrPk0uO2c1MvY6Po8VGSLtL5ergv+o5ZbC4r+sA++cfMAOSs ZMDOjOhiBSjx1dcUIX8XW+Bo71UOxtKVx0Nr/75p80EEXPmFHPBTr05Mp8c7m+LP98Ti svm9D67jAgkCj7aBBPfCrVV3UmxT0EWggS/doZhynTR98ixY/4DNZz28lCOdnv0q9v3d p1bO36LPVU4VMlWlxQWLtiiVII7jFwCDW7gmXK1apQmwabiBXE+I9xA3w/8Yqb+JrUsy dIiQEovE3D/Skiyc+7BKnF5tuA7TTPD4wt+upGxVUu3zL71GIERDO53KY62tScj/8Ebj Mlkw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=Cuhk1XdERu69A8XvjiWHEvjoeMu5OdiIwkUINeMnFcc=; b=tbSMkjOyLIZhe0Xy2WRX0skZxHIZ5t/SpKL1If51iGT0INB1485Vkv6VFAAbHZTVLs LT+3kuleVz5sNZLR/+rovagW86MFOV8w9tqM9ZZ3ICjpTMTvuJ5Y/8UShtxWmB+F/6ZN VMR0GbYMty/OtG7r4yr3QgGMMFA4LFASbF3KGdL3nqSE2N5f6+O6ID3kSZC15oF8P9Li o4fvXLHAlRNaYZ6HIT+kfh8MHeyvm6LeRuSNYHyAzDvkkih7GeUhhJ9clYRBBEmVRaaA VRdOHJvsR6D9wdKQWMrt8DYrmwl/UUycLikFCUrWiQ/R+TSYuxb6z4/rRvWJp2AcmMLd atCw== X-Gm-Message-State: AOAM532u9cHYIW4iuUBXOpRA2yo6b3uChp+iglYlrNbMcZs5P2bS+vkp zJJNR1j2NgaZWz5aq9Bk75Y= X-Google-Smtp-Source: ABdhPJwf2sNAcxgsLmbO/21Z8HUN/SDQK/R/iNTLlQUw9JfvSCENtjkJWMDTVzSouQlwsO7a1L/oUw== X-Received: by 2002:a17:902:e88e:b0:154:7562:176d with SMTP id w14-20020a170902e88e00b001547562176dmr30640930plg.13.1648569299518; Tue, 29 Mar 2022 08:54:59 -0700 (PDT) Received: from localhost.localdomain ([114.200.4.15]) by smtp.gmail.com with ESMTPSA id o3-20020a056a0015c300b004fb24adc4b8sm13922134pfu.159.2022.03.29.08.54.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 29 Mar 2022 08:54:58 -0700 (PDT) Date: Wed, 30 Mar 2022 00:54:53 +0900 From: Suwan Kim To: Stefan Hajnoczi Cc: mst@redhat.com, jasowang@redhat.com, pbonzini@redhat.com, mgurtovoy@nvidia.com, virtualization@lists.linux-foundation.org, linux-block@vger.kernel.org Subject: Re: [PATCH v3 2/2] virtio-blk: support mq_ops->queue_rqs() Message-ID: References: <20220324140450.33148-1-suwan.kim027@gmail.com> <20220324140450.33148-3-suwan.kim027@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue, Mar 29, 2022 at 04:01:46PM +0100, Stefan Hajnoczi wrote: > On Tue, Mar 29, 2022 at 10:48:16PM +0900, Suwan Kim wrote: > > On Tue, Mar 29, 2022 at 09:45:29AM +0100, Stefan Hajnoczi wrote: > > > On Tue, Mar 29, 2022 at 12:50:33AM +0900, Suwan Kim wrote: > > > > On Mon, Mar 28, 2022 at 02:16:13PM +0100, Stefan Hajnoczi wrote: > > > > > On Thu, Mar 24, 2022 at 11:04:50PM +0900, Suwan Kim wrote: > > > > > > +static void virtio_queue_rqs(struct request **rqlist) > > > > > > +{ > > > > > > + struct request *req, *next, *prev = NULL; > > > > > > + struct request *requeue_list = NULL; > > > > > > + > > > > > > + rq_list_for_each_safe(rqlist, req, next) { > > > > > > + struct virtio_blk_vq *vq = req->mq_hctx->driver_data; > > > > > > + unsigned long flags; > > > > > > + bool kick; > > > > > > + > > > > > > + if (!virtblk_prep_rq_batch(vq, req)) { > > > > > > + rq_list_move(rqlist, &requeue_list, req, prev); > > > > > > + req = prev; > > > > > > + > > > > > > + if (!req) > > > > > > + continue; > > > > > > + } > > > > > > + > > > > > > + if (!next || req->mq_hctx != next->mq_hctx) { > > > > > > + spin_lock_irqsave(&vq->lock, flags); > > > > > > > > > > Did you try calling virtblk_add_req() here to avoid acquiring and > > > > > releasing the lock multiple times? In other words, do virtblk_prep_rq() > > > > > but wait until we get here to do virtblk_add_req(). > > > > > > > > > > I don't know if it has any measurable effect on performance or maybe the > > > > > code would become too complex, but I noticed that we're not fully > > > > > exploiting batching. > > > > > > > > I tried as you said. I called virtlblk_add_req() and added requests > > > > of rqlist to virtqueue in this if statement with holding the lock > > > > only once. > > > > > > > > I attach the code at the end of this mail. > > > > Please refer the code. > > > > > > > > But I didn't see improvement. It showed slightly worse performance > > > > than the current patch. > > > > > > Okay, thanks for trying it! > > > > > > > > > + kick = virtqueue_kick_prepare(vq->vq); > > > > > > + spin_unlock_irqrestore(&vq->lock, flags); > > > > > > + if (kick) > > > > > > + virtqueue_notify(vq->vq); > > > > > > + > > > > > > + req->rq_next = NULL; > > > > > > > > Did you ask this part? > > > > > > > > > > + *rqlist = next; > > > > > > + prev = NULL; > > > > > > + } else > > > > > > + prev = req; > > > > > > > > > > What guarantees that req is still alive after we called > > > > > virtblk_add_req()? The device may have seen it and completed it already > > > > > by the time we get here. > > > > > > > > Isn't request completed after the kick? > > > > > > > > If you asked about "req->rq_next = NULL", > > > > I think it should be placed before > > > > "kick = virtqueue_kick_prepare(vq->vq);" > > > > > > > > ----------- > > > > req->rq_next = NULL; > > > > kick = virtqueue_kick_prepare(vq->vq); > > > > spin_unlock_irqrestore(&vq->lock, flags); > > > > if (kick) > > > > virtqueue_notify(vq->vq); > > > > ----------- > > > > > > No, virtqueue_add_sgs() exposes vring descriptors to the device. The > > > device may process immediately. In other words, VIRTIO devices may poll > > > the vring instead of waiting for virtqueue_notify(). There is no > > > guarantee that the request is alive until virtqueue_notify() is called. > > > > > > The code has to handle the case where the request is completed during > > > virtqueue_add_sgs(). > > > > Thanks for the explanation. > > > > We should not use req again after virtblk_add_req(). > > I understand... > > > > Then, as you commented in previous mail, is it ok that we do > > virtblk_add_req() in "if (!next || req->mq_hctx != next->mq_hctx)" > > statement to avoid use req again after virtblk_add_req() as below code? > > > > In this code, It adds reqs to virtqueue in batch just before > > virtqueue_notify(), and it doesn't use req again after calling > > virtblk_add_req(). > > > > If it is fine, I will try it again. > > This code is slightly different from the code I sent in previous mail. > > > > --- > > static void virtio_queue_rqs(struct request **rqlist) > > ... > > rq_list_for_each_safe(rqlist, req, next) { > > ... > > if (!next || req->mq_hctx != next->mq_hctx) { > > // Cut the list at current req > > req->rq_next = NULL; > > // Add req list to virtqueue in batch with holding lock once > > kick = virtblk_add_req_batch(vq, rqlist, &requeue_list); > > if (kick) > > virtqueue_notify(vq->vq); > > > > // setup new req list. Don't use previous req again. > > *rqlist = next; > > prev = NULL; > > ... > > Yes, that sounds good. > > (I noticed struct request has a reference count so that might be a way > to keep requests alive, if necessary, but I haven't investigated. See > req_ref_put_and_test() though it's not used by block drivers and maybe > virtio-blk shouldn't mess with it either.) I also think that using ref count is not a good idea. I will send the next version soon. Regards, Suwan Kim