From nobody Tue Feb 10 17:45:12 2026 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1541153044169448.2471815013091; Fri, 2 Nov 2018 03:04:04 -0700 (PDT) Received: from localhost ([::1]:50542 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gIWJ5-0004fv-Bt for importer@patchew.org; Fri, 02 Nov 2018 06:03:59 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34318) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gIWH9-0002XG-53 for qemu-devel@nongnu.org; Fri, 02 Nov 2018 06:02:01 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gIWH8-0000dE-5p for qemu-devel@nongnu.org; Fri, 02 Nov 2018 06:01:59 -0400 Received: from smtp03.citrix.com ([162.221.156.55]:1114) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gIWGv-000098-Ar; Fri, 02 Nov 2018 06:01:45 -0400 X-IronPort-AV: E=Sophos;i="5.54,455,1534809600"; d="scan'208";a="69460955" From: Tim Smith To: , , Date: Fri, 2 Nov 2018 10:01:04 +0000 Message-ID: <154115286450.11300.208978412106583040.stgit@dhcp-3-135.uk.xensource.com> In-Reply-To: <154115285434.11300.8459925605672823399.stgit@dhcp-3-135.uk.xensource.com> References: <154115285434.11300.8459925605672823399.stgit@dhcp-3-135.uk.xensource.com> User-Agent: StGit/0.17.1-dirty MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 162.221.156.55 Subject: [Qemu-devel] [PATCH 2/3] Improve xen_disk response latency X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Anthony Perard , Kevin Wolf , Paul Durrant , Stefano Stabellini , Max Reitz Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" If the I/O ring is full, the guest cannot send any more requests until some responses are sent. Only sending all available responses just before checking for new work does not leave much time for the guest to supply new work, so this will cause stalls if the ring gets full. Also, not completing reads as soon as possible adds latency to the guest. To alleviate that, complete IO requests as soon as they come back. blk_send_response() already returns a value indicating whether a notify should be sent, which is all the batching we need. Signed-off-by: Tim Smith Acked-by: Anthony PERARD Reviewed-by: Paul Durrant --- hw/block/xen_disk.c | 43 ++++++++++++------------------------------- 1 file changed, 12 insertions(+), 31 deletions(-) diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c index cb2881b7e6..b506e23868 100644 --- a/hw/block/xen_disk.c +++ b/hw/block/xen_disk.c @@ -83,11 +83,9 @@ struct XenBlkDev { =20 /* request lists */ QLIST_HEAD(inflight_head, ioreq) inflight; - QLIST_HEAD(finished_head, ioreq) finished; QLIST_HEAD(freelist_head, ioreq) freelist; int requests_total; int requests_inflight; - int requests_finished; unsigned int max_requests; =20 gboolean feature_discard; @@ -104,6 +102,9 @@ struct XenBlkDev { /* Threshold of in-flight requests above which we will start using * blk_io_plug()/blk_io_unplug() to batch requests */ #define IO_PLUG_THRESHOLD 1 + +static int blk_send_response(struct ioreq *ioreq); + /* ------------------------------------------------------------- */ =20 static void ioreq_reset(struct ioreq *ioreq) @@ -155,12 +156,10 @@ static void ioreq_finish(struct ioreq *ioreq) struct XenBlkDev *blkdev =3D ioreq->blkdev; =20 QLIST_REMOVE(ioreq, list); - QLIST_INSERT_HEAD(&blkdev->finished, ioreq, list); blkdev->requests_inflight--; - blkdev->requests_finished++; } =20 -static void ioreq_release(struct ioreq *ioreq, bool finish) +static void ioreq_release(struct ioreq *ioreq) { struct XenBlkDev *blkdev =3D ioreq->blkdev; =20 @@ -168,11 +167,7 @@ static void ioreq_release(struct ioreq *ioreq, bool fi= nish) ioreq_reset(ioreq); ioreq->blkdev =3D blkdev; QLIST_INSERT_HEAD(&blkdev->freelist, ioreq, list); - if (finish) { - blkdev->requests_finished--; - } else { - blkdev->requests_inflight--; - } + blkdev->requests_inflight--; } =20 /* @@ -351,6 +346,10 @@ static void qemu_aio_complete(void *opaque, int ret) default: break; } + if (blk_send_response(ioreq)) { + xen_pv_send_notify(&blkdev->xendev); + } + ioreq_release(ioreq); qemu_bh_schedule(blkdev->bh); =20 done: @@ -455,7 +454,7 @@ err: return -1; } =20 -static int blk_send_response_one(struct ioreq *ioreq) +static int blk_send_response(struct ioreq *ioreq) { struct XenBlkDev *blkdev =3D ioreq->blkdev; int send_notify =3D 0; @@ -504,22 +503,6 @@ static int blk_send_response_one(struct ioreq *ioreq) return send_notify; } =20 -/* walk finished list, send outstanding responses, free requests */ -static void blk_send_response_all(struct XenBlkDev *blkdev) -{ - struct ioreq *ioreq; - int send_notify =3D 0; - - while (!QLIST_EMPTY(&blkdev->finished)) { - ioreq =3D QLIST_FIRST(&blkdev->finished); - send_notify +=3D blk_send_response_one(ioreq); - ioreq_release(ioreq, true); - } - if (send_notify) { - xen_pv_send_notify(&blkdev->xendev); - } -} - static int blk_get_request(struct XenBlkDev *blkdev, struct ioreq *ioreq, = RING_IDX rc) { switch (blkdev->protocol) { @@ -554,7 +537,6 @@ static void blk_handle_requests(struct XenBlkDev *blkde= v) rp =3D blkdev->rings.common.sring->req_prod; xen_rmb(); /* Ensure we see queued requests up to 'rp'. */ =20 - blk_send_response_all(blkdev); /* If there was more than IO_PLUG_THRESHOLD ioreqs in flight * when we got here, this is an indication that there the bottleneck * is below us, so it's worth beginning to batch up I/O requests @@ -597,10 +579,10 @@ static void blk_handle_requests(struct XenBlkDev *blk= dev) break; }; =20 - if (blk_send_response_one(ioreq)) { + if (blk_send_response(ioreq)) { xen_pv_send_notify(&blkdev->xendev); } - ioreq_release(ioreq, false); + ioreq_release(ioreq); continue; } =20 @@ -646,7 +628,6 @@ static void blk_alloc(struct XenDevice *xendev) trace_xen_disk_alloc(xendev->name); =20 QLIST_INIT(&blkdev->inflight); - QLIST_INIT(&blkdev->finished); QLIST_INIT(&blkdev->freelist); =20 blkdev->iothread =3D iothread_create(xendev->name, &err);