From nobody Mon Feb 9 22:19:16 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.zoho.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 1492690157426225.23265104186441; Thu, 20 Apr 2017 05:09:17 -0700 (PDT) Received: from localhost ([::1]:53562 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1d1Atf-00050n-Fw for importer@patchew.org; Thu, 20 Apr 2017 08:09:15 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38742) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1d1Amp-0007gB-Re for qemu-devel@nongnu.org; Thu, 20 Apr 2017 08:02:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1d1Amd-00032H-4U for qemu-devel@nongnu.org; Thu, 20 Apr 2017 08:02:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34622) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1d1Am2-0002X3-5I; Thu, 20 Apr 2017 08:01:22 -0400 Received: from int-mx09.intmail.prod.int.phx2.redhat.com (int-mx09.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BB54E341577; Thu, 20 Apr 2017 12:01:20 +0000 (UTC) Received: from donizetti.redhat.com (ovpn-117-9.ams2.redhat.com [10.36.117.9]) by int-mx09.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id v3KC0w44002037; Thu, 20 Apr 2017 08:01:19 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com BB54E341577 Authentication-Results: ext-mx05.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx05.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=pbonzini@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com BB54E341577 From: Paolo Bonzini To: qemu-devel@nongnu.org Date: Thu, 20 Apr 2017 14:00:55 +0200 Message-Id: <20170420120058.28404-15-pbonzini@redhat.com> In-Reply-To: <20170420120058.28404-1-pbonzini@redhat.com> References: <20170420120058.28404-1-pbonzini@redhat.com> X-Scanned-By: MIMEDefang 2.68 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Thu, 20 Apr 2017 12:01:20 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PATCH 14/17] block: optimize access to reqs_lock 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: qemu-block@nongnu.org Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Hot path reqs_lock critical sections are very small; the only large critical sections happen when a request waits for serialising requests, and these should never happen in usual circumstances. We do not want these small critical sections to yield in any case, which calls for using a spinlock while writing the list. The reqs_lock is still used to protect the individual requests' CoQueue. For this purpose, serializing removals against concurrent walks of the request list can use lock_unlock for efficiency and determinism. The reqs_lock is also used to protect the flush generation counts, but that's unrelated. Signed-off-by: Paolo Bonzini --- block.c | 1 + block/io.c | 25 ++++++++++++++++++++----- include/block/block_int.h | 11 ++++++++--- 3 files changed, 29 insertions(+), 8 deletions(-) diff --git a/block.c b/block.c index 3b2ed29..7ba6afe 100644 --- a/block.c +++ b/block.c @@ -234,6 +234,7 @@ BlockDriverState *bdrv_new(void) QLIST_INIT(&bs->op_blockers[i]); } notifier_with_return_list_init(&bs->before_write_notifiers); + qemu_spin_init(&bs->reqs_list_write_lock); qemu_co_mutex_init(&bs->reqs_lock); bs->refcnt =3D 1; bs->aio_context =3D qemu_get_aio_context(); diff --git a/block/io.c b/block/io.c index 7af9d47..476807d 100644 --- a/block/io.c +++ b/block/io.c @@ -374,14 +374,29 @@ void bdrv_drain_all(void) */ static void tracked_request_end(BdrvTrackedRequest *req) { + BlockDriverState *bs =3D req->bs; + if (req->serialising) { - atomic_dec(&req->bs->serialising_in_flight); + atomic_dec(&bs->serialising_in_flight); } =20 - qemu_co_mutex_lock(&req->bs->reqs_lock); + /* Note that there can be a concurrent visit while we remove the list, + * so we need to... + */ + qemu_spin_lock(&bs->reqs_list_write_lock); QLIST_REMOVE(req, list); + qemu_spin_unlock(&bs->reqs_list_write_lock); + + /* ... wait for it to end before we leave. qemu_co_mutex_lock_unlock + * avoids cacheline bouncing in the common case of no concurrent + * reader. + */ + qemu_co_mutex_lock_unlock(&bs->reqs_lock); + + /* Now no coroutine can add itself to the wait queue, so it is + * safe to call qemu_co_queue_restart_all outside the reqs_lock. + */ qemu_co_queue_restart_all(&req->wait_queue); - qemu_co_mutex_unlock(&req->bs->reqs_lock); } =20 /** @@ -406,9 +421,9 @@ static void tracked_request_begin(BdrvTrackedRequest *r= eq, =20 qemu_co_queue_init(&req->wait_queue); =20 - qemu_co_mutex_lock(&bs->reqs_lock); + qemu_spin_lock(&bs->reqs_list_write_lock); QLIST_INSERT_HEAD(&bs->tracked_requests, req, list); - qemu_co_mutex_unlock(&bs->reqs_lock); + qemu_spin_unlock(&bs->reqs_list_write_lock); } =20 static void mark_request_serialising(BdrvTrackedRequest *req, uint64_t ali= gn) diff --git a/include/block/block_int.h b/include/block/block_int.h index 42b49f5..b298de8 100644 --- a/include/block/block_int.h +++ b/include/block/block_int.h @@ -78,9 +78,10 @@ typedef struct BdrvTrackedRequest { =20 QLIST_ENTRY(BdrvTrackedRequest) list; Coroutine *co; /* owner, used for deadlock detection */ - CoQueue wait_queue; /* coroutines blocked on this request */ - struct BdrvTrackedRequest *waiting_for; + + /* Protected by BlockDriverState's reqs_lock. */ + CoQueue wait_queue; /* coroutines blocked on this request */ } BdrvTrackedRequest; =20 struct BlockDriver { @@ -626,11 +627,15 @@ struct BlockDriverState { int quiesce_counter; unsigned int write_gen; /* Current data generation */ =20 - /* Protected by reqs_lock. */ + /* Writes are protected by reqs_list_write_lock. Reads take + * reqs_lock so that removals can easily synchronize with walks. + */ QLIST_HEAD(, BdrvTrackedRequest) tracked_requests; CoQueue flush_queue; /* Serializing flush queue */ bool active_flush_req; /* Flush request in flight? */ unsigned int flushed_gen; /* Flushed write generation */ + + QemuSpin reqs_list_write_lock; CoMutex reqs_lock; }; =20 --=20 2.9.3