From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751706AbaGFOjt (ORCPT ); Sun, 6 Jul 2014 10:39:49 -0400 Received: from mail-we0-f179.google.com ([74.125.82.179]:41956 "EHLO mail-we0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751515AbaGFOjn (ORCPT ); Sun, 6 Jul 2014 10:39:43 -0400 From: Paolo Bonzini To: linux-kernel@vger.kernel.org Cc: linux-scsi@vger.kernel.org, Ming Lei Subject: [PATCH 1/2] virtio-scsi: replace target spinlock with seqcount Date: Sun, 6 Jul 2014 16:39:26 +0200 Message-Id: <1404657567-17133-2-git-send-email-pbonzini@redhat.com> X-Mailer: git-send-email 1.8.3.1 In-Reply-To: <1404657567-17133-1-git-send-email-pbonzini@redhat.com> References: <1404657567-17133-1-git-send-email-pbonzini@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Ming Lei The spinlock of tgt_lock is only for serializing read and write req_vq, one lockless seqcount is enough for the purpose. On one 16core VM with vhost-scsi backend, the patch can improve IOPS with 3% on random read test. Signed-off-by: Ming Lei [Add initialization in virtscsi_target_alloc. - Paolo] Signed-off-by: Paolo Bonzini --- drivers/scsi/virtio_scsi.c | 42 +++++++++++++++++++++++++++++------------- 1 file changed, 29 insertions(+), 13 deletions(-) diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c index 308256b5e4cb..f67a872de14b 100644 --- a/drivers/scsi/virtio_scsi.c +++ b/drivers/scsi/virtio_scsi.c @@ -27,6 +27,7 @@ #include #include #include +#include #define VIRTIO_SCSI_MEMPOOL_SZ 64 #define VIRTIO_SCSI_EVENT_LEN 8 @@ -75,18 +76,16 @@ struct virtio_scsi_vq { * queue, and also lets the driver optimize the IRQ affinity for the virtqueues * (each virtqueue's affinity is set to the CPU that "owns" the queue). * - * tgt_lock is held to serialize reading and writing req_vq. Reading req_vq - * could be done locklessly, but we do not do it yet. + * tgt_seq is held to serialize reading and writing req_vq. * * Decrements of reqs are never concurrent with writes of req_vq: before the * decrement reqs will be != 0; after the decrement the virtqueue completion * routine will not use the req_vq so it can be changed by a new request. - * Thus they can happen outside the tgt_lock, provided of course we make reqs + * Thus they can happen outside the tgt_seq, provided of course we make reqs * an atomic_t. */ struct virtio_scsi_target_state { - /* This spinlock never held at the same time as vq_lock. */ - spinlock_t tgt_lock; + seqcount_t tgt_seq; /* Count of outstanding requests. */ atomic_t reqs; @@ -559,19 +558,33 @@ static struct virtio_scsi_vq *virtscsi_pick_vq(struct virtio_scsi *vscsi, unsigned long flags; u32 queue_num; - spin_lock_irqsave(&tgt->tgt_lock, flags); + local_irq_save(flags); + if (atomic_inc_return(&tgt->reqs) > 1) { + unsigned long seq; + + do { + seq = read_seqcount_begin(&tgt->tgt_seq); + vq = tgt->req_vq; + } while (read_seqcount_retry(&tgt->tgt_seq, seq)); + } else { + /* no writes can be concurrent because of atomic_t */ + write_seqcount_begin(&tgt->tgt_seq); + + /* keep previous req_vq if a reader just arrived */ + if (unlikely(atomic_read(&tgt->reqs) > 1)) { + vq = tgt->req_vq; + goto unlock; + } - if (atomic_inc_return(&tgt->reqs) > 1) - vq = tgt->req_vq; - else { queue_num = smp_processor_id(); while (unlikely(queue_num >= vscsi->num_queues)) queue_num -= vscsi->num_queues; - tgt->req_vq = vq = &vscsi->req_vqs[queue_num]; + unlock: + write_seqcount_end(&tgt->tgt_seq); } + local_irq_restore(flags); - spin_unlock_irqrestore(&tgt->tgt_lock, flags); return vq; } @@ -667,14 +680,17 @@ static int virtscsi_abort(struct scsi_cmnd *sc) static int virtscsi_target_alloc(struct scsi_target *starget) { + struct Scsi_Host *sh = dev_to_shost(starget->dev.parent); + struct virtio_scsi *vscsi = shost_priv(sh); + struct virtio_scsi_target_state *tgt = kmalloc(sizeof(*tgt), GFP_KERNEL); if (!tgt) return -ENOMEM; - spin_lock_init(&tgt->tgt_lock); + seqcount_init(&tgt->tgt_seq); atomic_set(&tgt->reqs, 0); - tgt->req_vq = NULL; + tgt->req_vq = &vscsi->req_vqs[0]; starget->hostdata = tgt; return 0; -- 1.8.3.1