From: "Andrea Parri (Microsoft)" <parri.andrea@gmail.com>
To: linux-kernel@vger.kernel.org
Cc: "K . Y . Srinivasan" <kys@microsoft.com>,
Haiyang Zhang <haiyangz@microsoft.com>,
Stephen Hemminger <sthemmin@microsoft.com>,
Wei Liu <wei.liu@kernel.org>,
linux-hyperv@vger.kernel.org,
Michael Kelley <mikelley@microsoft.com>,
Dexuan Cui <decui@microsoft.com>,
Boqun Feng <boqun.feng@gmail.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
"Andrea Parri (Microsoft)" <parri.andrea@gmail.com>
Subject: [PATCH 02/11] Drivers: hv: vmbus: Don't bind the offer&rescind works to a specific CPU
Date: Mon, 6 Apr 2020 02:15:05 +0200 [thread overview]
Message-ID: <20200406001514.19876-3-parri.andrea@gmail.com> (raw)
In-Reply-To: <20200406001514.19876-1-parri.andrea@gmail.com>
The offer and rescind works are currently scheduled on the so called
"connect CPU". However, this is not really needed: we can synchronize
the works by relying on the usage of the offer_in_progress counter and
of the channel_mutex mutex. This synchronization is already in place.
So, remove this unnecessary "bind to the connect CPU" constraint and
update the inline comments accordingly.
Suggested-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
---
drivers/hv/channel_mgmt.c | 21 ++++++++++++++++-----
drivers/hv/vmbus_drv.c | 39 ++++++++++++++++++++++++++++-----------
2 files changed, 44 insertions(+), 16 deletions(-)
diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 501c43c5851dc..aed8db59a5ff8 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -1028,11 +1028,22 @@ static void vmbus_onoffer_rescind(struct vmbus_channel_message_header *hdr)
* offer comes in first and then the rescind.
* Since we process these events in work elements,
* and with preemption, we may end up processing
- * the events out of order. Given that we handle these
- * work elements on the same CPU, this is possible only
- * in the case of preemption. In any case wait here
- * until the offer processing has moved beyond the
- * point where the channel is discoverable.
+ * the events out of order. We rely on the synchronization
+ * provided by offer_in_progress and by channel_mutex for
+ * ordering these events:
+ *
+ * { Initially: offer_in_progress = 1 }
+ *
+ * CPU1 CPU2
+ *
+ * [vmbus_process_offer()] [vmbus_onoffer_rescind()]
+ *
+ * LOCK channel_mutex WAIT_ON offer_in_progress == 0
+ * DECREMENT offer_in_progress LOCK channel_mutex
+ * INSERT chn_list SEARCH chn_list
+ * UNLOCK channel_mutex UNLOCK channel_mutex
+ *
+ * Forbids: CPU2's SEARCH from *not* seeing CPU1's INSERT
*/
while (atomic_read(&vmbus_connection.offer_in_progress) != 0) {
diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index a491d44362f9f..41ec0c95df33f 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1075,8 +1075,9 @@ void vmbus_on_msg_dpc(unsigned long data)
/*
* The host can generate a rescind message while we
* may still be handling the original offer. We deal with
- * this condition by ensuring the processing is done on the
- * same CPU.
+ * this condition by relying on the synchronization provided
+ * by offer_in_progress and by channel_mutex. See also the
+ * inline comments in vmbus_onoffer_rescind().
*/
switch (hdr->msgtype) {
case CHANNELMSG_RESCIND_CHANNELOFFER:
@@ -1098,16 +1099,34 @@ void vmbus_on_msg_dpc(unsigned long data)
* work queue: the RESCIND handler can not start to
* run before the OFFER handler finishes.
*/
- schedule_work_on(VMBUS_CONNECT_CPU,
- &ctx->work);
+ schedule_work(&ctx->work);
break;
case CHANNELMSG_OFFERCHANNEL:
+ /*
+ * The host sends the offer message of a given channel
+ * before sending the rescind message of the same
+ * channel. These messages are sent to the guest's
+ * connect CPU; the guest then starts processing them
+ * in the tasklet handler on this CPU:
+ *
+ * VMBUS_CONNECT_CPU
+ *
+ * [vmbus_on_msg_dpc()]
+ * atomic_inc() // CHANNELMSG_OFFERCHANNEL
+ * queue_work()
+ * ...
+ * [vmbus_on_msg_dpc()]
+ * schedule_work() // CHANNELMSG_RESCIND_CHANNELOFFER
+ *
+ * We rely on the memory-ordering properties of the
+ * queue_work() and schedule_work() primitives, which
+ * guarantee that the atomic increment will be visible
+ * to the CPUs which will execute the offer & rescind
+ * works by the time these works will start execution.
+ */
atomic_inc(&vmbus_connection.offer_in_progress);
- queue_work_on(VMBUS_CONNECT_CPU,
- vmbus_connection.work_queue,
- &ctx->work);
- break;
+ fallthrough;
default:
queue_work(vmbus_connection.work_queue, &ctx->work);
@@ -1151,9 +1170,7 @@ static void vmbus_force_channel_rescinded(struct vmbus_channel *channel)
INIT_WORK(&ctx->work, vmbus_onmessage_work);
- queue_work_on(VMBUS_CONNECT_CPU,
- vmbus_connection.work_queue,
- &ctx->work);
+ queue_work(vmbus_connection.work_queue, &ctx->work);
}
#endif /* CONFIG_PM_SLEEP */
--
2.24.0
next prev parent reply other threads:[~2020-04-06 0:16 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-06 0:15 [PATCH 00/11] VMBus channel interrupt reassignment Andrea Parri (Microsoft)
2020-04-06 0:15 ` [PATCH 01/11] Drivers: hv: vmbus: Always handle the VMBus messages on CPU0 Andrea Parri (Microsoft)
2020-04-10 17:18 ` Michael Kelley
2020-04-06 0:15 ` Andrea Parri (Microsoft) [this message]
2020-04-10 17:23 ` [PATCH 02/11] Drivers: hv: vmbus: Don't bind the offer&rescind works to a specific CPU Michael Kelley
2020-04-06 0:15 ` [PATCH 03/11] Drivers: hv: vmbus: Replace the per-CPU channel lists with a global array of channels Andrea Parri (Microsoft)
2020-04-10 17:29 ` Michael Kelley
2020-04-06 0:15 ` [PATCH 04/11] hv_netvsc: Disable NAPI before closing the VMBus channel Andrea Parri (Microsoft)
2020-04-10 19:20 ` Michael Kelley
2020-04-06 0:15 ` [PATCH 05/11] hv_utils: Always execute the fcopy and vss callbacks in a tasklet Andrea Parri (Microsoft)
2020-04-10 19:26 ` Michael Kelley
2020-04-06 0:15 ` [PATCH 06/11] Drivers: hv: vmbus: Use a spin lock for synchronizing channel scheduling vs. channel removal Andrea Parri (Microsoft)
2020-04-10 19:28 ` Michael Kelley
2020-04-06 0:15 ` [PATCH 07/11] PCI: hv: Prepare hv_compose_msi_msg() for the VMBus-channel-interrupt-to-vCPU reassignment functionality Andrea Parri (Microsoft)
2020-04-10 19:30 ` Michael Kelley
2020-04-06 0:15 ` [PATCH 08/11] Drivers: hv: vmbus: Remove the unused HV_LOCALIZED channel affinity logic Andrea Parri (Microsoft)
2020-04-10 19:32 ` Michael Kelley
2020-04-06 0:15 ` [PATCH 09/11] Drivers: hv: vmbus: Synchronize init_vp_index() vs. CPU hotplug Andrea Parri (Microsoft)
2020-04-10 19:33 ` Michael Kelley
2020-04-06 0:15 ` [PATCH 10/11] Drivers: hv: vmbus: Introduce the CHANNELMSG_MODIFYCHANNEL message type Andrea Parri (Microsoft)
2020-04-10 19:34 ` Michael Kelley
2020-04-06 0:15 ` [PATCH 11/11] scsi: storvsc: Re-init stor_chns when a channel interrupt is re-assigned Andrea Parri (Microsoft)
2020-04-06 19:54 ` Long Li
2020-04-07 0:35 ` Andrea Parri
2020-04-08 2:25 ` Long Li
2020-04-08 14:54 ` Andrea Parri
2020-04-10 19:35 ` Michael Kelley
2020-04-13 15:48 ` [PATCH 00/11] VMBus channel interrupt reassignment Wei Liu
2020-04-13 17:07 ` Andrea Parri
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