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Date:   Fri, 16 Oct 2020 12:03:01 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtualization@...ts.linux-foundation.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>
Subject: Re: [PATCH v1 09/29] virtio-mem: don't always trigger the workqueue
 when offlining memory

On Mon, Oct 12, 2020 at 02:53:03PM +0200, David Hildenbrand wrote:
>Let's trigger from offlining code when we're not allowed to touch online
>memory.

This describes the change in virtio_mem_memory_notifier_cb()?

>
>Handle the other case (memmap possibly freeing up another memory block)
>when actually removing memory. When removing via virtio_mem_remove(),
>virtio_mem_retry() is a NOP and safe to use.
>
>While at it, move retry handling when offlining out of
>virtio_mem_notify_offline(), to share it with Device Block Mode (DBM)
>soon.

I may not understand the logic fully. Here is my understanding of current
logic:


  virtio_mem_run_wq()
      virtio_mem_unplug_request()
          virtio_mem_mb_unplug_any_sb_offline()
	      virtio_mem_mb_remove()             --- 1
	  virtio_mem_mb_unplug_any_sb_online()
	      virtio_mem_mb_offline_and_remove() --- 2

This patch tries to trigger the wq at 1 and 2. And these two functions are
only valid during this code flow.

These two functions actually remove some memory from the system. So I am not
sure where extra unplug-able memory comes from. I guess those memory is from
memory block device and mem_sectioin, memmap? While those memory is still
marked as online, right?

In case we can gather extra memory at 1 and form a whole memory block. So that
we can unplug an online memory block (by moving data to a new place), this
just affect the process at 2. This means there is no need to trigger the wq at
1, and we can leave it at 2.

>
>Cc: "Michael S. Tsirkin" <mst@...hat.com>
>Cc: Jason Wang <jasowang@...hat.com>
>Cc: Pankaj Gupta <pankaj.gupta.linux@...il.com>
>Signed-off-by: David Hildenbrand <david@...hat.com>
>---
> drivers/virtio/virtio_mem.c | 40 ++++++++++++++++++++++++++-----------
> 1 file changed, 28 insertions(+), 12 deletions(-)
>
>diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
>index 5c93f8a65eba..8ea00f0b2ecd 100644
>--- a/drivers/virtio/virtio_mem.c
>+++ b/drivers/virtio/virtio_mem.c
>@@ -158,6 +158,7 @@ static DEFINE_MUTEX(virtio_mem_mutex);
> static LIST_HEAD(virtio_mem_devices);
> 
> static void virtio_mem_online_page_cb(struct page *page, unsigned int order);
>+static void virtio_mem_retry(struct virtio_mem *vm);
> 
> /*
>  * Register a virtio-mem device so it will be considered for the online_page
>@@ -435,9 +436,17 @@ static int virtio_mem_mb_add(struct virtio_mem *vm, unsigned long mb_id)
> static int virtio_mem_mb_remove(struct virtio_mem *vm, unsigned long mb_id)
> {
> 	const uint64_t addr = virtio_mem_mb_id_to_phys(mb_id);
>+	int rc;
> 
> 	dev_dbg(&vm->vdev->dev, "removing memory block: %lu\n", mb_id);
>-	return remove_memory(vm->nid, addr, memory_block_size_bytes());
>+	rc = remove_memory(vm->nid, addr, memory_block_size_bytes());
>+	if (!rc)
>+		/*
>+		 * We might have freed up memory we can now unplug, retry
>+		 * immediately instead of waiting.
>+		 */
>+		virtio_mem_retry(vm);
>+	return rc;
> }
> 
> /*
>@@ -452,11 +461,19 @@ static int virtio_mem_mb_offline_and_remove(struct virtio_mem *vm,
> 					    unsigned long mb_id)
> {
> 	const uint64_t addr = virtio_mem_mb_id_to_phys(mb_id);
>+	int rc;
> 
> 	dev_dbg(&vm->vdev->dev, "offlining and removing memory block: %lu\n",
> 		mb_id);
>-	return offline_and_remove_memory(vm->nid, addr,
>-					 memory_block_size_bytes());
>+	rc = offline_and_remove_memory(vm->nid, addr,
>+				       memory_block_size_bytes());
>+	if (!rc)
>+		/*
>+		 * We might have freed up memory we can now unplug, retry
>+		 * immediately instead of waiting.
>+		 */
>+		virtio_mem_retry(vm);
>+	return rc;
> }
> 
> /*
>@@ -534,15 +551,6 @@ static void virtio_mem_notify_offline(struct virtio_mem *vm,
> 		BUG();
> 		break;
> 	}
>-
>-	/*
>-	 * Trigger the workqueue, maybe we can now unplug memory. Also,
>-	 * when we offline and remove a memory block, this will re-trigger
>-	 * us immediately - which is often nice because the removal of
>-	 * the memory block (e.g., memmap) might have freed up memory
>-	 * on other memory blocks we manage.
>-	 */
>-	virtio_mem_retry(vm);
> }
> 
> static void virtio_mem_notify_online(struct virtio_mem *vm, unsigned long mb_id)
>@@ -679,6 +687,14 @@ static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
> 		break;
> 	case MEM_OFFLINE:
> 		virtio_mem_notify_offline(vm, mb_id);
>+
>+		/*
>+		 * Trigger the workqueue. Now that we have some offline memory,
>+		 * maybe we can handle pending unplug requests.
>+		 */
>+		if (!unplug_online)
>+			virtio_mem_retry(vm);
>+
> 		vm->hotplug_active = false;
> 		mutex_unlock(&vm->hotplug_mutex);
> 		break;
>-- 
>2.26.2

-- 
Wei Yang
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