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Date:   Thu, 1 Feb 2018 21:14:25 +0200
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc:     Wei Wang <wei.w.wang@intel.com>, virtio-dev@lists.oasis-open.org,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
        linux-mm@kvack.org, mhocko@kernel.org, akpm@linux-foundation.org,
        pbonzini@redhat.com, liliang.opensource@gmail.com,
        yang.zhang.wz@gmail.com, quan.xu0@gmail.com, nilal@redhat.com,
        riel@redhat.com
Subject: Re: [PATCH v24 2/2] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT
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On Thu, Jan 25, 2018 at 08:28:52PM +0900, Tetsuo Handa wrote:
> On 2018/01/25 12:32, Wei Wang wrote:
> > On 01/25/2018 01:15 AM, Michael S. Tsirkin wrote:
> >> On Wed, Jan 24, 2018 at 06:42:42PM +0800, Wei Wang wrote:
> >> +
> >> +static void report_free_page_func(struct work_struct *work)
> >> +{
> >> +    struct virtio_balloon *vb;
> >> +    unsigned long flags;
> >> +
> >> +    vb = container_of(work, struct virtio_balloon, report_free_page_work);
> >> +
> >> +    /* Start by sending the obtained cmd id to the host with an outbuf */
> >> +    send_cmd_id(vb, &vb->start_cmd_id);
> >> +
> >> +    /*
> >> +     * Set start_cmd_id to VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID to
> >> +     * indicate a new request can be queued.
> >> +     */
> >> +    spin_lock_irqsave(&vb->stop_update_lock, flags);
> >> +    vb->start_cmd_id = cpu_to_virtio32(vb->vdev,
> >> +                VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID);
> >> +    spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> >> +
> >> +    walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
> >> Can you teach walk_free_mem_block to return the && of all
> >> return calls, so caller knows whether it completed?
> > 
> > There will be two cases that can cause walk_free_mem_block to return without completing:
> > 1) host requests to stop in advance
> > 2) vq->broken
> > 
> > How about letting walk_free_mem_block simply return the value returned by its callback (i.e. virtio_balloon_send_free_pages)?
> > 
> > For host requests to stop, it returns "1", and the above only bails out when walk_free_mem_block return a "< 0" value.
> 
> I feel that virtio_balloon_send_free_pages is doing too heavy things.
> 
> It can be called for many times with IRQ disabled. Number of times
> it is called depends on amount of free pages (and fragmentation state).
> Generally, more free pages, more calls.
> 
> Then, why don't you allocate some pages for holding all pfn values
> and then call walk_free_mem_block() only for storing pfn values
> and then send pfn values without disabling IRQ?

So going over it, at some level we are talking about optimizations here.

I'll go over it one last time but at some level we might
be able to make progress faster if we start by enabling
the functionality in question.
If there are no other issues, I'm inclined to merge.

If nothing else, this
will make it possible for people to experiment and
report sources of overhead.

-- 
MST