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SubjectRe: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to handle THP spilt issue
On Tue, Mar 31, 2020 at 12:35:24PM +0200, David Hildenbrand wrote:
> On 26.03.20 10:49, Michael S. Tsirkin wrote:
> > On Thu, Mar 26, 2020 at 08:54:04AM +0100, David Hildenbrand wrote:
> >>
> >>
> >>> Am 26.03.2020 um 08:21 schrieb Michael S. Tsirkin <mst@redhat.com>:
> >>>
> >>> On Thu, Mar 12, 2020 at 09:51:25AM +0100, David Hildenbrand wrote:
> >>>>> On 12.03.20 09:47, Michael S. Tsirkin wrote:
> >>>>> On Thu, Mar 12, 2020 at 09:37:32AM +0100, David Hildenbrand wrote:
> >>>>>> 2. You are essentially stealing THPs in the guest. So the fastest
> >>>>>> mapping (THP in guest and host) is gone. The guest won't be able to make
> >>>>>> use of THP where it previously was able to. I can imagine this implies a
> >>>>>> performance degradation for some workloads. This needs a proper
> >>>>>> performance evaluation.
> >>>>>
> >>>>> I think the problem is more with the alloc_pages API.
> >>>>> That gives you exactly the given order, and if there's
> >>>>> a larger chunk available, it will split it up.
> >>>>>
> >>>>> But for balloon - I suspect lots of other users,
> >>>>> we do not want to stress the system but if a large
> >>>>> chunk is available anyway, then we could handle
> >>>>> that more optimally by getting it all in one go.
> >>>>>
> >>>>>
> >>>>> So if we want to address this, IMHO this calls for a new API.
> >>>>> Along the lines of
> >>>>>
> >>>>> struct page *alloc_page_range(gfp_t gfp, unsigned int min_order,
> >>>>> unsigned int max_order, unsigned int *order)
> >>>>>
> >>>>> the idea would then be to return at a number of pages in the given
> >>>>> range.
> >>>>>
> >>>>> What do you think? Want to try implementing that?
> >>>>
> >>>> You can just start with the highest order and decrement the order until
> >>>> your allocation succeeds using alloc_pages(), which would be enough for
> >>>> a first version. At least I don't see the immediate need for a new
> >>>> kernel API.
> >>>
> >>> OK I remember now. The problem is with reclaim. Unless reclaim is
> >>> completely disabled, any of these calls can sleep. After it wakes up,
> >>> we would like to get the larger order that has become available
> >>> meanwhile.
> >>>
> >>
> >> Yes, but that‘s a pure optimization IMHO.
> >> So I think we should do a trivial implementation first and then see what we gain from a new allocator API. Then we might also be able to justify it using real numbers.
> >>
> >
> > Well how do you propose implement the necessary semantics?
> > I think we are both agreed that alloc_page_range is more or
> > less what's necessary anyway - so how would you approximate it
> > on top of existing APIs?
>
> Looking at drivers/misc/vmw_balloon.c:vmballoon_inflate(), it first
> tries to allocate huge pages using
>
> alloc_pages(__GFP_HIGHMEM|__GFP_NOWARN| __GFP_NOMEMALLOC,
> VMW_BALLOON_2M_ORDER)
>
> And then falls back to 4k allocations (balloon_page_alloc()) in case
> allocation fails.
>
> I'm roughly thinking of something like the following, but with an
> optimized reporting interface/bigger pfn array so we can report >
> 1MB at a time. Also, it might make sense to remember the order that
> succeeded across some fill_balloon() calls.
>
> Don't even expect it to compile ...
>
>
>
> >From 4305f989672ccca4be9293e6d4167e929f3e299b Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <david@redhat.com>
> Date: Tue, 31 Mar 2020 12:28:07 +0200
> Subject: [PATCH RFC] tmp
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> drivers/virtio/virtio_balloon.c | 38 ++++++++++++++++++--------
> include/linux/balloon_compaction.h | 7 ++++-
> mm/balloon_compaction.c | 43 +++++++++++++++++++++++-------
> 3 files changed, 67 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 8511d258dbb4..0660b1b988f0 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -187,7 +187,7 @@ int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_info,
> }
>
> static void set_page_pfns(struct virtio_balloon *vb,
> - __virtio32 pfns[], struct page *page)
> + __virtio32 pfns[], struct page *page, int order)
> {
> unsigned int i;
>
> @@ -197,7 +197,7 @@ static void set_page_pfns(struct virtio_balloon *vb,
> * Set balloon pfns pointing at this page.
> * Note that the first pfn points at start of the page.
> */
> - for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++)
> + for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE * (1 << order); i++)
> pfns[i] = cpu_to_virtio32(vb->vdev,
> page_to_balloon_pfn(page) + i);
> }
> @@ -205,6 +205,7 @@ static void set_page_pfns(struct virtio_balloon *vb,
> static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
> {
> unsigned num_allocated_pages;
> + int order = MAX_ORDER - 1;
> unsigned num_pfns;
> struct page *page;
> LIST_HEAD(pages);
> @@ -212,9 +213,20 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
> /* We can only do one array worth at a time. */
> num = min(num, ARRAY_SIZE(vb->pfns));
>
> + /*
> + * Note: we will currently never allocate more than 1MB due to the
> + * pfn array size, so we will not allocate MAX_ORDER - 1 ...
> + */
> +
> for (num_pfns = 0; num_pfns < num;
> - num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
> - struct page *page = balloon_page_alloc();
> + num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE * (1 << order)) {
> + const unsigned long remaining = num - num_pfns;
> +
> + order = MIN(order,
> + get_order(remaining << VIRTIO_BALLOON_PFN_SHIFT));
> + if ((1 << order) * VIRTIO_BALLOON_PAGES_PER_PAGE > remaining)
> + order--;
> + page = balloon_pages_alloc(order);
>
> if (!page) {
> dev_info_ratelimited(&vb->vdev->dev,
> @@ -225,6 +237,8 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
> break;
> }
>
> + /* Continue with the actual order that succeeded. */
> + order = page_private(page);
> balloon_page_push(&pages, page);
> }
>
> @@ -233,14 +247,16 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
> vb->num_pfns = 0;
>
> while ((page = balloon_page_pop(&pages))) {
> + order = page_order(page);
> + /* enqueuing will split the page and clear the order */
> balloon_page_enqueue(&vb->vb_dev_info, page);
>
> - set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> - vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
> + set_page_pfns(vb, vb->pfns + vb->num_pfns, page, order);
> + vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE * (1 << order);
> if (!virtio_has_feature(vb->vdev,
> VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> - adjust_managed_page_count(page, -1);
> - vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;
> + adjust_managed_page_count(page, -1 * (1 << order));
> + vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE * (1 << order);
> }
>
> num_allocated_pages = vb->num_pfns;
> @@ -284,7 +300,7 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
> page = balloon_page_dequeue(vb_dev_info);
> if (!page)
> break;
> - set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> + set_page_pfns(vb, vb->pfns + vb->num_pfns, page, 0);
> list_add(&page->lru, &pages);
> vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
> }
> @@ -786,7 +802,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
> __count_vm_event(BALLOON_MIGRATE);
> spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
> vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> - set_page_pfns(vb, vb->pfns, newpage);
> + set_page_pfns(vb, vb->pfns, newpage, 0);
> tell_host(vb, vb->inflate_vq);
>
> /* balloon's page migration 2nd step -- deflate "page" */
> @@ -794,7 +810,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
> balloon_page_delete(page);
> spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
> vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> - set_page_pfns(vb, vb->pfns, page);
> + set_page_pfns(vb, vb->pfns, page, 0);
> tell_host(vb, vb->deflate_vq);
>
> mutex_unlock(&vb->balloon_lock);
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index 338aa27e4773..ed93fe5704d1 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -60,7 +60,7 @@ struct balloon_dev_info {
> struct inode *inode;
> };
>
> -extern struct page *balloon_page_alloc(void);
> +extern struct page *balloon_pages_alloc(int order);
> extern void balloon_page_enqueue(struct balloon_dev_info *b_dev_info,
> struct page *page);
> extern struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info);
> @@ -78,6 +78,11 @@ static inline void balloon_devinfo_init(struct balloon_dev_info *balloon)
> balloon->inode = NULL;
> }
>
> +static inline struct page *balloon_page_alloc(void)
> +{
> + return balloon_pages_alloc(0);
> +}
> +
> #ifdef CONFIG_BALLOON_COMPACTION
> extern const struct address_space_operations balloon_aops;
> extern bool balloon_page_isolate(struct page *page,
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index 26de020aae7b..067810b32813 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -112,23 +112,35 @@ size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
> EXPORT_SYMBOL_GPL(balloon_page_list_dequeue);
>
> /*
> - * balloon_page_alloc - allocates a new page for insertion into the balloon
> - * page list.
> + * balloon_pages_alloc - allocates a new page (of at most the given order)
> + * for insertion into the balloon page list.
> *
> * Driver must call this function to properly allocate a new balloon page.
> * Driver must call balloon_page_enqueue before definitively removing the page
> * from the guest system.
> *
> + * Will fall back to smaller orders if allocation fails. The order of the
> + * allocated page is stored in page->private.
> + *
> * Return: struct page for the allocated page or NULL on allocation failure.
> */
> -struct page *balloon_page_alloc(void)
> +struct page *balloon_pages_alloc(int order)
> {
> - struct page *page = alloc_page(balloon_mapping_gfp_mask() |
> - __GFP_NOMEMALLOC | __GFP_NORETRY |
> - __GFP_NOWARN);
> - return page;
> + struct page *page;
> +
> + while (order >= 0) {
> + page = alloc_pages(balloon_mapping_gfp_mask() |
> + __GFP_NOMEMALLOC | __GFP_NORETRY |
> + __GFP_NOWARN, order);
> + if (page) {
> + set_page_private(page, order);
> + return page;
> + }
> + order--;
> + }
> + return NULL;
> }
> -EXPORT_SYMBOL_GPL(balloon_page_alloc);
> +EXPORT_SYMBOL_GPL(balloon_pages_alloc);
>
> /*
> * balloon_page_enqueue - inserts a new page into the balloon page list.


I think this will try to invoke direct reclaim from the first iteration
to free up the max order.

> @@ -146,10 +158,23 @@ EXPORT_SYMBOL_GPL(balloon_page_alloc);
> void balloon_page_enqueue(struct balloon_dev_info *b_dev_info,
> struct page *page)
> {
> + const int order = page_private(page);
> unsigned long flags;
> + int i;
> +
> + /*
> + * We can only migrate single pages - and even if we could migrate
> + * bigger ones, we would want to split them on demand instead of
> + * trying to move around big chunks.
> + */
> + if (order > 0)
> + split_page(page, order);
> + set_page_private(page, order);
>
> spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> - balloon_page_enqueue_one(b_dev_info, page);
> + for (i = 0; i < (1 << order); i++)
> + balloon_page_enqueue_one(b_dev_info, page + i);
> +
> spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
> }
> EXPORT_SYMBOL_GPL(balloon_page_enqueue);
> --
> 2.25.1
>
> --
> Thanks,
>
> David / dhildenb

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