David Hildenbrand
2020-Apr-01 11:21 UTC
[RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to handle THP spilt issue
On 31.03.20 18:27, Nadav Amit wrote:>> On Mar 31, 2020, at 6:32 AM, David Hildenbrand <david at redhat.com> wrote: >> >> On 31.03.20 15:24, Michael S. Tsirkin wrote: >>> 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 at 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 at redhat.com> >>>> Date: Tue, 31 Mar 2020 12:28:07 +0200 >>>> Subject: [PATCH RFC] tmp >>>> >>>> Signed-off-by: David Hildenbrand <david at 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. >> >> %__GFP_NORETRY: The VM implementation will try only very lightweight >> memory direct reclaim to get some memory under memory pressure (thus it >> can sleep). It will avoid disruptive actions like OOM killer. >> >> Certainly good enough for a first version I would say, no? Looking at >> the vmware balloon, they don't even set __GFP_NORETRY. > > Yes, it does seem that we are missing __GFP_NORETRY. I really do not know > what I was thinking when I did not add it for huge-pages allocation. I will > send a patch. Thanks for noticing :) > > In regard to your patch, I would be happy to consolidate the allocation > mechanisms, so VMware balloon driver would also use your code. In general > your code looks good, take-away some style issues.Yeah, let's see in which direction we'll be bringing balloon_page_alloc(), I think there are still some questions to be answered (mostly performance implications). Cheers! -- Thanks, David / dhildenb