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[79.178.2.19]) by smtp.gmail.com with ESMTPSA id x6sm6089374qke.43.2020.03.11.13.22.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Mar 2020 13:22:58 -0700 (PDT) Date: Wed, 11 Mar 2020 16:22:51 -0400 From: "Michael S. Tsirkin" To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, virtualization@lists.linux-foundation.org, Jason Wang , Stephen Rothwell , Tyler Sanderson , David Rientjes , Wei Wang , Alexander Duyck , Nadav Amit , Michal Hocko , Andrew Morton Subject: Re: [PATCH v4] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM Message-ID: <20200311162218-mutt-send-email-mst@kernel.org> References: <20200311135523.18512-1-david@redhat.com> <20200311135523.18512-2-david@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200311135523.18512-2-david@redhat.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 11, 2020 at 02:55:23PM +0100, David Hildenbrand wrote: > Commit 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker") > changed the behavior when deflation happens automatically. Instead of > deflating when called by the OOM handler, the shrinker is used. > > However, the balloon is not simply some other slab cache that should be > shrunk when under memory pressure. The shrinker does not have a concept of > priorities yet, so this behavior cannot be configured. Eventually once > that is in place, we might want to switch back after doing proper > testing. > > There was a report that this results in undesired side effects when > inflating the balloon to shrink the page cache. [1] > "When inflating the balloon against page cache (i.e. no free memory > remains) vmscan.c will both shrink page cache, but also invoke the > shrinkers -- including the balloon's shrinker. So the balloon > driver allocates memory which requires reclaim, vmscan gets this > memory by shrinking the balloon, and then the driver adds the > memory back to the balloon. Basically a busy no-op." > > The name "deflate on OOM" makes it pretty clear when deflation should > happen - after other approaches to reclaim memory failed, not while > reclaiming. This allows to minimize the footprint of a guest - memory > will only be taken out of the balloon when really needed. > > Keep using the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT, because > this has no such side effects. Always register the shrinker with > VIRTIO_BALLOON_F_FREE_PAGE_HINT now. We are always allowed to reuse free > pages that are still to be processed by the guest. The hypervisor takes > care of identifying and resolving possible races between processing a > hinting request and the guest reusing a page. > > In contrast to pre commit 71994620bb25 ("virtio_balloon: replace oom > notifier with shrinker"), don't add a module parameter to configure the > number of pages to deflate on OOM. Can be re-added if really needed. > Also, pay attention that leak_balloon() returns the number of 4k pages - > convert it properly in virtio_balloon_oom_notify(). > > Testing done by Tyler for future reference: > Test setup: VM with 16 CPU, 64GB RAM. Running Debian 10. We have a 42 > GB file full of random bytes that we continually cat to /dev/null. > This fills the page cache as the file is read. Meanwhile, we trigger > the balloon to inflate, with a target size of 53 GB. This setup causes > the balloon inflation to pressure the page cache as the page cache is > also trying to grow. Afterwards we shrink the balloon back to zero (so > total deflate = total inflate). > > Without this patch (kernel 4.19.0-5): > Inflation never reaches the target until we stop the "cat file > > /dev/null" process. Total inflation time was 542 seconds. The longest > period that made no net forward progress was 315 seconds. > Result of "grep balloon /proc/vmstat" after the test: > balloon_inflate 154828377 > balloon_deflate 154828377 > > With this patch (kernel 5.6.0-rc4+): > Total inflation duration was 63 seconds. No deflate-queue activity > occurs when pressuring the page-cache. > Result of "grep balloon /proc/vmstat" after the test: > balloon_inflate 12968539 > balloon_deflate 12968539 > > Conclusion: This patch fixes the issue. In the test it reduced > inflate/deflate activity by 12x, and reduced inflation time by 8.6x. > But more importantly, if we hadn't killed the "cat file > /dev/null" > process then, without the patch, the inflation process would never reach > the target. > > [1] https://www.spinics.net/lists/linux-virtualization/msg40863.html > > Reported-by: Tyler Sanderson > Tested-by: Tyler Sanderson > Acked-by: David Rientjes > Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker") > Cc: Michael S. Tsirkin > Cc: Wei Wang > Cc: Alexander Duyck > Cc: David Rientjes > Cc: Nadav Amit > Cc: Michal Hocko > Cc: Andrew Morton > Signed-off-by: David Hildenbrand Acked-by: Michael S. Tsirkin Andrew, pls merge through your tree. > --- > drivers/virtio/virtio_balloon.c | 103 +++++++++++++++----------------- > 1 file changed, 47 insertions(+), 56 deletions(-) > > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c > index 9e0177529bad..0ef16566c3f3 100644 > --- a/drivers/virtio/virtio_balloon.c > +++ b/drivers/virtio/virtio_balloon.c > @@ -14,6 +14,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -28,7 +29,9 @@ > */ > #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT) > #define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256 > -#define VIRTBALLOON_OOM_NOTIFY_PRIORITY 80 > +/* Maximum number of (4k) pages to deflate on OOM notifications. */ > +#define VIRTIO_BALLOON_OOM_NR_PAGES 256 > +#define VIRTIO_BALLOON_OOM_NOTIFY_PRIORITY 80 > > #define VIRTIO_BALLOON_FREE_PAGE_ALLOC_FLAG (__GFP_NORETRY | __GFP_NOWARN | \ > __GFP_NOMEMALLOC) > @@ -114,9 +117,12 @@ struct virtio_balloon { > /* Memory statistics */ > struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR]; > > - /* To register a shrinker to shrink memory upon memory pressure */ > + /* Shrinker to return free pages - VIRTIO_BALLOON_F_FREE_PAGE_HINT */ > struct shrinker shrinker; > > + /* OOM notifier to deflate on OOM - VIRTIO_BALLOON_F_DEFLATE_ON_OOM */ > + struct notifier_block oom_nb; > + > /* Free page reporting device */ > struct virtqueue *reporting_vq; > struct page_reporting_dev_info pr_dev_info; > @@ -830,50 +836,13 @@ static unsigned long shrink_free_pages(struct virtio_balloon *vb, > return blocks_freed * VIRTIO_BALLOON_HINT_BLOCK_PAGES; > } > > -static unsigned long leak_balloon_pages(struct virtio_balloon *vb, > - unsigned long pages_to_free) > -{ > - return leak_balloon(vb, pages_to_free * VIRTIO_BALLOON_PAGES_PER_PAGE) / > - VIRTIO_BALLOON_PAGES_PER_PAGE; > -} > - > -static unsigned long shrink_balloon_pages(struct virtio_balloon *vb, > - unsigned long pages_to_free) > -{ > - unsigned long pages_freed = 0; > - > - /* > - * One invocation of leak_balloon can deflate at most > - * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it > - * multiple times to deflate pages till reaching pages_to_free. > - */ > - while (vb->num_pages && pages_freed < pages_to_free) > - pages_freed += leak_balloon_pages(vb, > - pages_to_free - pages_freed); > - > - update_balloon_size(vb); > - > - return pages_freed; > -} > - > static unsigned long virtio_balloon_shrinker_scan(struct shrinker *shrinker, > struct shrink_control *sc) > { > - unsigned long pages_to_free, pages_freed = 0; > struct virtio_balloon *vb = container_of(shrinker, > struct virtio_balloon, shrinker); > > - pages_to_free = sc->nr_to_scan; > - > - if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) > - pages_freed = shrink_free_pages(vb, pages_to_free); > - > - if (pages_freed >= pages_to_free) > - return pages_freed; > - > - pages_freed += shrink_balloon_pages(vb, pages_to_free - pages_freed); > - > - return pages_freed; > + return shrink_free_pages(vb, sc->nr_to_scan); > } > > static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker, > @@ -881,12 +850,22 @@ static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker, > { > struct virtio_balloon *vb = container_of(shrinker, > struct virtio_balloon, shrinker); > - unsigned long count; > > - count = vb->num_pages / VIRTIO_BALLOON_PAGES_PER_PAGE; > - count += vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES; > + return vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES; > +} > + > +static int virtio_balloon_oom_notify(struct notifier_block *nb, > + unsigned long dummy, void *parm) > +{ > + struct virtio_balloon *vb = container_of(nb, > + struct virtio_balloon, oom_nb); > + unsigned long *freed = parm; > + > + *freed += leak_balloon(vb, VIRTIO_BALLOON_OOM_NR_PAGES) / > + VIRTIO_BALLOON_PAGES_PER_PAGE; > + update_balloon_size(vb); > > - return count; > + return NOTIFY_OK; > } > > static void virtio_balloon_unregister_shrinker(struct virtio_balloon *vb) > @@ -971,7 +950,23 @@ static int virtballoon_probe(struct virtio_device *vdev) > VIRTIO_BALLOON_CMD_ID_STOP); > spin_lock_init(&vb->free_page_list_lock); > INIT_LIST_HEAD(&vb->free_page_list); > + /* > + * We're allowed to reuse any free pages, even if they are > + * still to be processed by the host. > + */ > + err = virtio_balloon_register_shrinker(vb); > + if (err) > + goto out_del_balloon_wq; > + } > + > + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) { > + vb->oom_nb.notifier_call = virtio_balloon_oom_notify; > + vb->oom_nb.priority = VIRTIO_BALLOON_OOM_NOTIFY_PRIORITY; > + err = register_oom_notifier(&vb->oom_nb); > + if (err < 0) > + goto out_unregister_shrinker; > } > + > if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON)) { > /* Start with poison val of 0 representing general init */ > __u32 poison_val = 0; > @@ -986,15 +981,6 @@ static int virtballoon_probe(struct virtio_device *vdev) > virtio_cwrite(vb->vdev, struct virtio_balloon_config, > poison_val, &poison_val); > } > - /* > - * We continue to use VIRTIO_BALLOON_F_DEFLATE_ON_OOM to decide if a > - * shrinker needs to be registered to relieve memory pressure. > - */ > - if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) { > - err = virtio_balloon_register_shrinker(vb); > - if (err) > - goto out_del_balloon_wq; > - } > > vb->pr_dev_info.report = virtballoon_free_page_report; > if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) { > @@ -1003,12 +989,12 @@ static int virtballoon_probe(struct virtio_device *vdev) > capacity = virtqueue_get_vring_size(vb->reporting_vq); > if (capacity < PAGE_REPORTING_CAPACITY) { > err = -ENOSPC; > - goto out_unregister_shrinker; > + goto out_unregister_oom; > } > > err = page_reporting_register(&vb->pr_dev_info); > if (err) > - goto out_unregister_shrinker; > + goto out_unregister_oom; > } > > virtio_device_ready(vdev); > @@ -1017,8 +1003,11 @@ static int virtballoon_probe(struct virtio_device *vdev) > virtballoon_changed(vdev); > return 0; > > -out_unregister_shrinker: > +out_unregister_oom: > if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) > + unregister_oom_notifier(&vb->oom_nb); > +out_unregister_shrinker: > + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) > virtio_balloon_unregister_shrinker(vb); > out_del_balloon_wq: > if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) > @@ -1061,6 +1050,8 @@ static void virtballoon_remove(struct virtio_device *vdev) > if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) > page_reporting_unregister(&vb->pr_dev_info); > if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) > + unregister_oom_notifier(&vb->oom_nb); > + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) > virtio_balloon_unregister_shrinker(vb); > spin_lock_irq(&vb->stop_update_lock); > vb->stop_update = true; > -- > 2.24.1