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Re: [PATCH v2 1/5] virtio-mem: Probe THP size to determine default block
From: |
Pankaj Gupta |
Subject: |
Re: [PATCH v2 1/5] virtio-mem: Probe THP size to determine default block size |
Date: |
Tue, 29 Sep 2020 15:54:53 +0200 |
> Let's allow a minimum block size of 1 MiB in all configurations. Select
> the default block size based on
> - The page size of the memory backend.
> - The THP size if the memory backend size corresponds to the real hsot
s/hsot/host
> page size.
> - The global minimum of 1 MiB.
> and warn if something smaller is configured by the user.
>
> VIRTIO_MEM only supports Linux (depends on LINUX), so we can probe the
> THP size unconditionally.
>
> For now we only support virtio-mem on x86-64 - there isn't a user-visiable
s/visiable/visible
> change (x86-64 only supports 2 MiB THP on the PMD level) - the default
> was, and will be 2 MiB.
>
> If we ever have THP on the PUD level (e.g., 1 GiB THP on x86-64), we
> expect to have a trigger to explicitly opt-in for the new THP granularity.
>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Wei Yang <richardw.yang@linux.intel.com>
> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> hw/virtio/virtio-mem.c | 105 +++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 101 insertions(+), 4 deletions(-)
>
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index 8fbec77ccc..9b1461cf9d 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/hw/virtio/virtio-mem.c
> @@ -33,10 +33,83 @@
> #include "trace.h"
>
> /*
> - * Use QEMU_VMALLOC_ALIGN, so no THP will have to be split when unplugging
> - * memory (e.g., 2MB on x86_64).
> + * Let's not allow blocks smaller than 1 MiB, for example, to keep the
> tracking
> + * bitmap small.
> */
> -#define VIRTIO_MEM_MIN_BLOCK_SIZE ((uint32_t)QEMU_VMALLOC_ALIGN)
> +#define VIRTIO_MEM_MIN_BLOCK_SIZE ((uint32_t)(1 * MiB))
> +
> +#if defined(__x86_64__) || defined(__arm__) || defined(__aarch64__) || \
> + defined(__powerpc64__)
> +#define VIRTIO_MEM_DEFAULT_THP_SIZE ((uint32_t)(2 * MiB))
> +#else
> + /* fallback to 1 MiB (e.g., the THP size on s390x) */
> +#define VIRTIO_MEM_DEFAULT_THP_SIZE VIRTIO_MEM_MIN_BLOCK_SIZE
> +#endif
> +
> +/*
> + * We want to have a reasonable default block size such that
> + * 1. We avoid splitting THPs when unplugging memory, which degrades
> + * performance.
> + * 2. We avoid placing THPs for plugged blocks that also cover unplugged
> + * blocks.
> + *
> + * The actual THP size might differ between Linux kernels, so we try to probe
> + * it. In the future (if we ever run into issues regarding 2.), we might want
> + * to disable THP in case we fail to properly probe the THP size, or if the
> + * block size is configured smaller than the THP size.
> + */
> +static uint32_t thp_size;
> +
> +#define HPAGE_PMD_SIZE_PATH
> "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
> +static uint32_t virtio_mem_thp_size(void)
> +{
> + gchar *content = NULL;
> + const char *endptr;
> + uint64_t tmp;
> +
> + if (thp_size) {
> + return thp_size;
> + }
> +
> + /*
> + * Try to probe the actual THP size, fallback to (sane but eventually
> + * incorrect) default sizes.
> + */
> + if (g_file_get_contents(HPAGE_PMD_SIZE_PATH, &content, NULL, NULL) &&
> + !qemu_strtou64(content, &endptr, 0, &tmp) &&
> + (!endptr || *endptr == '\n')) {
> + /*
> + * Sanity-check the value, if it's too big (e.g., aarch64 with 64k
> base
> + * pages) or weird, fallback to something smaller.
> + */
> + if (!tmp || !is_power_of_2(tmp) || tmp > 16 * MiB) {
> + warn_report("Read unsupported THP size: %" PRIx64, tmp);
> + } else {
> + thp_size = tmp;
> + }
> + }
> +
> + if (!thp_size) {
> + thp_size = VIRTIO_MEM_DEFAULT_THP_SIZE;
> + warn_report("Could not detect THP size, falling back to %" PRIx64
> + " MiB.", thp_size / MiB);
> + }
> +
> + g_free(content);
> + return thp_size;
> +}
> +
> +static uint64_t virtio_mem_default_block_size(RAMBlock *rb)
> +{
> + const uint64_t page_size = qemu_ram_pagesize(rb);
> +
> + /* We can have hugetlbfs with a page size smaller than the THP size. */
> + if (page_size == qemu_real_host_page_size) {
> + return MAX(page_size, virtio_mem_thp_size());
> + }
This condition is special, can think of hugetlbfs smaller in size than THP size
configured.
> + return MAX(page_size, VIRTIO_MEM_MIN_BLOCK_SIZE);
Do we still need this? Or we can have only one return for both the cases?
Probably, I am missing something here.
> +}
> +
> /*
> * Size the usable region bigger than the requested size if possible. Esp.
> * Linux guests will only add (aligned) memory blocks in case they fully
> @@ -437,10 +510,23 @@ static void virtio_mem_device_realize(DeviceState *dev,
> Error **errp)
> rb = vmem->memdev->mr.ram_block;
> page_size = qemu_ram_pagesize(rb);
>
> + /*
> + * If the block size wasn't configured by the user, use a sane default.
> This
> + * allows using hugetlbfs backends of any page size without manual
> + * intervention.
> + */
> + if (!vmem->block_size) {
> + vmem->block_size = virtio_mem_default_block_size(rb);
> + }
> +
> if (vmem->block_size < page_size) {
> error_setg(errp, "'%s' property has to be at least the page size
> (0x%"
> PRIx64 ")", VIRTIO_MEM_BLOCK_SIZE_PROP, page_size);
> return;
> + } else if (vmem->block_size < virtio_mem_default_block_size(rb)) {
> + warn_report("'%s' property is smaller than the default block size (%"
> + PRIx64 " MiB)", VIRTIO_MEM_BLOCK_SIZE_PROP,
> + virtio_mem_default_block_size(rb) / MiB);
> } else if (!QEMU_IS_ALIGNED(vmem->requested_size, vmem->block_size)) {
> error_setg(errp, "'%s' property has to be multiples of '%s' (0x%"
> PRIx64
> ")", VIRTIO_MEM_REQUESTED_SIZE_PROP,
> @@ -731,6 +817,18 @@ static void virtio_mem_get_block_size(Object *obj,
> Visitor *v, const char *name,
> const VirtIOMEM *vmem = VIRTIO_MEM(obj);
> uint64_t value = vmem->block_size;
>
> + /*
> + * If not configured by the user (and we're not realized yet), use the
> + * default block size we would use with the current memory backend.
> + */
> + if (!value) {
> + if (vmem->memdev && memory_region_is_ram(&vmem->memdev->mr)) {
> + value =
> virtio_mem_default_block_size(vmem->memdev->mr.ram_block);
> + } else {
> + value = virtio_mem_thp_size();
> + }
> + }
> +
> visit_type_size(v, name, &value, errp);
> }
>
> @@ -810,7 +908,6 @@ static void virtio_mem_instance_init(Object *obj)
> {
> VirtIOMEM *vmem = VIRTIO_MEM(obj);
>
> - vmem->block_size = VIRTIO_MEM_MIN_BLOCK_SIZE;
> notifier_list_init(&vmem->size_change_notifiers);
> vmem->precopy_notifier.notify = virtio_mem_precopy_notify;
- [PATCH v2 0/5] virtio-mem: block size and address-assignment optimizations, David Hildenbrand, 2020/09/28
- [PATCH v2 2/5] virtio-mem: Check that "memaddr" is multiples of the block size, David Hildenbrand, 2020/09/28
- [PATCH v2 3/5] memory-device: Support big alignment requirements, David Hildenbrand, 2020/09/28
- [PATCH v2 4/5] memory-device: Add get_min_alignment() callback, David Hildenbrand, 2020/09/28
- [PATCH v2 5/5] virito-mem: Implement get_min_alignment(), David Hildenbrand, 2020/09/28