* Pavel Butsykin (pbutsy...@virtuozzo.com) wrote:
> This feature frees the migrated memory on the source during postcopy-ram
> migration. In the second step of postcopy-ram migration when the source vm
> is put on pause we can free unnecessary memory. It will allow, in particular,
> to start relaxing the memory stress on the source host in a load-balancing
> scenario.
>
> Signed-off-by: Pavel Butsykin <pbutsy...@virtuozzo.com>
Hi Pavel,
Firstly a higher-level thing, can we use a different word than 'discard'
because I already use 'discard' in postcopy to mean the request from the source
to the destination to discard pages that are redirtied. I suggest 'release-ram'
to just pick a different word that means the same thing.
Also, see patchew's build error it spotted.
> ---
> include/migration/migration.h | 1 +
> include/migration/qemu-file.h | 3 ++-
> migration/migration.c | 9 +++++++
> migration/qemu-file.c | 59
> ++++++++++++++++++++++++++++++++++++++-----
> migration/ram.c | 24 +++++++++++++++++-
> qapi-schema.json | 5 +++-
> 6 files changed, 91 insertions(+), 10 deletions(-)
>
> diff --git a/include/migration/migration.h b/include/migration/migration.h
> index c309d23370..d7bd404365 100644
> --- a/include/migration/migration.h
> +++ b/include/migration/migration.h
> @@ -294,6 +294,7 @@ void migrate_add_blocker(Error *reason);
> */
> void migrate_del_blocker(Error *reason);
>
> +bool migrate_discard_ram(void);
> bool migrate_postcopy_ram(void);
> bool migrate_zero_blocks(void);
>
> diff --git a/include/migration/qemu-file.h b/include/migration/qemu-file.h
> index abedd466c9..0cd648a733 100644
> --- a/include/migration/qemu-file.h
> +++ b/include/migration/qemu-file.h
> @@ -132,7 +132,8 @@ void qemu_put_byte(QEMUFile *f, int v);
> * put_buffer without copying the buffer.
> * The buffer should be available till it is sent asynchronously.
> */
> -void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size);
> +void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
> + bool may_free);
> bool qemu_file_mode_is_not_valid(const char *mode);
> bool qemu_file_is_writable(QEMUFile *f);
>
> diff --git a/migration/migration.c b/migration/migration.c
> index f498ab84f2..391db6f28b 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1251,6 +1251,15 @@ void qmp_migrate_set_downtime(double value, Error
> **errp)
> qmp_migrate_set_parameters(&p, errp);
> }
>
> +bool migrate_discard_ram(void)
> +{
> + MigrationState *s;
> +
> + s = migrate_get_current();
> +
> + return s->enabled_capabilities[MIGRATION_CAPABILITY_DISCARD_RAM];
> +}
> +
> bool migrate_postcopy_ram(void)
> {
> MigrationState *s;
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index e9fae31158..f85a0ecd9e 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -30,6 +30,7 @@
> #include "qemu/coroutine.h"
> #include "migration/migration.h"
> #include "migration/qemu-file.h"
> +#include "sysemu/sysemu.h"
> #include "trace.h"
>
> #define IO_BUF_SIZE 32768
> @@ -49,6 +50,7 @@ struct QEMUFile {
> int buf_size; /* 0 when writing */
> uint8_t buf[IO_BUF_SIZE];
>
> + DECLARE_BITMAP(may_free, MAX_IOV_SIZE);
> struct iovec iov[MAX_IOV_SIZE];
> unsigned int iovcnt;
>
> @@ -132,6 +134,40 @@ bool qemu_file_is_writable(QEMUFile *f)
> return f->ops->writev_buffer;
> }
>
> +static void qemu_iovec_discard_ram(QEMUFile *f)
> +{
> + struct iovec iov;
> + unsigned long idx;
> +
> + if (!migrate_discard_ram() || !runstate_check(RUN_STATE_FINISH_MIGRATE))
> {
> + return;
> + }
Can we split this out into a separate function please; so qemu_iovec_discard_ram
always does it, and then you have something that only calls it if enabled.
> + idx = find_next_bit(f->may_free, f->iovcnt, 0);
> + if (idx >= f->iovcnt) {
> + return;
> + }
> + iov = f->iov[idx];
> +
> + while ((idx = find_next_bit(f->may_free, f->iovcnt, idx + 1)) <
> f->iovcnt) {
> + /* check for adjacent buffer and coalesce them */
> + if (iov.iov_base + iov.iov_len == f->iov[idx].iov_base) {
> + iov.iov_len += f->iov[idx].iov_len;
> + continue;
> + }
> + if (qemu_madvise(iov.iov_base, iov.iov_len, QEMU_MADV_DONTNEED) < 0)
> {
> + error_report("migrate: madvise DONTNEED failed %p %ld: %s",
> + iov.iov_base, iov.iov_len, strerror(errno));
> + }
> + iov = f->iov[idx];
Can you add some comments in here please; it took me a while to understand it;
I think what you're doing is that the madvise in the loop is called for the last
iov within a continuous range and then you fall through to deal with the last
one.
Also, see my 'postcopy: enhance ram_discard_range for hugepages' - these
madvise's
get a bit more complex with hugepage.
> + }
> + if (qemu_madvise(iov.iov_base, iov.iov_len, QEMU_MADV_DONTNEED) < 0) {
> + error_report("migrate: madvise DONTNEED failed %p %ld: %s",
> + iov.iov_base, iov.iov_len, strerror(errno));
> + }
> + memset(f->may_free, 0, sizeof(f->may_free));
> +}
> +
> /**
> * Flushes QEMUFile buffer
> *
> @@ -151,6 +187,8 @@ void qemu_fflush(QEMUFile *f)
> if (f->iovcnt > 0) {
> expect = iov_size(f->iov, f->iovcnt);
> ret = f->ops->writev_buffer(f->opaque, f->iov, f->iovcnt, f->pos);
> +
> + qemu_iovec_discard_ram(f);
> }
>
> if (ret >= 0) {
> @@ -304,13 +342,19 @@ int qemu_fclose(QEMUFile *f)
> return ret;
> }
>
> -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size)
> +static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> + bool may_free)
> {
> /* check for adjacent buffer and coalesce them */
> if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
> - f->iov[f->iovcnt - 1].iov_len) {
> + f->iov[f->iovcnt - 1].iov_len &&
> + may_free == test_bit(f->iovcnt - 1, f->may_free))
> + {
> f->iov[f->iovcnt - 1].iov_len += size;
> } else {
> + if (may_free) {
> + set_bit(f->iovcnt, f->may_free);
> + }
> f->iov[f->iovcnt].iov_base = (uint8_t *)buf;
> f->iov[f->iovcnt++].iov_len = size;
> }
> @@ -320,14 +364,15 @@ static void add_to_iovec(QEMUFile *f, const uint8_t
> *buf, size_t size)
> }
> }
>
> -void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size)
> +void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
> + bool may_free)
> {
> if (f->last_error) {
> return;
> }
>
> f->bytes_xfer += size;
> - add_to_iovec(f, buf, size);
> + add_to_iovec(f, buf, size, may_free);
> }
>
> void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
> @@ -345,7 +390,7 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf,
> size_t size)
> }
> memcpy(f->buf + f->buf_index, buf, l);
> f->bytes_xfer += l;
> - add_to_iovec(f, f->buf + f->buf_index, l);
> + add_to_iovec(f, f->buf + f->buf_index, l, false);
> f->buf_index += l;
> if (f->buf_index == IO_BUF_SIZE) {
> qemu_fflush(f);
> @@ -366,7 +411,7 @@ void qemu_put_byte(QEMUFile *f, int v)
>
> f->buf[f->buf_index] = v;
> f->bytes_xfer++;
> - add_to_iovec(f, f->buf + f->buf_index, 1);
> + add_to_iovec(f, f->buf + f->buf_index, 1, false);
> f->buf_index++;
> if (f->buf_index == IO_BUF_SIZE) {
> qemu_fflush(f);
> @@ -647,7 +692,7 @@ ssize_t qemu_put_compression_data(QEMUFile *f, const
> uint8_t *p, size_t size,
> }
> qemu_put_be32(f, blen);
> if (f->ops->writev_buffer) {
> - add_to_iovec(f, f->buf + f->buf_index, blen);
> + add_to_iovec(f, f->buf + f->buf_index, blen, false);
> }
> f->buf_index += blen;
> if (f->buf_index == IO_BUF_SIZE) {
> diff --git a/migration/ram.c b/migration/ram.c
> index a1c8089010..b0322a0b5c 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -27,6 +27,7 @@
> */
> #include "qemu/osdep.h"
> #include "qemu-common.h"
> +#include "sysemu/sysemu.h"
> #include "cpu.h"
> #include <zlib.h>
> #include "qapi-event.h"
> @@ -713,6 +714,18 @@ static int save_zero_page(QEMUFile *f, RAMBlock *block,
> ram_addr_t offset,
> return pages;
> }
>
> +static void ram_discard_page(uint8_t *addr, int pages)
> +{
> + if (!migrate_discard_ram() || !runstate_check(RUN_STATE_FINISH_MIGRATE))
> {
> + return;
> + }
> +
> + if (qemu_madvise(addr, pages << TARGET_PAGE_BITS, QEMU_MADV_DONTNEED) <
> 0) {
> + error_report("migrate: madvise DONTNEED failed %p %d: %s",
> + addr, pages << TARGET_PAGE_BITS, strerror(errno));
> + }
> +}
Would it make sense to pick up my 'Fold postcopy_ram_discard_range into
ram_discard_range'
patch and then just call that wrapped with your test?
Again, this is another reason to change the word 'discard'.
> /**
> * ram_save_page: Send the given page to the stream
> *
> @@ -772,6 +785,7 @@ static int ram_save_page(QEMUFile *f, PageSearchStatus
> *pss,
> * page would be stale
> */
> xbzrle_cache_zero_page(current_addr);
> + ram_discard_page(p, pages);
> } else if (!ram_bulk_stage &&
> !migration_in_postcopy(migrate_get_current()) &&
> migrate_use_xbzrle()) {
> @@ -791,9 +805,11 @@ static int ram_save_page(QEMUFile *f, PageSearchStatus
> *pss,
> *bytes_transferred += save_page_header(f, block,
> offset | RAM_SAVE_FLAG_PAGE);
> if (send_async) {
> - qemu_put_buffer_async(f, p, TARGET_PAGE_SIZE);
> + qemu_put_buffer_async(
> + f, p, TARGET_PAGE_SIZE, migrate_discard_ram());
> } else {
> qemu_put_buffer(f, p, TARGET_PAGE_SIZE);
> + ram_discard_page(p, 1);
Does this case ever happen? Isn't the only time we hit non-async
if xbzrle is enabled, and then 'p' can be a pointer into the xbzrle
cache? (and we don't support xbzrle with postcopy).
> }
> *bytes_transferred += TARGET_PAGE_SIZE;
> pages = 1;
> @@ -821,6 +837,7 @@ static int do_compress_ram_page(QEMUFile *f, RAMBlock
> *block,
> error_report("compressed data failed!");
> } else {
> bytes_sent += blen;
> + ram_discard_page(p, 1);
> }
>
> return bytes_sent;
> @@ -959,12 +976,17 @@ static int ram_save_compressed_page(QEMUFile *f,
> PageSearchStatus *pss,
> error_report("compressed data failed!");
> }
> }
> + if (pages > 0) {
> + ram_discard_page(p, pages);
> + }
> } else {
> offset |= RAM_SAVE_FLAG_CONTINUE;
> pages = save_zero_page(f, block, offset, p, bytes_transferred);
> if (pages == -1) {
> pages = compress_page_with_multi_thread(f, block, offset,
> bytes_transferred);
> + } else {
> + ram_discard_page(p, pages);
> }
> }
Not that we support compressed pages on postcopy yet (destination code needs
writing).
> }
> diff --git a/qapi-schema.json b/qapi-schema.json
> index ce20f16757..f02b434765 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -588,11 +588,14 @@
> # side, this process is called COarse-Grain LOck Stepping (COLO) for
> # Non-stop Service. (since 2.8)
> #
> +# @discard-ram: if enabled, qemu will free the migrated ram pages on the
> source
> +# during postcopy-ram migration. (since 2.9)
> +#
> # Since: 1.2
> ##
> { 'enum': 'MigrationCapability',
> 'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
> - 'compress', 'events', 'postcopy-ram', 'x-colo'] }
> + 'compress', 'events', 'postcopy-ram', 'x-colo', 'discard-ram'] }
Interestingly, it's not directly tied to postcopy, but I guess postcopy
is the only time you get the bulk of pages sent in FINISH_MIGRATE mode;
if enabled it would trigger for the final stage of a precopy migration.
I suppose we have to worry about a case like:
postcopy is enabled
discard-ram is enabled
migration starts
migration goes into FINISH_MIGRATE very quickly in precopy
discard-ram throws source pages away.
migration fails (some screwup as it starts destination)
management tells source to carry on (after all it never actually entered
postcopy)
-> source is using some emptied ram.
The fix for that would be to tie it to postcopy.
Dave
>
> ##
> # @MigrationCapabilityStatus:
> --
> 2.11.0
>
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
