Hao Xiang <hao.xi...@bytedance.com> writes:
> On Wed, Feb 21, 2024 at 1:06 PM Fabiano Rosas <faro...@suse.de> wrote:
>>
>> Hao Xiang <hao.xi...@bytedance.com> writes:
>>
>> > This change adds a dedicated handler for
>> > MigrationOps::ram_save_target_page in
>>
>> nit: Add a dedicated handler...
>>
>> Usually "this patch/change" is used only when necessary to avoid
>> ambiguity.
>
> Will do.
>
>>
>> > multifd live migration. Now zero page checking can be done in the multifd
>> > threads
>> > and this becomes the default configuration. We still provide backward
>> > compatibility
>> > where zero page checking is done from the migration main thread.
>> >
>> > Signed-off-by: Hao Xiang <hao.xi...@bytedance.com>
>> > ---
>> > migration/multifd.c | 1 +
>> > migration/options.c | 2 +-
>> > migration/ram.c | 53 ++++++++++++++++++++++++++++++++++-----------
>> > 3 files changed, 42 insertions(+), 14 deletions(-)
>> >
>> > diff --git a/migration/multifd.c b/migration/multifd.c
>> > index fbb40ea10b..ef5dad1019 100644
>> > --- a/migration/multifd.c
>> > +++ b/migration/multifd.c
>> > @@ -13,6 +13,7 @@
>> > #include "qemu/osdep.h"
>> > #include "qemu/cutils.h"
>>
>> This include...
>>
>> > #include "qemu/rcu.h"
>> > +#include "qemu/cutils.h"
>>
>> is there already.
>>
>> > #include "exec/target_page.h"
>> > #include "sysemu/sysemu.h"
>> > #include "exec/ramblock.h"
>> > diff --git a/migration/options.c b/migration/options.c
>> > index 3c603391b0..3c79b6ccd4 100644
>> > --- a/migration/options.c
>> > +++ b/migration/options.c
>> > @@ -181,7 +181,7 @@ Property migration_properties[] = {
>> > MIG_MODE_NORMAL),
>> > DEFINE_PROP_ZERO_PAGE_DETECTION("zero-page-detection", MigrationState,
>> > parameters.zero_page_detection,
>> > - ZERO_PAGE_DETECTION_LEGACY),
>> > + ZERO_PAGE_DETECTION_MULTIFD),
>>
>> I think we'll need something to avoid a 9.0 -> 8.2 migration with this
>> enabled. Otherwise it will go along happily until we get data corruption
>> because the new QEMU didn't send any zero pages on the migration thread
>> and the old QEMU did not look for them in the multifd packet.
>>
>> Perhaps bumping the MULTIFD_VERSION when ZERO_PAGE_DETECTION_MULTIFD is
>> in use. We'd just need to fix the test in the new QEMU to check
>> (msg.version > MULTIFD_VERSION) instead of (msg.version != MULTIFD_VERSION).
>>
>> >
>> > /* Migration capabilities */
>> > DEFINE_PROP_MIG_CAP("x-xbzrle", MIGRATION_CAPABILITY_XBZRLE),
>> > diff --git a/migration/ram.c b/migration/ram.c
>> > index 5ece9f042e..b088c5a98c 100644
>> > --- a/migration/ram.c
>> > +++ b/migration/ram.c
>> > @@ -1123,10 +1123,6 @@ static int save_zero_page(RAMState *rs,
>> > PageSearchStatus *pss,
>> > QEMUFile *file = pss->pss_channel;
>> > int len = 0;
>> >
>> > - if (migrate_zero_page_detection() != ZERO_PAGE_DETECTION_LEGACY) {
>> > - return 0;
>> > - }
>>
>> How does 'none' work now?
>
> I tested it and all pages are transferred with payload (including the
> zero pages).
>
>>
>> > -
>> > if (!buffer_is_zero(p, TARGET_PAGE_SIZE)) {
>> > return 0;
>> > }
>> > @@ -1256,6 +1252,10 @@ static int ram_save_page(RAMState *rs,
>> > PageSearchStatus *pss)
>> >
>> > static int ram_save_multifd_page(RAMBlock *block, ram_addr_t offset)
>> > {
>> > + assert(migrate_multifd());
>> > + assert(!migrate_compress());
>> > + assert(!migration_in_postcopy());
>>
>> Drop these, please. Keep only the asserts that are likely to trigger
>> during development, such as the existing ones at multifd_send_pages.
>
> I think I have got enough feedback regarding too many asserts. I will
> drop these. assert is not compiled into the free build, correct?
>
>From include/qemu/osdep.h:
/*
* We have a lot of unaudited code that may fail in strange ways, or
* even be a security risk during migration, if you disable assertions
* at compile-time. You may comment out these safety checks if you
* absolutely want to disable assertion overhead, but it is not
* supported upstream so the risk is all yours. Meanwhile, please
* submit patches to remove any side-effects inside an assertion, or
* fixing error handling that should use Error instead of assert.
*/
#ifdef NDEBUG
#error building with NDEBUG is not supported
#endif
#ifdef G_DISABLE_ASSERT
#error building with G_DISABLE_ASSERT is not supported
#endif
>>
>> > +
>> > if (!multifd_queue_page(block, offset)) {
>> > return -1;
>> > }
>> > @@ -2046,7 +2046,6 @@ static bool save_compress_page(RAMState *rs,
>> > PageSearchStatus *pss,
>> > */
>> > static int ram_save_target_page_legacy(RAMState *rs, PageSearchStatus
>> > *pss)
>> > {
>> > - RAMBlock *block = pss->block;
>> > ram_addr_t offset = ((ram_addr_t)pss->page) << TARGET_PAGE_BITS;
>> > int res;
>> >
>> > @@ -2062,17 +2061,40 @@ static int ram_save_target_page_legacy(RAMState
>> > *rs, PageSearchStatus *pss)
>> > return 1;
>> > }
>> >
>> > + return ram_save_page(rs, pss);
>>
>> Look at where git put this! Are you using the default diff algorithm? If
>> not try using --patience to see if it improves the diff.
>
> I used the default diff algorithm.
>
>>
>> > +}
>> > +
>> > +/**
>> > + * ram_save_target_page_multifd: save one target page
>> > + *
>> > + * Returns the number of pages written
>>
>> We could be more precise here:
>>
>> ram_save_target_page_multifd: send one target page to multifd workers
>>
>> Returns 1 if the page was queued, -1 otherwise.
>
> Will do.
>
>>
>> > + *
>> > + * @rs: current RAM state
>> > + * @pss: data about the page we want to send
>> > + */
>> > +static int ram_save_target_page_multifd(RAMState *rs, PageSearchStatus
>> > *pss)
>> > +{
>> > + RAMBlock *block = pss->block;
>> > + ram_addr_t offset = ((ram_addr_t)pss->page) << TARGET_PAGE_BITS;
>> > +
>> > + /* Multifd is not compatible with old compression. */
>> > + assert(!migrate_compress());
>>
>> This should already be enforced at options.c.
>>
>> > +
>> > + /* Multifd is not compabible with postcopy. */
>> > + assert(!migration_in_postcopy());
>>
>> Same here.
>>
>> > +
>> > /*
>> > - * Do not use multifd in postcopy as one whole host page should be
>> > - * placed. Meanwhile postcopy requires atomic update of pages, so
>> > even
>> > - * if host page size == guest page size the dest guest during run may
>> > - * still see partially copied pages which is data corruption.
>> > + * Backward compatibility support. While using multifd live
>> > + * migration, we still need to handle zero page checking on the
>> > + * migration main thread.
>> > */
>> > - if (migrate_multifd() && !migration_in_postcopy()) {
>> > - return ram_save_multifd_page(block, offset);
>> > + if (migrate_zero_page_detection() == ZERO_PAGE_DETECTION_LEGACY) {
>> > + if (save_zero_page(rs, pss, offset)) {
>> > + return 1;
>> > + }
>> > }
>> >
>> > - return ram_save_page(rs, pss);
>> > + return ram_save_multifd_page(block, offset);
>> > }
>> >
>> > /* Should be called before sending a host page */
>> > @@ -2984,7 +3006,12 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>> > }
>> >
>> > migration_ops = g_malloc0(sizeof(MigrationOps));
>> > - migration_ops->ram_save_target_page = ram_save_target_page_legacy;
>> > +
>> > + if (migrate_multifd()) {
>> > + migration_ops->ram_save_target_page =
>> > ram_save_target_page_multifd;
>> > + } else {
>> > + migration_ops->ram_save_target_page = ram_save_target_page_legacy;
>> > + }
>> >
>> > bql_unlock();
>> > ret = multifd_send_sync_main();
