Re: [PATCH RFC 2/2] migration: Avoid doing RAMBlock dirty sync in the initial iteration

[Yong Huang <yong.huang@smartx.com>]

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On Wed, Oct 30, 2024 at 12:21 AM Peter Xu <peterx@redhat.com> wrote:

> On Wed, Oct 23, 2024 at 10:09:51AM +0800, yong.huang@smartx.com wrote:
> > From: Hyman Huang <yong.huang@smartx.com>
> >
> > KVM always returns 1 when userspace retrieves a dirty bitmap for
> > the first time when KVM_DIRTY_LOG_INITIALLY_SET is enabled; in such
> > scenario, the RAMBlock dirty sync of the initial iteration can be
> > skipped.
> >
> > Signed-off-by: Hyman Huang <yong.huang@smartx.com>
> > ---
> >  migration/cpu-throttle.c |  3 ++-
> >  migration/ram.c          | 30 +++++++++++++++++++++++++++---
> >  2 files changed, 29 insertions(+), 4 deletions(-)
> >
> > diff --git a/migration/cpu-throttle.c b/migration/cpu-throttle.c
> > index 342681cdd4..06e3b1be78 100644
> > --- a/migration/cpu-throttle.c
> > +++ b/migration/cpu-throttle.c
> > @@ -27,6 +27,7 @@
> >  #include "hw/core/cpu.h"
> >  #include "qemu/main-loop.h"
> >  #include "sysemu/cpus.h"
> > +#include "sysemu/kvm.h"
> >  #include "cpu-throttle.h"
> >  #include "migration.h"
> >  #include "migration-stats.h"
> > @@ -141,7 +142,7 @@ void cpu_throttle_dirty_sync_timer_tick(void *opaque)
> >       * effect on guest performance, therefore omit it to avoid
> >       * paying extra for the sync penalty.
> >       */
> > -    if (sync_cnt <= 1) {
> > +    if (sync_cnt <= (kvm_dirty_log_manual_enabled() ? 0 : 1)) {
> >          goto end;
> >      }
> >
> > diff --git a/migration/ram.c b/migration/ram.c
> > index d284f63854..b312ebd69d 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -2718,7 +2718,7 @@ static void ram_list_init_bitmaps(void)
> >  {
> >      MigrationState *ms = migrate_get_current();
> >      RAMBlock *block;
> > -    unsigned long pages;
> > +    unsigned long pages, clear_bmap_pages;
> >      uint8_t shift;
> >
> >      /* Skip setting bitmap if there is no RAM */
> > @@ -2736,6 +2736,7 @@ static void ram_list_init_bitmaps(void)
> >
> >          RAMBLOCK_FOREACH_NOT_IGNORED(block) {
> >              pages = block->max_length >> TARGET_PAGE_BITS;
> > +            clear_bmap_pages = clear_bmap_size(pages, shift);
> >              /*
> >               * The initial dirty bitmap for migration must be set with
> all
> >               * ones to make sure we'll migrate every guest RAM page to
> > @@ -2751,7 +2752,17 @@ static void ram_list_init_bitmaps(void)
> >                  block->file_bmap = bitmap_new(pages);
> >              }
> >              block->clear_bmap_shift = shift;
> > -            block->clear_bmap = bitmap_new(clear_bmap_size(pages,
> shift));
> > +            block->clear_bmap = bitmap_new(clear_bmap_pages);
> > +            /*
> > +             * Set the clear bitmap by default to enable dirty logging.
> > +             *
> > +             * Note that with KVM_DIRTY_LOG_INITIALLY_SET, dirty logging
> > +             * will be enabled gradually in small chunks using
> > +             * KVM_CLEAR_DIRTY_LOG
> > +             */
> > +            if (kvm_dirty_log_manual_enabled()) {
> > +                bitmap_set(block->clear_bmap, 0, clear_bmap_pages);
> > +            }
>
> Why it needs to be relevant to whether DIRTY_LOG is enabled?
>
> I wonder if we should always set clear_bmap to 1 unconditionally, as we
> always set bmap to all 1s by default.
>

OK, this works. We can drop it.


>
> Then we skip sync always during setup, dropping patch 1.
>

IIUC, KVM initializes the slot->dirty_bitmap with 1 only when
KVM_DIRTY_LOG_INITIALLY_SET is enabled, 0 otherwize.
This means that if KVM does not support the
KVM_DIRTY_LOG_INITIALLY_SET feature, userspace should
do the first sync so that KVM could set the WP bit and clear
the D-bit of the PTE.

Skipping first sync could handle this scenario?


> >          }
> >      }
> >  }
> > @@ -2771,6 +2782,7 @@ static void
> migration_bitmap_clear_discarded_pages(RAMState *rs)
> >
> >  static bool ram_init_bitmaps(RAMState *rs, Error **errp)
> >  {
> > +    Error *local_err = NULL;
> >      bool ret = true;
> >
> >      qemu_mutex_lock_ramlist();
> > @@ -2783,7 +2795,19 @@ static bool ram_init_bitmaps(RAMState *rs, Error
> **errp)
> >              if (!ret) {
> >                  goto out_unlock;
> >              }
> > -            migration_bitmap_sync_precopy(false);
> > +
> > +            if (kvm_dirty_log_manual_enabled()) {
> > +                /*
> > +                 * Bypass the RAMBlock dirty sync and still publish a
> > +                 * notification.
> > +                 */
> > +                if (precopy_notify(PRECOPY_NOTIFY_AFTER_BITMAP_SYNC,
> > +                            &local_err)) {
> > +                    error_report_err(local_err);
> > +                }
> > +            } else {
> > +                migration_bitmap_sync_precopy(false);
> > +            }
> >          }
> >      }
> >  out_unlock:
> > --
> > 2.27.0
> >
>
> --
> Peter Xu
>
>

-- 
Best regards

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