RE: [PATCH] migration: Move bitmap_mutex out of migration_bitmap_clear_dirty()

[Wang, Wei W]

On Thursday, July 1, 2021 4:08 AM, Peter Xu wrote:
> Taking the mutex every time for each dirty bit to clear is too slow, 
> especially we'll
> take/release even if the dirty bit is cleared.  So far it's only used to sync 
> with
> special cases with qemu_guest_free_page_hint() against migration thread,
> nothing really that serious yet.  Let's move the lock to be upper.
> 
> There're two callers of migration_bitmap_clear_dirty().
> 
> For migration, move it into ram_save_iterate().  With the help of MAX_WAIT
> logic, we'll only run ram_save_iterate() for no more than 50ms-ish time, so 
> taking
> the lock once there at the entry.  It also means any call sites to
> qemu_guest_free_page_hint() can be delayed; but it should be very rare, only
> during migration, and I don't see a problem with it.
> 
> For COLO, move it up to colo_flush_ram_cache().  I think COLO forgot to take
> that lock even when calling ramblock_sync_dirty_bitmap(), where another
> example is migration_bitmap_sync() who took it right.  So let the mutex cover
> both the
> ramblock_sync_dirty_bitmap() and migration_bitmap_clear_dirty() calls.
> 
> It's even possible to drop the lock so we use atomic operations upon rb->bmap
> and the variable migration_dirty_pages.  I didn't do it just to still be 
> safe, also
> not predictable whether the frequent atomic ops could bring overhead too e.g.
> on huge vms when it happens very often.  When that really comes, we can
> keep a local counter and periodically call atomic ops.  Keep it simple for 
> now.
> 
> Cc: Wei Wang <wei.w.wang@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Hailiang Zhang <zhang.zhanghailiang@huawei.com>
> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Cc: Juan Quintela <quintela@redhat.com>
> Cc: Leonardo Bras Soares Passos <lsoaresp@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

FWIW
Reviewed-by: Wei Wang <wei.w.wang@intel.com>

If no one could help do a regression test of free page hint, please document 
something like this in the patch:
Locking at the coarser granularity is possible to minimize the improvement 
brought by free page hints, but seems not causing critical issues.
We will let users of free page hints to report back any requirement and come up 
with a better solution later.

Best,
Wei