[Xen-devel] [PATCHv13 0/2] gnttab: Improve scaleability

[David Vrabel <david.vrabel@xxxxxxxxxx>]

Performance results for aggregate intrahost network throughput
(between 20 VM pairs, with 16 dom0 VCPUs) show substantial
improvements.

                             Throughput/Gbit/s

Base                          9.7
Split locks                  25.8
Split locks                  42.9
 + per-VCPU maptrack lists

v13:
  * When adding new frames, update v->maptrack_head last.
  * Each VCPU no longer gets a reserved number of frames -- they're
    allocated first-come, first-served.
  * Use original gt->maptrack_limit (instead of gt->maptrack_pages).
  * Steal an extra entry for the sentinel if a VCPU's free list is
    unitialized.

v12:
  * Check for same refs in __gnttab_swap_grant_ref() to avoid deadlock.
  * Assert local grant table lock is held in mapcount().
  * Document that maptrack_lock is only for the free list.
  * Document locking needed for maptrack entries (the active entry
    lock).
  * Try and steal maptrack entries if a VCPU runs out (RFC).

v11:
  * Fix exists test in gnttab_map_exists().
  * Call gnttab_need_iommu_mapping() once for each lock/unlock.
  * Take active entry lock in gnttab_transfer().

  * Use read_atomic() when checking maptrack flags for validity (note:
    all other reads of map->flags are either after it is validated as
    stable or protected by the active entry lock and thus don't need
    read_atomic()).
  * Add comment to double_gt_lock().
  * Allocate maptrack array with vzalloc() since it may be >1 page.

v10:
  * Reduce scope of act in grant_map_exists().
  * Make unlock sequence in error paths consistent in
    __acquire_grant_for_copy().
  * gnt_unlock_out -> to gt_unlock_out in __acquire_grant_for_copy().
  * In gnttab_map_grant_ref(), keep double lock around maptrack update
    if gnttab_need_iommu_mapping().  Use a wmb(), otherwise.
  * Divide max_maptrack_frames evenly amongst the VCPUs.
  * Increase default max_maptrack_frames to compensate.

v9:
  * Refactor the locking patches into three commits:
      1. Add per-active entry locking.
      2. Add maptrack_lock.
      3. Make the grant table lock a read-write lock.
  * Keep the double lock around IOMMU updates, but adjust their scope
    to only the IOMMU updates.

v8:
  * Misc locking fixes.
  * Use read/write_atomic() in per-VCPU maptrack free list.
  * Improve locking docs.

v7:
  * Re-add grant table rwlock (locking was broken without it and the
    gains are minimal anyway).
  * Remove unneeded locks from grant_table_destroy().
  * Fix get_maptrack_handle() locking.
v6:
  * Remove most uses of the grant table lock.
  * Make the grant table lock a spin lock again (there were only
    writers left after the above)
  * Add per-VCPU maptrack free lists.
v5:
  * Addressed locking issue pointed out by Jan Beulich
  * Fixed git rebase merge issue introduced in v4
    (acquiring locking twice)
  * Change for ()-loop in grant_map_exists
  * Coding style fixes
v4:
  * Coding style nits from Jan Beulich
  * Fixup read locks pointed out by Jan Beulich
  * renamed double_gt_(un)lock to double_maptrack_(un)lock
    per request from Jan Beulich
  * Addressed ASSERT()'s from Jan Beulich
  * Addressed locking issues in unmap_common pointed out
    by Jan Beulich
v3:
  * Addressed gnttab_swap_grant_ref() comment from Andrew Cooper
v2:
  * Add arm part per request from Julien Grall

David


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