• Paul Turner's avatar
    sched: Guarantee new group-entities always have weight · 9af6b695
    Paul Turner authored
    commit 0ac9b1c2 upstream.
    
    Currently, group entity load-weights are initialized to zero. This
    admits some races with respect to the first time they are re-weighted in
    earlty use. ( Let g[x] denote the se for "g" on cpu "x". )
    
    Suppose that we have root->a and that a enters a throttled state,
    immediately followed by a[0]->t1 (the only task running on cpu[0])
    blocking:
    
      put_prev_task(group_cfs_rq(a[0]), t1)
      put_prev_entity(..., t1)
      check_cfs_rq_runtime(group_cfs_rq(a[0]))
      throttle_cfs_rq(group_cfs_rq(a[0]))
    
    Then, before unthrottling occurs, let a[0]->b[0]->t2 wake for the first
    time:
    
      enqueue_task_fair(rq[0], t2)
      enqueue_entity(group_cfs_rq(b[0]), t2)
      enqueue_entity_load_avg(group_cfs_rq(b[0]), t2)
      account_entity_enqueue(group_cfs_ra(b[0]), t2)
      update_cfs_shares(group_cfs_rq(b[0]))
      < skipped because b is part of a throttled hierarchy >
      enqueue_entity(group_cfs_rq(a[0]), b[0])
      ...
    
    We now have b[0] enqueued, yet group_cfs_rq(a[0])->load.weight == 0
    which violates invariants in several code-paths. Eliminate the
    possibility of this by initializing group entity weight.
    Signed-off-by: default avatarPaul Turner <pjt@google.com>
    Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/20131016181627.22647.47543.stgit@sword-of-the-dawn.mtv.corp.google.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    Cc: Chris J Arges <chris.j.arges@canonical.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    9af6b695
fair.c 141 KB