• Xuewei Zhang's avatar
    sched/fair: Scale bandwidth quota and period without losing quota/period ratio precision · 4929a4e6
    Xuewei Zhang authored
    The quota/period ratio is used to ensure a child task group won't get
    more bandwidth than the parent task group, and is calculated as:
    
      normalized_cfs_quota() = [(quota_us << 20) / period_us]
    
    If the quota/period ratio was changed during this scaling due to
    precision loss, it will cause inconsistency between parent and child
    task groups.
    
    See below example:
    
    A userspace container manager (kubelet) does three operations:
    
     1) Create a parent cgroup, set quota to 1,000us and period to 10,000us.
     2) Create a few children cgroups.
     3) Set quota to 1,000us and period to 10,000us on a child cgroup.
    
    These operations are expected to succeed. However, if the scaling of
    147/128 happens before step 3, quota and period of the parent cgroup
    will be changed:
    
      new_quota: 1148437ns,   1148us
     new_period: 11484375ns, 11484us
    
    And when step 3 comes in, the ratio of the child cgroup will be
    104857, which will be larger than the parent cgroup ratio (104821),
    and will fail.
    
    Scaling them by a factor of 2 will fix the problem.
    Tested-by: default avatarPhil Auld <pauld@redhat.com>
    Signed-off-by: default avatarXuewei Zhang <xueweiz@google.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Acked-by: default avatarPhil Auld <pauld@redhat.com>
    Cc: Anton Blanchard <anton@ozlabs.org>
    Cc: Ben Segall <bsegall@google.com>
    Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
    Cc: Juri Lelli <juri.lelli@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Vincent Guittot <vincent.guittot@linaro.org>
    Fixes: 2e8e1922 ("sched/fair: Limit sched_cfs_period_timer() loop to avoid hard lockup")
    Link: https://lkml.kernel.org/r/20191004001243.140897-1-xueweiz@google.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    4929a4e6
fair.c 279 KB