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From Megha Sharma <mshar...@apple.com>
Subject Re: Review Request 58898: Do not kill non partition aware tasks.
Date Sun, 06 Aug 2017 05:40:13 GMT


> On May 10, 2017, 11:24 p.m., Neil Conway wrote:
> > My apologies for the delay in reviewing this.
> > 
> > High-level comments:
> > 
> > (a) Can we improve the description of the problem in the commit summary? It took
me quite a while to figure out what is actually going on here. My understanding is:
> > 
> > (1) Agent re-registers
> > (2) Master sends `ShutdownFrameworkMessage` for non-PA frameworks on the agent
> > (3) Master offers agent resources to framework
> > (4) Framework launches new task on agent _before the agent has finished shutting
down the framework_
> > (5) Agent ignores launch task because it believes the framework is still terminating.
> > 
> > This is a race condition, because if the agent finished shutting down the framework
(in #4) before the launch task was received, there would not be a problem. Is my understanding
correct?
> > 
> > (2) I'd prefer a new unit test that constructs exactly this scenario, rather than
changing existing unit tests.
> > 
> > (3) The new behavior is that the framework will receive `TASK_KILLED` for any non-PA
tasks running on a partitioned agent that re-registers. This does not seem ideal, because
`TASK_KILLED` _normally_ corresponds to a framework-initiated `killTask` operation.
> > 
> > (4) Thinking out loud, what if a custom executor ignores the `killTask` request?
When shutting down a framework, it seems we forcibly destroy the container (via `containerizer->destroy()`),
if the executor doesn't exit promptly upon receiving the framework shutdown request. AFAIK
we don't have similar logic for `killTask` requests.
> > 
> > 3 + 4 suggests that maybe we want a different way to terminate the task on the agent
-- let's discuss.
> 
> Megha Sharma wrote:
>     Summarizing the 2 approaches we talked about.
>     
>     Approach 1: ShutdownFrameworkMessage
>     
>     1. Upon agent re-registration, master will add tasks even for non-PA frameworks on
this agent. This is needed by the master to do correct resource accounting and not offer resources
already in use on this agent. We need to mutate the TaskState on the Task before adding them
to the master's data structures since the TaskState might be non-terminal when the agent sends
these tasks with ReregisterSlaveMessage. And the master has already sent TASK_LOST for these
tasks to the frameworks so we need to set the TaskState to TASK_LOST so that any future reconciliations
with the framework doesn't have this task transitioning from TASK_LOST to TASK_RUNNNG/TASK_STARTING.
This is to avoid unnecessary confusion about task state as observed by the framework but indeed
this could have happened with non-strict registry as well where the framework can actually
receive a non terminal task state update after receiving a TASK_LOST for the same task in
the past.
>     
>     2. When the agent re-registers, the master will continue to send a ShutdownFrameworkMessage
to the agent to kill the tasks pertaining to non-PA frameworks on the agent as it does today.
An additional optional field will be added to the ShutdownFrameworkMessage to indicate whether
or not the shutdown was initiated internally.
>     
>     3. During framework shutdown the state of the framework is set to Framework::TERMINATING
which prevents it from launching new tasks. Here, since the framework is not really terminating
so in order to allow it to launch new tasks, the agent will not set the state to terminating
if the ShutdownFrameworkMessage is generated internally.
>     
>     4. The framework shutdown today doesn't generate any status updates which needs to
change. The status updates will be sent if the framework shutdown is triggered internally,
this is needed to remove the tasks of non-PA frameworks that got added when the agent re-registered
(1).
>     
>     Approach 2: Do not shutdown non-PA framework when agent re-registers and let the
frameworks make the decision on what needs to be done when they receive non-terminal status
updates for tasks for which they have already received a TASK_LOST. This hopefully won't break
any frameworks since it could have happened in the past with non-strict registry as well and
frameworks should be resilient enough to handle this scenario.
>     
>     Let me know if I have missed anything. Personally, I am in favor of approach 1 as
it seems less catastrophic for the frameworks. What do you think?
> 
> Megha Sharma wrote:
>     The proposed solution to fix the race between new task launches and shutdown framework
on the agent, was to not kill the non-partition aware tasks when an unreachable agent re-registers
with the master. So now as far as Mesos internals are concerned, the tasks from non-partition
aware frameworks are to be treated the same way as partition aware tasks and one way to do
it cleanly in Mesos was to transition such non-partition aware tasks to TASK_UNREACHABLE state
instead of TASK_LOST when the agent becomes unreachable. Internally such tasks will be in
Framework#unreachableTasks cache instead of Framework#completedTasks and their state would
be TASK_UNREACHABLE but to be backwards compatible we will do some transformations when the
data is being exposed to the users so there is no difference in how these tasks are presented
before and after this patch. I will post the patch soon but do let me know what do you think
about the approach.

Since we are now adding the tasks from non-partition aware frameworks to unreachableTasks
cache instead of completedTasks when the agent goes unreachable so we needed to do some transformations
when the completed_tasks and unreachable_tasks metric are being presented to the user. To
achieve this we needed an additional cache nonParitionAwareTasks to remember the taskIDs for
non-partition aware tasks which have been added to unreachableTasks so they can be filtered
out when the unreachable_tasks are shown and added when we present the completed_tasks (code
snippet below).
```    
    writer->field("completed_tasks", [this](JSON::ArrayWriter* writer) {
      foreach (const Owned<Task>& task, framework_->completedTasks) {
        // Skip unauthorized tasks.
        if (!authorizeTask_->accept(*task.get(), framework_->info)) {
          continue;
        }

        writer->element(*task.get());
      }
      foreach (const TaskID& taskID, framework_->nonParitionAwareTasks) {
        if (framework_->unreachableTasks.contains(taskID)) {
          Task task =
            framework_->transformNonPartitionAwareTask(
                *framework_->unreachableTasks.at(taskID).get());
          // Skip unauthorized tasks.
          if (!authorizeTask_->accept(task, framework_->info)) {
            continue;
          }
          writer->element(task);
        }
      }
    });
```


- Megha


-----------------------------------------------------------
This is an automatically generated e-mail. To reply, visit:
https://reviews.apache.org/r/58898/#review174194
-----------------------------------------------------------


On Aug. 6, 2017, 4:35 a.m., Megha Sharma wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/58898/
> -----------------------------------------------------------
> 
> (Updated Aug. 6, 2017, 4:35 a.m.)
> 
> 
> Review request for mesos, Neil Conway and Jiang Yan Xu.
> 
> 
> Bugs: MESOS-7215
>     https://issues.apache.org/jira/browse/MESOS-7215
> 
> 
> Repository: mesos
> 
> 
> Description
> -------
> 
> Master will not kill the tasks for non-Partition aware frameworks
> when an unreachable agent re-registers with the master.
> Master used to send a ShutdownFrameworkMessages to the agent
> to kill the tasks from non partition aware frameworks including the
> ones that are still registered which was problematic because the offer
> from this agent could still go to the same framework which could then
> launch new tasks. The agent would then receive tasks of the same
> framework and ignore them because it thinks the framework is shutting
> down. The framework is not shutting down of course, so from the master
> and the scheduler’s perspective the task is pending in STAGING forever
> until the next agent reregistration, which could happen much later.
> This commit fixes the problem by not shutting down the non-partition
> aware frameworks on such an agent.
> 
> 
> Diffs
> -----
> 
>   src/master/http.cpp 959091c8ec03b6ac7bcb5d21b04d2f7d5aff7d54 
>   src/master/master.hpp b802fd153a10f6012cea381f153c28cc78cae995 
>   src/master/master.cpp 7f38a5e21884546d4b4c866ca5918db779af8f99 
>   src/tests/partition_tests.cpp 62a84f797201ccd18b71490949e3130d2b9c3668 
> 
> 
> Diff: https://reviews.apache.org/r/58898/diff/3/
> 
> 
> Testing
> -------
> 
> make check
> 
> 
> Thanks,
> 
> Megha Sharma
> 
>


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