Re: [PATCH 08/10] posix-cpu-timers: Migrate to use new tick dependency mask model

[Chris Metcalf <cmetcalf@ezchip.com>]

On 07/29/2015 08:44 PM, Frederic Weisbecker wrote:
> On Wed, Jul 29, 2015 at 01:24:16PM -0400, Chris Metcalf wrote:
>> On 07/29/2015 09:23 AM, Frederic Weisbecker wrote:
>>>> At a higher level, is the posix-cpu-timers code here really providing the
>>>>> right semantics? It seems like before, the code was checking a struct
>>>>> task-specific state, and now you are setting a global state such that if ANY
>>>>> task anywhere in the system (even on housekeeping cores) has a pending posix
>>>>> cpu timer, then nothing can go into nohz_full mode.
>>>>>
>>>>> Perhaps what is needed is a task_struct->tick_dependency to go along with
>>>>> the system-wide and per-cpu flag words?
>>> That's an excellent point! Indeed the tick dependency check on posix-cpu-timers
>>> was made on task granularity before and now it's a global dependency.
>>>
>>> Which means that if any task in the system has a posix-cpu-timer enqueued, it
>>> prevents all CPUs from shutting down the tick. I need to mention that in the
>>> changelog.
>>>
>>> Now here is the rationale: I expect that nohz full users are not interested in
>>> posix cpu timers at all. The only chance for one to run without breaking the
>>> isolation is on housekeeping CPUs. So perhaps there is a corner case somewhere
>>> but I assume there isn't until somebody reports an issue.
>>>
>>> Keeping a task level dependency check means that we need to update it on context
>>> switch. Plus it's not only about task but also process. So that means two
>>> states to update on context switch and to check from interrupts. I don't think
>>> it's worth the effort if there is no user at all.
>> I really worry about this!  The vision EZchip offers our customers is
>> that they can run whatever they want on the slow path housekeeping
>> cores, i.e. random control-plane code.  Then, on the fast-path cores,
>> they run their nohz_full stuff without interruption.  Often they don't
>> even know what the hell is running on their control plane cores - SNMP
>> or random third-party crap or god knows what.  And there is a decent
>> likelihood that some posix cpu timer code might sneak in.
> I see. But note that installing a posix cpu timer ends up triggering an
> IPI to all nohz full CPUs. That's how nohz full has always behaved.
> So users running posix timers on nohz should already suffer issues anyway.

True now, yes, I'm just looking ahead to doing better when we have
a chance to improve things.

>> You mentioned needing two fields, for task and for process, but in
>> fact let's just add the one field to the one thing that needs it and
>> not worry about additional possible future needs.  And note that it's
>> the task_struct->signal where we need to add the field for posix cpu
>> timers (the signal_struct) since that's where the sharing occurs, and
>> given CLONE_SIGHAND I imagine it could be different from the general
>> "process" model anyway.
> Well, posix cpu timers can be install per process (signal struct) or
> per thread (task struct).
>
> But we can certainly simplify that with a per process flag and expand
> the thread dependency to the process scope.
>
> Still there is the issue of telling the CPUs where a process runs when
> a posix timer is installed there. There is no process-like tsk->cpus_allowed.
> Either we send an IPI everywhere like we do now or we iterate through all
> threads in the process to OR all their cpumasks in order to send that IPI.

Is there a reason the actual timer can't run on a housekeeping
core?  Then when it does wake_up_process() or whatever, the
specific target task will get an IPI to wake up at that point.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com