Re: [RFC PATCH v2] sched_pair_cpu: Introduce scheduler task pairing system call

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jun 26, 2020, at 11:16 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Jun 25, 2020, at 12:34 PM, Mathieu Desnoyers
> mathieu.desnoyers@efficios.com wrote:
> 
>> ----- On Jun 25, 2020, at 10:56 AM, Mathieu Desnoyers
>> mathieu.desnoyers@efficios.com wrote:
>> 
>>> ----- On Jun 24, 2020, at 3:50 PM, Peter Zijlstra peterz@infradead.org wrote:
>>> 
>>>> On Wed, Jun 24, 2020 at 02:31:33PM -0400, Mathieu Desnoyers wrote:
>>>> 
>>> [...]
>>>> The other alternative is using a preempt_notifier for the worker I
>>>> suppose.
>>> 
>> [...]
>>>> 
>>>> preempt_notifier could work here too I suppose, install it on yourself
>>>> when you do the pear syscall and take it away again when you're finished
>>>> with it.
>> 
>> The issue I currently have with preempt notifiers is that I need to
>> send an IPI from a sched_out notifier, which has interrupts off and
>> hold the rq lock. smp_call_function_single() warns due to irq off, and
>> indeed it triggers deadlocks.
>> 
>> Before using preempt notifiers, I was touching the "prev" task after
>> irqs were reenabled and rq lock was released, which allowed me to
>> send an IPI from that context.
>> 
>> Any thoughts on how to best solve this ?
> 
> I think I may have found a way out of this: I may not need to use
> smp_call_function_single() at all.
> 
> When preempting a paired task, I think we can rely on memory barrier at the
> beginning of scheduling of the paired task to match the memory barrier at
> the end of scheduling of the kworker thread to provide memory ordering.
> Therefore,
> the IPI is not needed at all in this case.
> 
> When preempting the kworker thread, things are a bit trickier. AFAIU I can
> simply
> queue task work on the paired task directly without an IPI, and then use
> kick_process() on the paired task.
> 
> The remaining concern is whether kick_process() (and thus smp_send_reschedule())
> is sufficient to guarantee a memory barrier before smp_send_reschedule returns ?
> I suspect not, because it only raises the IPI, and does not appear to wait for
> its handler to complete. In that case I need a release on the paired task and
> an acquire in sched_out of the kworker. The memory barrier at the end of
> schedule
> fulfills the acquire, but I don't see how the acquire is done on the paired
> task,
> because execution of its scheduler does not necessarily happen immediately when
> the IPI is raised.

Hrm, smp_cond_load_acquire(&running_task->on_cpu, !VAL); is probably all I need
after that kick_process.

Thanks,

Mathieu

> 
> Thanks,
> 
> Mathieu
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com