Re: [PATCH 1/2] sched/fair: Record the average duration of a task

[Raghavendra K T]

On 7/1/2024 8:27 PM, Chen Yu wrote:
> Hi Mike,
>
> On 2024-07-01 at 08:57:25 +0200, Mike Galbraith wrote:
> > On Sun, 2024-06-30 at 21:09 +0800, Chen Yu wrote:
> > > Hi Mike,
> > >
> > > Thanks for your time and giving the insights.
>
> According to a test conducted last month on a system with 500+ CPUs where 4 CPUs
> share the same L2 cache, around 20% improvement was noticed (though not as much
> as on the non-L2 shared platform). I haven't delved into the details yet, but my
> understanding is that L1 cache-to-cache latency within the L2 domain might also
> matter on large servers (which I need to investigate further).
>
> > 1:N or M:N
> > tasks can approach its wakeup frequency range, and there's nothing you can do
> > about the very same cache to cache latency you're trying to duck, it
> > just is what it is, and is considered perfectly fine as it is.  That's
> > a bit of a red flag, but worse is the lack of knowledge wrt what tasks
> > are actually up to at any given time.  We rashly presume that tasks
> > waking one another implies a 1:1 relationship, we routinely call them
> > buddies and generally get away with it.. but during any overlap they
> > can be doing anything including N way data share, and regardless of
> > what that is and section size, needless stacking flushes concurrency,
> > injecting service latency in its place, cost unknown.
>
> I believe this is a generic issue that the current scheduler faces, where
> it attempts to predict the task's behavior based on its runtime. For instance,
> task_hot() checks the task runtime to predict whether the task is cache-hot,
> regardless of what the task does during its time slice. This is also the case
> with WF_SYNC, which provides the scheduler with a hint to wake up on the current
> CPU to potentially benefit from cache locality.
>
> A thought occurred to me that one possible method to determine if the waker
> and wakee share data could be to leverage the NUMA balance's numa_group data structure.
> As numa balance periodically scans the task's VMA space and groups tasks accessing
> the same physical page into one numa_group, we can infer that if the waker and wakee
> are within the same numa_group, they are likely to share data, and it might be
> appropriate to place the wakee on top of the waker.
>
> CC Raghavendra here in case he has any insights.

Agree with your thought here,

So I imagine two possible things to explore here.

1) Use task1, task2 numa_group and check if they belong to same
numa_group, also check if there is a possibility of M:N relationship
by checking if t1/t2->numa_group->nr_tasks > 1 etc

2) Given a VMA we can use vma_numab_state pids_active[] if task1, task2
(threads) possibly interested in same VMA.
Latter one looks to be practically difficult because we don't want to
sweep across VMAs perhaps..

> thanks,
> Chenyu