Re: sched/fair: scheduler not running high priority process on idle cpu

[Steven Rostedt]

On Wed, 15 Jan 2020 15:11:32 +0000
David Laight <David.Laight@xxxxxxxxxx> wrote:

> From: Steven Rostedt
> > Sent: 15 January 2020 13:19
> > On Wed, 15 Jan 2020 12:44:19 +0000
> > David Laight <David.Laight@xxxxxxxxxx> wrote:
> >   
> > > > Yes, even with CONFIG_PREEMPT, Linux has no guarantees of latency for
> > > > any task regardless of priority. If you have latency requirements, then
> > > > you need to apply the PREEMPT_RT patch (which may soon make it to
> > > > mainline this year!), which spin locks and bh wont stop a task from
> > > > scheduling (unless they need the same lock)  
> > 
> > Every time you add something to allow higher priority processes to run
> > with less latency you add overhead. By just adding that spinlock check
> > or to migrate a process to a idle cpu will add a measurable overhead,
> > and as you state, distros won't like that.
> > 
> > It's a constant game of give and take.  
> 
> I know exactly how much effect innocuous changes can have...
> 
> Sorting out process migration on a 1024 cpu NUMA system must be a PITA.
> 
> For this case an idle cpu doing a unlocked check for a processes that has
> been waiting 'ages' to preempt the running process may not be too
> expensive.

How do you measure a process waiting for ages on another CPU? And then
by the time you get the information to pull it, there's always the race
that the process will get the chance to run. And if you think about it,
by looking for a process waiting for a long time, it is likely it will
start to run because "ages" means it's probably close to being released.

> I presume the locks are in place for the migrate itself.

Note, by grabbing locks on another CPU will incur overhead on that
other CPU. I've seen huge latency caused by doing just this.

> The only downside is that the process's data is likely to be in the wrong cache,
> but unless the original cpu becomes available just after the migrate it is
> probably still a win.

If you are doing this with just tasks that are waiting for the CPU to
be preemptable, then it is most likely not a win at all.

Now, the RT tasks do have an aggressive push / pull logic, that keeps
track of which CPUs are running lower priority tasks and will work hard
to keep all RT tasks running (and aggressively migrate them). But this
logic still only takes place at preemption points (cond_resched(), etc).

-- Steve