Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends

[peterz@infradead.org]

On Wed, Sep 23, 2020 at 11:52:51AM -0400, Steven Rostedt wrote:
> On Wed, 23 Sep 2020 10:40:32 +0200
> peterz@infradead.org wrote:
> 
> > However, with migrate_disable() we can have each task preempted in a
> > migrate_disable() region, worse we can stack them all on the _same_ CPU
> > (super ridiculous odds, sure). And then we end up only able to run one
> > task, with the rest of the CPUs picking their nose.
> 
> What if we just made migrate_disable() a local_lock() available for !RT?

Can't, neiter migrate_disable() nor migrate_enable() are allowed to
block -- which is what makes their implementation so 'interesting'.

> This should lower the SHC in theory, if you can't have stacked migrate
> disables on the same CPU.

See this email in that other thread (you're on Cc too IIRC):

  https://lkml.kernel.org/r/20200923170809.GY1362448@hirez.programming.kicks-ass.net

I think that is we 'frob' the balance PULL, we'll end up with something
similar.

Whichever way around we turn this thing, the migrate_disable() runtime
(we'll have to add a tracer for that), will be an interference term on
the lower priority task, exactly like preempt_disable() would be. We'll
just not exclude a higher priority task from running.


AFAICT; the best case is a single migrate_disable() nesting, where a
higher priority task preempts in a migrate_disable() section -- this is
per design.

When this preempted task becomes elegible to run under the ideal model
(IOW it becomes one of the M highest priority tasks), it might still
have to wait for the preemptee's migrate_disable() section to complete.
Thereby suffering interference in the exact duration of
migrate_disable() section.

Per this argument, the change from preempt_disable() to
migrate_disable() gets us:

 - higher priority tasks gain reduced wake-up latency
 - lower priority tasks are unchanged and are subject to the exact same
   interference term as if the higher priority task were using
   preempt_disable().

Since we've already established this term is unbounded, any task but the
highest priority task is basically buggered.

TL;DR, if we get balancing fixed and achieve (near) the optimal case
above, migrate_disable() is an over-all win. But it's provably
non-deterministic as long as the migrate_disable() sections are
non-deterministic.


The reason this all mostly works in practise is (I think) because:

 - People care most about the higher prio RT tasks and craft them to
   mostly avoid the migrate_disable() infected code.

 - The preemption scenario is most pronounced at higher utilization
   scenarios, and I suspect this is fairly rare to begin with.

 - And while many of these migrate_disable() regions are unbound in
   theory, in practise they're often fairly reasonable.


So my current todo list is:

 - Change RT PULL
 - Change DL PULL
 - Add migrate_disable() tracer; exactly like preempt/irqoff, except
   measuring task-runtime instead of cpu-time.
 - Add a mode that measures actual interference.
 - Add a traceevent to detect preemption in migrate_disable().


And then I suppose I should twist Daniel's arm to update his model to
include these scenarios and numbers.