Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

Peter Zijlstra
2023-09-12T08:26:45Z

On Mon, Sep 11, 2023 at 10:04:17AM -0700, Ankur Arora wrote:
>
> Peter Zijlstra <peterz@infradead.org> writes:
>
> > On Sun, Sep 10, 2023 at 11:32:32AM -0700, Linus Torvalds wrote:
> >
> >> I was hoping that we'd have some generic way to deal with this where
> >> we could just say "this thing is reschedulable", and get rid of - or
> >> at least not increasingly add to - the cond_resched() mess.
> >
> > Isn't that called PREEMPT=y ? That tracks precisely all the constraints
> > required to know when/if we can preempt.
> >
> > The whole voluntary preempt model is basically the traditional
> > co-operative preemption model and that fully relies on manual yields.
>
> Yeah, but as Linus says, this means a lot of code is just full of
> cond_resched(). For instance a loop the process_huge_page() uses
> this pattern:
>
> for (...) {
> cond_resched();
> clear_page(i);
>
> cond_resched();
> clear_page(j);
> }

Yeah, that's what co-operative preemption gets you.

> > The problem with the REP prefix (and Xen hypercalls) is that
> > they're long running instructions and it becomes fundamentally
> > impossible to put a cond_resched() in.
> >
> >> Yes. I'm starting to think that that the only sane solution is to
> >> limit cases that can do this a lot, and the "instruciton pointer
> >> region" approach would certainly work.
> >
> > From a code locality / I-cache POV, I think a sorted list of
> > (non overlapping) ranges might be best.
>
> Yeah, agreed. There are a few problems with doing that though.
>
> I was thinking of using a check of this kind to schedule out when
> it is executing in this "reschedulable" section:
> !preempt_count() && in_resched_function(regs->rip);
>
> For preemption=full, this should mostly work.
> For preemption=voluntary, though this'll only work with out-of-line
> locks, not if the lock is inlined.
>
> (Both, should have problems with __this_cpu_* and the like, but
> maybe we can handwave that away with sparse/objtool etc.)

So one thing we can do is combine the TIF_ALLOW_RESCHED with the ranges
thing, and then only search the range when TIF flag is set.

And I'm thinking it might be a good idea to have objtool validate the
range only contains simple instructions, the moment it contains control
flow I'm thinking it's too complicated.

> How expensive would be always having PREEMPT_COUNT=y?

Effectively I think that is true today. At the very least Debian and
SuSE (I can't find a RHEL .config in a hurry but I would think they too)
ship with PREEMPT_DYNAMIC=y.

Mel, I'm sure you ran numbers at the time (you always do), what if any
was the measured overhead from PREEMPT_DYNAMIC vs 'regular' voluntary
preemption?