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

Ankur Arora
2023-09-11T17:05:53Z

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);
}

> 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.)

How expensive would be always having PREEMPT_COUNT=y?

--
ankur