Messages in this thread | | | From | Ankur Arora <> | Subject | Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED | Date | Mon, 11 Sep 2023 10:04:17 -0700 |
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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
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