Re: [RFC PATCH 1/4] rseq: Add sched_state field to struct rseq

[Noah Goldstein]

On Tue, May 23, 2023 at 12:30 PM Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> On 2023-05-23 12:32, Noah Goldstein wrote:
> > On Tue, May 23, 2023 at 7:49 AM Mathieu Desnoyers
> > <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> >>
> >> On 2023-05-19 16:51, Noah Goldstein wrote:
> >>> On Wed, May 17, 2023 at 10:28 AM Mathieu Desnoyers via Libc-alpha
> >>> <libc-alpha@xxxxxxxxxxxxxx> wrote:
> >>>>
> >>>> Expose the "on-cpu" state for each thread through struct rseq to allow
> >>>> adaptative mutexes to decide more accurately between busy-waiting and
> >>>> calling sys_futex() to release the CPU, based on the on-cpu state of the
> >>>> mutex owner.
> >>>>
> >>>> It is only provided as an optimization hint, because there is no
> >>>> guarantee that the page containing this field is in the page cache, and
> >>>> therefore the scheduler may very well fail to clear the on-cpu state on
> >>>> preemption. This is expected to be rare though, and is resolved as soon
> >>>> as the task returns to user-space.
> >>>>
> >>>> The goal is to improve use-cases where the duration of the critical
> >>>> sections for a given lock follows a multi-modal distribution, preventing
> >>>> statistical guesses from doing a good job at choosing between busy-wait
> >>>> and futex wait behavior.
> >>>>
> >>>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> >>>> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> >>>> Cc: Jonathan Corbet <corbet@xxxxxxx>
> >>>> Cc: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> >>>> Cc: Carlos O'Donell <carlos@xxxxxxxxxx>
> >>>> Cc: Florian Weimer <fweimer@xxxxxxxxxx>
> >>>> Cc: libc-alpha@xxxxxxxxxxxxxx
> >>>> ---
> >>>>    include/linux/sched.h     | 12 ++++++++++++
> >>>>    include/uapi/linux/rseq.h | 17 +++++++++++++++++
> >>>>    kernel/rseq.c             | 14 ++++++++++++++
> >>>>    3 files changed, 43 insertions(+)
> >>>>
> >>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
> >>>> index eed5d65b8d1f..c7e9248134c1 100644
> >>>> --- a/include/linux/sched.h
> >>>> +++ b/include/linux/sched.h
> >>>> @@ -2351,11 +2351,20 @@ static inline void rseq_signal_deliver(struct ksignal *ksig,
> >>>>           rseq_handle_notify_resume(ksig, regs);
> >>>>    }
> >>>>
> >>>> +void __rseq_set_sched_state(struct task_struct *t, unsigned int state);
> >>>> +
> >>>> +static inline void rseq_set_sched_state(struct task_struct *t, unsigned int state)
> >>>> +{
> >>>> +       if (t->rseq)
> >>>> +               __rseq_set_sched_state(t, state);
> >>>> +}
> >>>> +
> >>>>    /* rseq_preempt() requires preemption to be disabled. */
> >>>>    static inline void rseq_preempt(struct task_struct *t)
> >>>>    {
> >>>>           __set_bit(RSEQ_EVENT_PREEMPT_BIT, &t->rseq_event_mask);
> >>>>           rseq_set_notify_resume(t);
> >>>> +       rseq_set_sched_state(t, 0);
> >>>
> >>> Should rseq_migrate also be made to update the cpu_id of the new core?
> >>> I imagine the usage of this will be something along the lines of:
> >>>
> >>> if(!on_cpu(mutex->owner_rseq_struct) &&
> >>>      cpu(mutex->owner_rseq_struct) == this_threads_cpu)
> >>>      // goto futex
> >>>
> >>> So I would think updating on migrate would be useful as well.
> >>
> >> I don't think we want to act differently based on the cpu on which the
> >> owner is queued.
> >>
> >> If the mutex owner is not on-cpu, and queued on the same cpu as the
> >> current thread, we indeed want to call sys_futex WAIT.
> >>
> >> If the mutex owner is not on-cpu, but queued on a different cpu than the
> >> current thread, we *still* want to call sys_futex WAIT, because
> >> busy-waiting for a thread which is queued but not currently running is
> >> wasteful.
> >>
> > I think this is less clear. In some cases sure but not always. Going
> > to the futex
> > has more latency that userland waits, and if the system is not busy (other than
> > the one process) most likely less latency that yield. Also going to the futex
> > requires a syscall on unlock.
> >
> > For example if the critical section is expected to be very small, it
> > would be easy
> > to imagine the lock be better implemented with:
> > while(is_locked)
> >    if (owner->on_cpu || owner->cpu != my_cpu)
> >      exponential backoff
> >    else
> >      yield
> >
> > Its not that "just go to futex" doesn't ever make sense, but I don't
> > think its fair
> > to say that *always* the case.
> >
> > Looking at the kernel code, it doesn't seem to be a particularly high cost to
> > keep the CPU field updated during migration so seems like a why not
> > kind of question.
>
> We already have the owner rseq_abi cpu_id field populated on every
> return-to-userspace. I wonder if it's really relevant that migration
> populates an updated value in this field immediately ? It's another case
> where this would be provided as a hint updated only if the struct rseq
> is in the page cache, because AFAIU the scheduler migration path cannot
> take a page fault.
>

Ah, thats a good point. And probably as probability the page is in the cache
goes down a fair bit as the task is idle / bounced around for longer.

> Also, if a thread bounces around many runqueues before being scheduled
> again, we would be adding those useless stores to the rseq_abi structure
> at each migration between runqueues.
>
> Given this would add some complexity to the scheduler migration code, I
> would want to see metrics/benchmarks showing that it indeed improves
> real-world use-cases before adding this to the rseq ABI.
>
> It's not only a question of added lines of code as of today, but also a
> question of added userspace ABI guarantees which can prevent future
> scheduler optimizations. I'm *very* careful about keeping those to a
> strict minimum, which I hope Peter Zijlstra appreciates.

Well, this entire thing is moreso a hint than a guarantee. Even on_cpu
is only updated if the page happens to be in the pagecache so I don't
see how you could ever be *having* to do anything.

But fair enough, thought I'd throw the idea out there, but enough valid
concerns seem to make it not such a good idea.
>
> Thanks,
>
> Mathieu
>
>
> >> Or am I missing something ?
> >>
> >> Thanks,
> >>
> >> Mathieu
> >>
> >> --
> >> Mathieu Desnoyers
> >> EfficiOS Inc.
> >> https://www.efficios.com
> >>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>