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    SubjectRe: [RFC PATCH 1/4] rseq: Add sched_state field to struct rseq
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    On 2023-05-23 12:32, Noah Goldstein wrote:
    > On Tue, May 23, 2023 at 7:49 AM Mathieu Desnoyers
    > <mathieu.desnoyers@efficios.com> 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@sourceware.org> 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@efficios.com>
    >>>> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
    >>>> Cc: Jonathan Corbet <corbet@lwn.net>
    >>>> Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
    >>>> Cc: Carlos O'Donell <carlos@redhat.com>
    >>>> Cc: Florian Weimer <fweimer@redhat.com>
    >>>> Cc: libc-alpha@sourceware.org
    >>>> ---
    >>>> 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.

    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.

    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

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