From: Noah Goldstein <goldstein.w.n@gmail.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Peter Zijlstra" <peterz@infradead.org>,
linux-kernel@vger.kernel.org,
"Thomas Gleixner" <tglx@linutronix.de>,
"Paul E . McKenney" <paulmck@kernel.org>,
"Boqun Feng" <boqun.feng@gmail.com>,
"H . Peter Anvin" <hpa@zytor.com>, "Paul Turner" <pjt@google.com>,
linux-api@vger.kernel.org,
"Christian Brauner" <brauner@kernel.org>,
"Florian Weimer" <fw@deneb.enyo.de>,
David.Laight@aculab.com, carlos@redhat.com,
"Peter Oskolkov" <posk@posk.io>,
"Alexander Mikhalitsyn" <alexander@mihalicyn.com>,
"Chris Kennelly" <ckennelly@google.com>,
"Ingo Molnar" <mingo@redhat.com>,
"Darren Hart" <dvhart@infradead.org>,
"Davidlohr Bueso" <dave@stgolabs.net>,
"André Almeida" <andrealmeid@igalia.com>,
libc-alpha@sourceware.org, "Steven Rostedt" <rostedt@goodmis.org>,
"Jonathan Corbet" <corbet@lwn.net>,
"Florian Weimer" <fweimer@redhat.com>
Subject: Re: [RFC PATCH 1/4] rseq: Add sched_state field to struct rseq
Date: Tue, 23 May 2023 15:10:48 -0500 [thread overview]
Message-ID: <CAFUsyfKqxiYt3+G0-wHm05nwUZQ6b_i_998YvkSnnvKmFH9=XA@mail.gmail.com> (raw)
In-Reply-To: <21f3d5e4-44ba-65de-5180-f059c145deef@efficios.com>
On Tue, May 23, 2023 at 12:30 PM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> 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.
>
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
>
next prev parent reply other threads:[~2023-05-23 20:11 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-17 15:26 [RFC PATCH 0/4] Extend rseq with sched_state field Mathieu Desnoyers
2023-05-17 15:26 ` [RFC PATCH 1/4] rseq: Add sched_state field to struct rseq Mathieu Desnoyers
2023-05-17 16:03 ` Davidlohr Bueso
2023-05-18 21:49 ` Boqun Feng
2023-05-19 14:15 ` Mathieu Desnoyers
2023-05-19 17:18 ` Boqun Feng
2023-05-23 14:10 ` Mathieu Desnoyers
2023-05-19 20:51 ` Noah Goldstein
2023-05-23 12:49 ` Mathieu Desnoyers
2023-05-23 16:32 ` Noah Goldstein
2023-05-23 17:30 ` Mathieu Desnoyers
2023-05-23 20:10 ` Noah Goldstein [this message]
2023-05-17 15:26 ` [RFC PATCH 2/4] selftests/rseq: Add sched_state rseq field and getter Mathieu Desnoyers
2023-05-17 15:26 ` [RFC PATCH 3/4] selftests/rseq: Implement sched state test program Mathieu Desnoyers
2023-05-17 15:26 ` [RFC PATCH 4/4] selftests/rseq: Implement rseq_mutex " Mathieu Desnoyers
2023-05-17 16:07 ` [RFC PATCH 0/4] Extend rseq with sched_state field Davidlohr Bueso
2023-05-17 18:36 ` Steven Rostedt
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