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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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.

Or am I missing something ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com