Re: [ISSUE] The time cost of IOSQE_IO_LINK

[Jens Axboe <axboe@kernel.dk>]

On 2/17/20 5:09 AM, Peter Zijlstra wrote:
> On Fri, Feb 14, 2020 at 01:44:32PM -0700, Jens Axboe wrote:
> 
> I've not looked at git trees yet, but the below doesn't apply to
> anything I have at hand.
> 
> Anyway, I think I can still make sense of it -- just a rename or two
> seems to be missing.
> 
> A few notes on the below...

Thanks for continuing to look at it, while we both try and make sense of
it :-)

>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 04278493bf15..447b06c6bed0 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -685,6 +685,11 @@ struct task_struct {
>>  #endif
>>  	struct sched_dl_entity		dl;
>>  
>> +#ifdef CONFIG_IO_URING
>> +	struct list_head		uring_work;
>> +	raw_spinlock_t			uring_lock;
>> +#endif
>> +
> 
> Could we pretty please use struct callback_head for this, just like
> task_work() and RCU ? Look at task_work_add() for inspiration.

Sure, so add a new one, sched_work, and have it get this sched-in or
sched-out behavior.

Only potential hitch I see there is related to ordering, which is more
of a fairness thab correctness issue. I'm going to ignore that for now,
and we can always revisit later.

> And maybe remove the uring naming form this.

No problem

>>  #ifdef CONFIG_UCLAMP_TASK
>>  	/* Clamp values requested for a scheduling entity */
>>  	struct uclamp_se		uclamp_req[UCLAMP_CNT];
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 51ca491d99ed..170fefa1caf8 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2717,6 +2717,11 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
>>  	INIT_HLIST_HEAD(&p->preempt_notifiers);
>>  #endif
>>  
>> +#ifdef CONFIG_IO_URING
>> +	INIT_LIST_HEAD(&p->uring_work);
>> +	raw_spin_lock_init(&p->uring_lock);
>> +#endif
>> +
>>  #ifdef CONFIG_COMPACTION
>>  	p->capture_control = NULL;
>>  #endif
>> @@ -4104,6 +4109,20 @@ void __noreturn do_task_dead(void)
>>  		cpu_relax();
>>  }
>>  
>> +#ifdef CONFIG_IO_URING
>> +extern void io_uring_task_handler(struct task_struct *tsk);
>> +
>> +static inline void io_uring_handler(struct task_struct *tsk)
>> +{
>> +	if (!list_empty(&tsk->uring_work))
>> +		io_uring_task_handler(tsk);
>> +}
>> +#else /* !CONFIG_IO_URING */
>> +static inline void io_uring_handler(struct task_struct *tsk)
>> +{
>> +}
>> +#endif
>> +
>>  static void sched_out_update(struct task_struct *tsk)
>>  {
>>  	/*
>> @@ -4121,6 +4140,7 @@ static void sched_out_update(struct task_struct *tsk)
>>  			io_wq_worker_sleeping(tsk);
>>  		preempt_enable_no_resched();
>>  	}
>> +	io_uring_handler(tsk);
>>  }
>>  
>>  static void sched_in_update(struct task_struct *tsk)
>> @@ -4131,6 +4151,7 @@ static void sched_in_update(struct task_struct *tsk)
>>  		else
>>  			io_wq_worker_running(tsk);
>>  	}
>> +	io_uring_handler(tsk);
>>  }
> 
> The problem I have here is that we have an unconditional load of the
> cacheline that has ->uring_work in.
> 
> /me curses about how nobody seems interested in building useful
> cacheline analyis tools :/
> 
> Lemme see if I can find a spot for this... perhaps something like so?
> 
> 
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 0918904c939d..4fba93293fa1 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -649,6 +649,7 @@ struct task_struct {
>  	/* Per task flags (PF_*), defined further below: */
>  	unsigned int			flags;
>  	unsigned int			ptrace;
> +	int				on_rq;
>  
>  #ifdef CONFIG_SMP
>  	struct llist_node		wake_entry;
> @@ -671,14 +672,16 @@ struct task_struct {
>  	int				recent_used_cpu;
>  	int				wake_cpu;
>  #endif
> -	int				on_rq;
>  
>  	int				prio;
>  	int				static_prio;
>  	int				normal_prio;
>  	unsigned int			rt_priority;
>  
> +	struct callbach_head		*sched_work;
> +
>  	const struct sched_class	*sched_class;
> +
>  	struct sched_entity		se;
>  	struct sched_rt_entity		rt;
>  #ifdef CONFIG_CGROUP_SCHED

Thanks, I'll kick off the series with doing it based on this instead.

-- 
Jens Axboe