Re: [ISSUE] The time cost of IOSQE_IO_LINK

Peter Zijlstra
2020-02-17T12:09:26Z

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...

> 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.

And maybe remove the uring naming form this.

> #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