Re: [PATCH] sched/rt: Clean up usage of rt_task()

Phil Auld
2024-05-14T23:59:15Z

Hi Qais,

On Wed, May 15, 2024 at 12:41:12AM +0100 Qais Yousef wrote:
> rt_task() checks if a task has RT priority. But depends on your
> dictionary, this could mean it belongs to RT class, or is a 'realtime'
> task, which includes RT and DL classes.
>
> Since this has caused some confusion already on discussion [1], it
> seemed a clean up is due.
>
> I define the usage of rt_task() to be tasks that belong to RT class.
> Make sure that it returns true only for RT class and audit the users and
> replace them with the new realtime_task() which returns true for RT and
> DL classes - the old behavior. Introduce similar realtime_prio() to
> create similar distinction to rt_prio() and update the users.

I think making the difference clear is good. However, I think rt_task() is
a better name. We have dl_task() still. And rt tasks are things managed
by rt.c, basically. Not realtime.c :) I know that doesn't work for deadline.c
and dl_ but this change would be the reverse of that pattern.

>
> Move MAX_DL_PRIO to prio.h so it can be used in the new definitions.
>
> Document the functions to make it more obvious what is the difference
> between them. PI-boosted tasks is a factor that must be taken into
> account when choosing which function to use.
>
> Rename task_is_realtime() to task_has_realtime_policy() as the old name
> is confusing against the new realtime_task().

Keeping it rt_task() above could mean this stays as it was but this
change makes sense as you have written it too.

Cheers,
Phil

>
> No functional changes were intended.
>
> [1] https://lore.kernel.org/lkml/20240506100509.GL40213@noisy.programming.kicks-ass.net/
>
> Signed-off-by: Qais Yousef <qyousef@layalina.io>
> ---
> fs/select.c | 2 +-
> include/linux/ioprio.h | 2 +-
> include/linux/sched/deadline.h | 6 ++++--
> include/linux/sched/prio.h | 1 +
> include/linux/sched/rt.h | 27 ++++++++++++++++++++++++++-
> kernel/locking/rtmutex.c | 4 ++--
> kernel/locking/rwsem.c | 4 ++--
> kernel/locking/ww_mutex.h | 2 +-
> kernel/sched/core.c | 6 +++---
> kernel/time/hrtimer.c | 6 +++---
> kernel/trace/trace_sched_wakeup.c | 2 +-
> mm/page-writeback.c | 4 ++--
> mm/page_alloc.c | 2 +-
> 13 files changed, 48 insertions(+), 20 deletions(-)
>
> diff --git a/fs/select.c b/fs/select.c
> index 9515c3fa1a03..8d5c1419416c 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -82,7 +82,7 @@ u64 select_estimate_accuracy(struct timespec64 *tv)
> * Realtime tasks get a slack of 0 for obvious reasons.
> */
>
> - if (rt_task(current))
> + if (realtime_task(current))
> return 0;
>
> ktime_get_ts64(&now);
> diff --git a/include/linux/ioprio.h b/include/linux/ioprio.h
> index db1249cd9692..6c00342b6166 100644
> --- a/include/linux/ioprio.h
> +++ b/include/linux/ioprio.h
> @@ -40,7 +40,7 @@ static inline int task_nice_ioclass(struct task_struct *task)
> {
> if (task->policy == SCHED_IDLE)
> return IOPRIO_CLASS_IDLE;
> - else if (task_is_realtime(task))
> + else if (task_has_realtime_policy(task))
> return IOPRIO_CLASS_RT;
> else
> return IOPRIO_CLASS_BE;
> diff --git a/include/linux/sched/deadline.h b/include/linux/sched/deadline.h
> index df3aca89d4f5..5cb88b748ad6 100644
> --- a/include/linux/sched/deadline.h
> +++ b/include/linux/sched/deadline.h
> @@ -10,8 +10,6 @@
>
> #include <linux/sched.h>
>
> -#define MAX_DL_PRIO 0
> -
> static inline int dl_prio(int prio)
> {
> if (unlikely(prio < MAX_DL_PRIO))
> @@ -19,6 +17,10 @@ static inline int dl_prio(int prio)
> return 0;
> }
>
> +/*
> + * Returns true if a task has a priority that belongs to DL class. PI-boosted
> + * tasks will return true. Use dl_policy() to ignore PI-boosted tasks.
> + */
> static inline int dl_task(struct task_struct *p)
> {
> return dl_prio(p->prio);
> diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
> index ab83d85e1183..6ab43b4f72f9 100644
> --- a/include/linux/sched/prio.h
> +++ b/include/linux/sched/prio.h
> @@ -14,6 +14,7 @@
> */
>
> #define MAX_RT_PRIO 100
> +#define MAX_DL_PRIO 0
>
> #define MAX_PRIO (MAX_RT_PRIO + NICE_WIDTH)
> #define DEFAULT_PRIO (MAX_RT_PRIO + NICE_WIDTH / 2)
> diff --git a/include/linux/sched/rt.h b/include/linux/sched/rt.h
> index b2b9e6eb9683..b31be3c50152 100644
> --- a/include/linux/sched/rt.h
> +++ b/include/linux/sched/rt.h
> @@ -7,18 +7,43 @@
> struct task_struct;
>
> static inline int rt_prio(int prio)
> +{
> + if (unlikely(prio < MAX_RT_PRIO && prio >= MAX_DL_PRIO))
> + return 1;
> + return 0;
> +}
> +
> +static inline int realtime_prio(int prio)
> {
> if (unlikely(prio < MAX_RT_PRIO))
> return 1;
> return 0;
> }
>
> +/*
> + * Returns true if a task has a priority that belongs to RT class. PI-boosted
> + * tasks will return true. Use rt_policy() to ignore PI-boosted tasks.
> + */
> static inline int rt_task(struct task_struct *p)
> {
> return rt_prio(p->prio);
> }
>
> -static inline bool task_is_realtime(struct task_struct *tsk)
> +/*
> + * Returns true if a task has a priority that belongs to RT or DL classes.
> + * PI-boosted tasks will return true. Use task_has_realtime_policy() to ignore
> + * PI-boosted tasks.
> + */
> +static inline int realtime_task(struct task_struct *p)
> +{
> + return realtime_prio(p->prio);
> +}
> +
> +/*
> + * Returns true if a task has a policy that belongs to RT or DL classes.
> + * PI-boosted tasks will return false.
> + */
> +static inline bool task_has_realtime_policy(struct task_struct *tsk)
> {
> int policy = tsk->policy;
>
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 88d08eeb8bc0..55c9dab37f33 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -347,7 +347,7 @@ static __always_inline int __waiter_prio(struct task_struct *task)
> {
> int prio = task->prio;
>
> - if (!rt_prio(prio))
> + if (!realtime_prio(prio))
> return DEFAULT_PRIO;
>
> return prio;
> @@ -435,7 +435,7 @@ static inline bool rt_mutex_steal(struct rt_mutex_waiter *waiter,
> * Note that RT tasks are excluded from same priority (lateral)
> * steals to prevent the introduction of an unbounded latency.
> */
> - if (rt_prio(waiter->tree.prio) || dl_prio(waiter->tree.prio))
> + if (realtime_prio(waiter->tree.prio))
> return false;
>
> return rt_waiter_node_equal(&waiter->tree, &top_waiter->tree);
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index c6d17aee4209..ad8d4438bc91 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -631,7 +631,7 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
> * if it is an RT task or wait in the wait queue
> * for too long.
> */
> - if (has_handoff || (!rt_task(waiter->task) &&
> + if (has_handoff || (!realtime_task(waiter->task) &&
> !time_after(jiffies, waiter->timeout)))
> return false;
>
> @@ -914,7 +914,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
> if (owner_state != OWNER_WRITER) {
> if (need_resched())
> break;
> - if (rt_task(current) &&
> + if (realtime_task(current) &&
> (prev_owner_state != OWNER_WRITER))
> break;
> }
> diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
> index 3ad2cc4823e5..fa4b416a1f62 100644
> --- a/kernel/locking/ww_mutex.h
> +++ b/kernel/locking/ww_mutex.h
> @@ -237,7 +237,7 @@ __ww_ctx_less(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
> int a_prio = a->task->prio;
> int b_prio = b->task->prio;
>
> - if (rt_prio(a_prio) || rt_prio(b_prio)) {
> + if (realtime_prio(a_prio) || realtime_prio(b_prio)) {
>
> if (a_prio > b_prio)
> return true;
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1a914388144a..27f15de3d099 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -162,7 +162,7 @@ static inline int __task_prio(const struct task_struct *p)
> if (p->sched_class == &stop_sched_class) /* trumps deadline */
> return -2;
>
> - if (rt_prio(p->prio)) /* includes deadline */
> + if (realtime_prio(p->prio)) /* includes deadline */
> return p->prio; /* [-1, 99] */
>
> if (p->sched_class == &idle_sched_class)
> @@ -2198,7 +2198,7 @@ static int effective_prio(struct task_struct *p)
> * keep the priority unchanged. Otherwise, update priority
> * to the normal priority:
> */
> - if (!rt_prio(p->prio))
> + if (!realtime_prio(p->prio))
> return p->normal_prio;
> return p->prio;
> }
> @@ -10282,7 +10282,7 @@ void normalize_rt_tasks(void)
> schedstat_set(p->stats.sleep_start, 0);
> schedstat_set(p->stats.block_start, 0);
>
> - if (!dl_task(p) && !rt_task(p)) {
> + if (!realtime_task(p)) {
> /*
> * Renice negative nice level userspace
> * tasks back to 0:
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 70625dff62ce..4150e98847fa 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -1996,7 +1996,7 @@ static void __hrtimer_init_sleeper(struct hrtimer_sleeper *sl,
> * expiry.
> */
> if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> - if (task_is_realtime(current) && !(mode & HRTIMER_MODE_SOFT))
> + if (task_has_realtime_policy(current) && !(mode & HRTIMER_MODE_SOFT))
> mode |= HRTIMER_MODE_HARD;
> }
>
> @@ -2096,7 +2096,7 @@ long hrtimer_nanosleep(ktime_t rqtp, const enum hrtimer_mode mode,
> u64 slack;
>
> slack = current->timer_slack_ns;
> - if (rt_task(current))
> + if (realtime_task(current))
> slack = 0;
>
> hrtimer_init_sleeper_on_stack(&t, clockid, mode);
> @@ -2301,7 +2301,7 @@ schedule_hrtimeout_range_clock(ktime_t *expires, u64 delta,
> * Override any slack passed by the user if under
> * rt contraints.
> */
> - if (rt_task(current))
> + if (realtime_task(current))
> delta = 0;
>
> hrtimer_init_sleeper_on_stack(&t, clock_id, mode);
> diff --git a/kernel/trace/trace_sched_wakeup.c b/kernel/trace/trace_sched_wakeup.c
> index 0469a04a355f..19d737742e29 100644
> --- a/kernel/trace/trace_sched_wakeup.c
> +++ b/kernel/trace/trace_sched_wakeup.c
> @@ -545,7 +545,7 @@ probe_wakeup(void *ignore, struct task_struct *p)
> * - wakeup_dl handles tasks belonging to sched_dl class only.
> */
> if (tracing_dl || (wakeup_dl && !dl_task(p)) ||
> - (wakeup_rt && !dl_task(p) && !rt_task(p)) ||
> + (wakeup_rt && !realtime_task(p)) ||
> (!dl_task(p) && (p->prio >= wakeup_prio || p->prio >= current->prio)))
> return;
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3e19b87049db..7372e40f225d 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -418,7 +418,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
> if (bg_thresh >= thresh)
> bg_thresh = thresh / 2;
> tsk = current;
> - if (rt_task(tsk)) {
> + if (realtime_task(tsk)) {
> bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
> thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
> }
> @@ -468,7 +468,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
> else
> dirty = vm_dirty_ratio * node_memory / 100;
>
> - if (rt_task(tsk))
> + if (realtime_task(tsk))
> dirty += dirty / 4;
>
> return dirty;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 14d39f34d336..0af24a60ade0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3877,7 +3877,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
> */
> if (alloc_flags & ALLOC_MIN_RESERVE)
> alloc_flags &= ~ALLOC_CPUSET;
> - } else if (unlikely(rt_task(current)) && in_task())
> + } else if (unlikely(realtime_task(current)) && in_task())
> alloc_flags |= ALLOC_MIN_RESERVE;
>
> alloc_flags = gfp_to_alloc_flags_cma(gfp_mask, alloc_flags);
> --
> 2.34.1
>
>

--