commit ed4fb6d7ef68111bb539283561953e5c6e9a6e38 upstream.

The timerslack_ns setting is used to specify how much the hardware timers should be delayed, to potentially dispatch multiple timers in a single interrupt. This is a performance optimization. Timers of realtime tasks (having a realtime scheduling policy) should not be delayed.

This logic was inconsitently applied to the hrtimers, leading to delays of realtime tasks which used timed waits for events (e.g. condition variables). Due to the downstream override of the slack for rt tasks, the procfs reported incorrect (non-zero) timerslack_ns values.

This is changed by setting the timer_slack_ns task attribute to 0 for all tasks with a rt policy. By that, downstream users do not need to specially handle rt tasks (w.r.t. the slack), and the procfs entry shows the correct value of "0". Setting non-zero slack values (either via procfs or PR_SET_TIMERSLACK) on tasks with a rt policy is ignored, as stated in "man 2 PR_SET_TIMERSLACK":

Timer slack is not applied to threads that are scheduled under a real-time scheduling policy (see sched_setscheduler(2)).

The special handling of timerslack on rt tasks in downstream users is removed as well.

Signed-off-by: Felix Moessbauer felix.moessbauer@siemens.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Link: https://lore.kernel.org/all/20240814121032.368444-2-felix.moessbauer@siemens... --- fs/proc/base.c | 9 +++++---- fs/select.c | 11 ++++------- kernel/sched/core.c | 8 ++++++++ kernel/sys.c | 2 ++ kernel/time/hrtimer.c | 18 +++--------------- 5 files changed, 22 insertions(+), 26 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c index 699f085d4de7d..91fe20b7657c0 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -2633,10 +2633,11 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf, }

task_lock(p); - if (slack_ns == 0) - p->timer_slack_ns = p->default_timer_slack_ns; - else - p->timer_slack_ns = slack_ns; + if (task_is_realtime(p)) + slack_ns = 0; + else if (slack_ns == 0) + slack_ns = p->default_timer_slack_ns; + p->timer_slack_ns = slack_ns; task_unlock(p);

out: diff --git a/fs/select.c b/fs/select.c index 3f730b8581f65..e66b6189845ea 100644 --- a/fs/select.c +++ b/fs/select.c @@ -77,19 +77,16 @@ u64 select_estimate_accuracy(struct timespec64 *tv) { u64 ret; struct timespec64 now; + u64 slack = current->timer_slack_ns;

- /* - * Realtime tasks get a slack of 0 for obvious reasons. - */ - - if (rt_task(current)) + if (slack == 0) return 0;

ktime_get_ts64(&now); now = timespec64_sub(*tv, now); ret = __estimate_accuracy(&now); - if (ret < current->timer_slack_ns) - return current->timer_slack_ns; + if (ret < slack) + return slack; return ret; }

diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 784a4f8409453..3d6dc03e4a7d3 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7530,6 +7530,14 @@ static void __setscheduler_params(struct task_struct *p, else if (fair_policy(policy)) p->static_prio = NICE_TO_PRIO(attr->sched_nice);

+ /* rt-policy tasks do not have a timerslack */ + if (task_is_realtime(p)) { + p->timer_slack_ns = 0; + } else if (p->timer_slack_ns == 0) { + /* when switching back to non-rt policy, restore timerslack */ + p->timer_slack_ns = p->default_timer_slack_ns; + } + /* * __sched_setscheduler() ensures attr->sched_priority == 0 when * !rt_policy. Always setting this ensures that things like diff --git a/kernel/sys.c b/kernel/sys.c index 44b5759903332..355de0b65c235 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2535,6 +2535,8 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, error = current->timer_slack_ns; break; case PR_SET_TIMERSLACK: + if (task_is_realtime(current)) + break; if (arg2 <= 0) current->timer_slack_ns = current->default_timer_slack_ns; diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c index e99b1305e1a5f..5db6912b8f6e1 100644 --- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c @@ -2093,14 +2093,9 @@ long hrtimer_nanosleep(ktime_t rqtp, const enum hrtimer_mode mode, struct restart_block *restart; struct hrtimer_sleeper t; int ret = 0; - u64 slack; - - slack = current->timer_slack_ns; - if (rt_task(current)) - slack = 0;

hrtimer_init_sleeper_on_stack(&t, clockid, mode); - hrtimer_set_expires_range_ns(&t.timer, rqtp, slack); + hrtimer_set_expires_range_ns(&t.timer, rqtp, current->timer_slack_ns); ret = do_nanosleep(&t, mode); if (ret != -ERESTART_RESTARTBLOCK) goto out; @@ -2281,7 +2276,7 @@ void __init hrtimers_init(void) /** * schedule_hrtimeout_range_clock - sleep until timeout * @expires: timeout value (ktime_t) - * @delta: slack in expires timeout (ktime_t) for SCHED_OTHER tasks + * @delta: slack in expires timeout (ktime_t) * @mode: timer mode * @clock_id: timer clock to be used */ @@ -2308,13 +2303,6 @@ schedule_hrtimeout_range_clock(ktime_t *expires, u64 delta, return -EINTR; }

- /* - * Override any slack passed by the user if under - * rt contraints. - */ - if (rt_task(current)) - delta = 0; - hrtimer_init_sleeper_on_stack(&t, clock_id, mode); hrtimer_set_expires_range_ns(&t.timer, *expires, delta); hrtimer_sleeper_start_expires(&t, mode); @@ -2334,7 +2322,7 @@ EXPORT_SYMBOL_GPL(schedule_hrtimeout_range_clock); /** * schedule_hrtimeout_range - sleep until timeout * @expires: timeout value (ktime_t) - * @delta: slack in expires timeout (ktime_t) for SCHED_OTHER tasks + * @delta: slack in expires timeout (ktime_t) * @mode: timer mode * * Make the current task sleep until the given expiry time has