[tip: sched/core] sched/core: Introduce SM_IDLE and an idle re-entry fast-path in __schedule()
From: tip-bot2 for Peter Zijlstra
Date: Tue Sep 10 2024 - 04:14:55 EST
The following commit has been merged into the sched/core branch of tip:
Commit-ID: 3dcac251b066b60dba6d44c97d76faeb00bf19c5
Gitweb: https://git.kernel.org/tip/3dcac251b066b60dba6d44c97d76faeb00bf19c5
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Fri, 09 Aug 2024 09:22:40
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 10 Sep 2024 09:51:14 +02:00
sched/core: Introduce SM_IDLE and an idle re-entry fast-path in __schedule()
Since commit b2a02fc43a1f ("smp: Optimize
send_call_function_single_ipi()") an idle CPU in TIF_POLLING_NRFLAG mode
can be pulled out of idle by setting TIF_NEED_RESCHED flag to service an
IPI without actually sending an interrupt. Even in cases where the IPI
handler does not queue a task on the idle CPU, do_idle() will call
__schedule() since need_resched() returns true in these cases.
Introduce and use SM_IDLE to identify call to __schedule() from
schedule_idle() and shorten the idle re-entry time by skipping
pick_next_task() when nr_running is 0 and the previous task is the idle
task.
With the SM_IDLE fast-path, the time taken to complete a fixed set of
IPIs using ipistorm improves noticeably. Following are the numbers
from a dual socket Intel Ice Lake Xeon server (2 x 32C/64T) and
3rd Generation AMD EPYC system (2 x 64C/128T) (boost on, C2 disabled)
running ipistorm between CPU8 and CPU16:
cmdline: insmod ipistorm.ko numipi=100000 single=1 offset=8 cpulist=8 wait=1
==================================================================
Test : ipistorm (modified)
Units : Normalized runtime
Interpretation: Lower is better
Statistic : AMean
======================= Intel Ice Lake Xeon ======================
kernel: time [pct imp]
tip:sched/core 1.00 [baseline]
tip:sched/core + SM_IDLE 0.80 [20.51%]
==================== 3rd Generation AMD EPYC =====================
kernel: time [pct imp]
tip:sched/core 1.00 [baseline]
tip:sched/core + SM_IDLE 0.90 [10.17%]
==================================================================
[ kprateek: Commit message, SM_RTLOCK_WAIT fix ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Not-yet-signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20240809092240.6921-1-kprateek.nayak@xxxxxxx
---
kernel/sched/core.c | 45 +++++++++++++++++++++++++-------------------
1 file changed, 26 insertions(+), 19 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ffcd637..2922fac 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6410,19 +6410,12 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
* Constants for the sched_mode argument of __schedule().
*
* The mode argument allows RT enabled kernels to differentiate a
- * preemption from blocking on an 'sleeping' spin/rwlock. Note that
- * SM_MASK_PREEMPT for !RT has all bits set, which allows the compiler to
- * optimize the AND operation out and just check for zero.
+ * preemption from blocking on an 'sleeping' spin/rwlock.
*/
-#define SM_NONE 0x0
-#define SM_PREEMPT 0x1
-#define SM_RTLOCK_WAIT 0x2
-
-#ifndef CONFIG_PREEMPT_RT
-# define SM_MASK_PREEMPT (~0U)
-#else
-# define SM_MASK_PREEMPT SM_PREEMPT
-#endif
+#define SM_IDLE (-1)
+#define SM_NONE 0
+#define SM_PREEMPT 1
+#define SM_RTLOCK_WAIT 2
/*
* __schedule() is the main scheduler function.
@@ -6463,9 +6456,14 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
*
* WARNING: must be called with preemption disabled!
*/
-static void __sched notrace __schedule(unsigned int sched_mode)
+static void __sched notrace __schedule(int sched_mode)
{
struct task_struct *prev, *next;
+ /*
+ * On PREEMPT_RT kernel, SM_RTLOCK_WAIT is noted
+ * as a preemption by schedule_debug() and RCU.
+ */
+ bool preempt = sched_mode > SM_NONE;
unsigned long *switch_count;
unsigned long prev_state;
struct rq_flags rf;
@@ -6476,13 +6474,13 @@ static void __sched notrace __schedule(unsigned int sched_mode)
rq = cpu_rq(cpu);
prev = rq->curr;
- schedule_debug(prev, !!sched_mode);
+ schedule_debug(prev, preempt);
if (sched_feat(HRTICK) || sched_feat(HRTICK_DL))
hrtick_clear(rq);
local_irq_disable();
- rcu_note_context_switch(!!sched_mode);
+ rcu_note_context_switch(preempt);
/*
* Make sure that signal_pending_state()->signal_pending() below
@@ -6511,12 +6509,20 @@ static void __sched notrace __schedule(unsigned int sched_mode)
switch_count = &prev->nivcsw;
+ /* Task state changes only considers SM_PREEMPT as preemption */
+ preempt = sched_mode == SM_PREEMPT;
+
/*
* We must load prev->state once (task_struct::state is volatile), such
* that we form a control dependency vs deactivate_task() below.
*/
prev_state = READ_ONCE(prev->__state);
- if (!(sched_mode & SM_MASK_PREEMPT) && prev_state) {
+ if (sched_mode == SM_IDLE) {
+ if (!rq->nr_running) {
+ next = prev;
+ goto picked;
+ }
+ } else if (!preempt && prev_state) {
if (signal_pending_state(prev_state, prev)) {
WRITE_ONCE(prev->__state, TASK_RUNNING);
} else {
@@ -6547,6 +6553,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
}
next = pick_next_task(rq, prev, &rf);
+picked:
clear_tsk_need_resched(prev);
clear_preempt_need_resched();
#ifdef CONFIG_SCHED_DEBUG
@@ -6588,7 +6595,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
psi_account_irqtime(rq, prev, next);
psi_sched_switch(prev, next, !task_on_rq_queued(prev));
- trace_sched_switch(sched_mode & SM_MASK_PREEMPT, prev, next, prev_state);
+ trace_sched_switch(preempt, prev, next, prev_state);
/* Also unlocks the rq: */
rq = context_switch(rq, prev, next, &rf);
@@ -6664,7 +6671,7 @@ static void sched_update_worker(struct task_struct *tsk)
}
}
-static __always_inline void __schedule_loop(unsigned int sched_mode)
+static __always_inline void __schedule_loop(int sched_mode)
{
do {
preempt_disable();
@@ -6709,7 +6716,7 @@ void __sched schedule_idle(void)
*/
WARN_ON_ONCE(current->__state);
do {
- __schedule(SM_NONE);
+ __schedule(SM_IDLE);
} while (need_resched());
}