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Message-ID: <CAKfTPtCNJUC-gNNPkEBRT5a2UVcPUHLdzUJ+-egZGQ5ihnU0Kw@mail.gmail.com>
Date: Thu, 11 Jul 2024 10:00:15 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>, linux-kernel@...r.kernel.org,
Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
"Paul E. McKenney" <paulmck@...nel.org>, Imran Khan <imran.f.khan@...cle.com>,
Leonardo Bras <leobras@...hat.com>, Guo Ren <guoren@...nel.org>, Rik van Riel <riel@...riel.com>,
Tejun Heo <tj@...nel.org>, Cruz Zhao <CruzZhao@...ux.alibaba.com>,
Lai Jiangshan <jiangshanlai@...il.com>, Joel Fernandes <joel@...lfernandes.org>,
Zqiang <qiang.zhang1211@...il.com>, Julia Lawall <julia.lawall@...ia.fr>,
"Gautham R. Shenoy" <gautham.shenoy@....com>
Subject: Re: [PATCH 2/3] sched/core: Introduce SM_IDLE and an idle re-entry
fast-path in __schedule()
On Wed, 10 Jul 2024 at 11:03, K Prateek Nayak <kprateek.nayak@....com> wrote:
>
> From: Peter Zijlstra <peterz@...radead.org>
>
> 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 significantly. Following are the numbers
> from a dual socket 3rd Generation 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
> ==================================================================
> kernel: time [pct imp]
> tip:sched/core 1.00 [baseline]
> tip:sched/core + SM_IDLE 0.25 [75.11%]
>
> [ kprateek: Commit log and testing ]
>
> Link: https://lore.kernel.org/lkml/20240615012814.GP8774@noisy.programming.kicks-ass.net/
> Not-yet-signed-off-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
> ---
> kernel/sched/core.c | 38 +++++++++++++++++++-------------------
> 1 file changed, 19 insertions(+), 19 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1e0c77eac65a..417d3ebbdf60 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6343,19 +6343,12 @@ 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.
> @@ -6396,11 +6389,12 @@ 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;
> unsigned long *switch_count;
> unsigned long prev_state;
> + bool preempt = sched_mode > 0;
> struct rq_flags rf;
> struct rq *rq;
> int cpu;
> @@ -6409,13 +6403,13 @@ 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
> @@ -6449,7 +6443,12 @@ static void __sched notrace __schedule(unsigned int sched_mode)
> * 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) {
With CONFIG_PREEMPT_RT, it was only for SM_PREEMPT but not for SM_RTLOCK_WAIT
> if (signal_pending_state(prev_state, prev)) {
> WRITE_ONCE(prev->__state, TASK_RUNNING);
> } else {
> @@ -6483,6 +6482,7 @@ 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
> @@ -6523,7 +6523,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
> migrate_disable_switch(rq, prev);
> 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);
> @@ -6599,7 +6599,7 @@ 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();
> @@ -6644,7 +6644,7 @@ void __sched schedule_idle(void)
> */
> WARN_ON_ONCE(current->__state);
> do {
> - __schedule(SM_NONE);
> + __schedule(SM_IDLE);
> } while (need_resched());
> }
>
> --
> 2.34.1
>
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