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Date: Thu, 5 Mar 2020 09:24:30 -0800
From: Kees Cook <keescook@...omium.org>
To: cl@...k-chips.com
Cc: heiko@...ech.de, mingo@...hat.com, peterz@...radead.org,
juri.lelli@...hat.com, vincent.guittot@...aro.org,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, akpm@...ux-foundation.org, tglx@...utronix.de,
mpe@...erman.id.au, surenb@...gle.com, ben.dooks@...ethink.co.uk,
anshuman.khandual@....com, catalin.marinas@....com,
will@...nel.org, luto@...capital.net, wad@...omium.org,
mark.rutland@....com, geert+renesas@...der.be,
george_davis@...tor.com, sudeep.holla@....com,
linux@...linux.org.uk, gregkh@...uxfoundation.org, info@...ux.net,
kstewart@...uxfoundation.org, allison@...utok.net,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
huangtao@...k-chips.com
Subject: Re: [PATCH v2 1/1] sched/fair: do not preempt current task if it is
going to call schedule()
On Thu, Mar 05, 2020 at 05:59:48PM +0800, cl@...k-chips.com wrote:
> From: Liang Chen <cl@...k-chips.com>
>
> when we create a kthread with ktrhead_create_on_cpu(),the child thread
> entry is ktread.c:ktrhead() which will be preempted by the parent after
> call complete(done) while schedule() is not called yet,then the parent
> will call wait_task_inactive(child) but the child is still on the runqueue,
> so the parent will schedule_hrtimeout() for 1 jiffy,it will waste a lot of
> time,especially on startup.
>
> parent child
> ktrhead_create_on_cpu()
> wait_fo_completion(&done) -----> ktread.c:ktrhead()
> |----- complete(done);--wakeup and preempted by parent
> kthread_bind() <------------| |-> schedule();--dequeue here
> wait_task_inactive(child) |
> schedule_hrtimeout(1 jiffy) -|
>
> So we hope the child just wakeup parent but not preempted by parent, and the
> child is going to call schedule() soon,then the parent will not call
> schedule_hrtimeout(1 jiffy) as the child is already dequeue.
>
> The same issue for ktrhead_park()&&kthread_parkme().
> This patch can save 120ms on rk312x startup with CONFIG_HZ=300.
>
> Signed-off-by: Liang Chen <cl@...k-chips.com>
I'm not familiar with the subtleties of scheduler internals
(e.g. is there a race between the end of "schedule();" and calling
"task_clear_going_to_sched();" that effects the preemption test logic?),
so I'll leave that review to the others. But speaking to the PFA change,
it looks sane to me:
Reviewed-by: Kees Cook <keescook@...omium.org>
-Kees
> ---
> include/linux/sched.h | 5 +++++
> kernel/kthread.c | 4 ++++
> kernel/sched/fair.c | 13 +++++++++++++
> 3 files changed, 22 insertions(+)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 04278493bf15..54bf336f5790 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1533,6 +1533,7 @@ static inline bool is_percpu_thread(void)
> #define PFA_SPEC_IB_DISABLE 5 /* Indirect branch speculation restricted */
> #define PFA_SPEC_IB_FORCE_DISABLE 6 /* Indirect branch speculation permanently restricted */
> #define PFA_SPEC_SSB_NOEXEC 7 /* Speculative Store Bypass clear on execve() */
> +#define PFA_GOING_TO_SCHED 8 /* task is going to call schedule() */
>
> #define TASK_PFA_TEST(name, func) \
> static inline bool task_##func(struct task_struct *p) \
> @@ -1575,6 +1576,10 @@ TASK_PFA_CLEAR(SPEC_IB_DISABLE, spec_ib_disable)
> TASK_PFA_TEST(SPEC_IB_FORCE_DISABLE, spec_ib_force_disable)
> TASK_PFA_SET(SPEC_IB_FORCE_DISABLE, spec_ib_force_disable)
>
> +TASK_PFA_TEST(GOING_TO_SCHED, going_to_sched)
> +TASK_PFA_SET(GOING_TO_SCHED, going_to_sched)
> +TASK_PFA_CLEAR(GOING_TO_SCHED, going_to_sched)
> +
> static inline void
> current_restore_flags(unsigned long orig_flags, unsigned long flags)
> {
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index b262f47046ca..bc96de2648f6 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -199,8 +199,10 @@ static void __kthread_parkme(struct kthread *self)
> if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
> break;
>
> + task_set_going_to_sched(current);
> complete(&self->parked);
> schedule();
> + task_clear_going_to_sched(current);
> }
> __set_current_state(TASK_RUNNING);
> }
> @@ -245,8 +247,10 @@ static int kthread(void *_create)
> /* OK, tell user we're spawned, wait for stop or wakeup */
> __set_current_state(TASK_UNINTERRUPTIBLE);
> create->result = current;
> + task_set_going_to_sched(current);
> complete(done);
> schedule();
> + task_clear_going_to_sched(current);
>
> ret = -EINTR;
> if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3c8a379c357e..78666cec794a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4330,6 +4330,12 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
> hrtimer_active(&rq_of(cfs_rq)->hrtick_timer))
> return;
> #endif
> + /*
> + * current task is going to call schedule(), do not preempt it or
> + * it will casue more useless contex_switch().
> + */
> + if (task_going_to_sched(rq_of(cfs_rq)->curr))
> + return;
>
> if (cfs_rq->nr_running > 1)
> check_preempt_tick(cfs_rq, curr);
> @@ -6634,6 +6640,13 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
> if (test_tsk_need_resched(curr))
> return;
>
> + /*
> + * current task is going to call schedule(), do not preempt it or
> + * it will casue more useless contex_switch().
> + */
> + if (task_going_to_sched(curr))
> + return;
> +
> /* Idle tasks are by definition preempted by non-idle tasks. */
> if (unlikely(task_has_idle_policy(curr)) &&
> likely(!task_has_idle_policy(p)))
> --
> 2.17.1
>
>
>
--
Kees Cook
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