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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id t4sm10049060pfd.52.2020.03.05.09.24.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Mar 2020 09:24:31 -0800 (PST) Date: Thu, 5 Mar 2020 09:24:30 -0800 From: Kees Cook To: cl@rock-chips.com Subject: Re: [PATCH v2 1/1] sched/fair: do not preempt current task if it is going to call schedule() Message-ID: <202003050921.5559A8C3@keescook> References: <20200305095948.10873-1-cl@rock-chips.com> <20200305095948.10873-2-cl@rock-chips.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200305095948.10873-2-cl@rock-chips.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200305_092433_837876_64B88F13 X-CRM114-Status: GOOD ( 25.31 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: juri.lelli@redhat.com, mark.rutland@arm.com, heiko@sntech.de, geert+renesas@glider.be, peterz@infradead.org, catalin.marinas@arm.com, bsegall@google.com, will@kernel.org, mpe@ellerman.id.au, linux@armlinux.org.uk, dietmar.eggemann@arm.com, ben.dooks@codethink.co.uk, mgorman@suse.de, huangtao@rock-chips.com, anshuman.khandual@arm.com, rostedt@goodmis.org, tglx@linutronix.de, surenb@google.com, mingo@redhat.com, allison@lohutok.net, linux-arm-kernel@lists.infradead.org, wad@chromium.org, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, luto@amacapital.net, george_davis@mentor.com, sudeep.holla@arm.com, akpm@linux-foundation.org, info@metux.net, kstewart@linuxfoundation.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Mar 05, 2020 at 05:59:48PM +0800, cl@rock-chips.com wrote: > From: Liang Chen > > 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 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 -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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel