From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932092AbbI3Hb3 (ORCPT ); Wed, 30 Sep 2015 03:31:29 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:57074 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754448AbbI3HbR (ORCPT ); Wed, 30 Sep 2015 03:31:17 -0400 Message-Id: <20150930072303.844400616@infradead.org> User-Agent: quilt/0.61-1 Date: Wed, 30 Sep 2015 09:10:38 +0200 From: Peter Zijlstra To: mingo@kernel.org Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, fweisbec@gmail.com, oleg@redhat.com, umgwanakikbuti@gmail.com, tglx@linutronix.de, rostedt@goodmis.org, Peter Zijlstra Subject: [PATCH v2 03/12] sched: Create preempt_count invariant References: <20150930071035.514587432@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline; filename=peterz-kill-preempt_active-2.patch Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Assuming units of PREEMPT_DISABLE_OFFSET for preempt_count() numbers. Now that TASK_DEAD no longer results in preempt_count() == 3 during scheduling, we will always call context_switch() with preempt_count() == 2. However, we don't always end up with preempt_count() == 2 in finish_task_switch() because new tasks get created with preempt_count() == 1. Create FORK_PREEMPT_COUNT and set it to 2 and use that in the right places. Note that we cannot use INIT_PREEMPT_COUNT as that serves another purpose (boot). After this, preempt_count() is invariant across the context switch, with exception of PREEMPT_ACTIVE. Signed-off-by: Peter Zijlstra (Intel) --- arch/x86/include/asm/preempt.h | 2 +- include/asm-generic/preempt.h | 2 +- include/linux/sched.h | 17 ++++++++++++----- kernel/sched/core.c | 23 +++++++++++++++++++++-- 4 files changed, 35 insertions(+), 9 deletions(-) --- a/arch/x86/include/asm/preempt.h +++ b/arch/x86/include/asm/preempt.h @@ -31,7 +31,7 @@ static __always_inline void preempt_coun * must be macros to avoid header recursion hell */ #define init_task_preempt_count(p) do { \ - task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED; \ + task_thread_info(p)->saved_preempt_count = FORK_PREEMPT_COUNT; \ } while (0) #define init_idle_preempt_count(p, cpu) do { \ --- a/include/asm-generic/preempt.h +++ b/include/asm-generic/preempt.h @@ -24,7 +24,7 @@ static __always_inline void preempt_coun * must be macros to avoid header recursion hell */ #define init_task_preempt_count(p) do { \ - task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \ + task_thread_info(p)->preempt_count = FORK_PREEMPT_COUNT; \ } while (0) #define init_idle_preempt_count(p, cpu) do { \ --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -599,11 +599,7 @@ struct task_cputime_atomic { .sum_exec_runtime = ATOMIC64_INIT(0), \ } -#ifdef CONFIG_PREEMPT_COUNT -#define PREEMPT_DISABLED (1 + PREEMPT_ENABLED) -#else -#define PREEMPT_DISABLED PREEMPT_ENABLED -#endif +#define PREEMPT_DISABLED (PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED) /* * Disable preemption until the scheduler is running -- use an unconditional @@ -613,6 +609,17 @@ struct task_cputime_atomic { */ #define INIT_PREEMPT_COUNT PREEMPT_OFFSET +/* + * Initial preempt_count value; reflects the preempt_count schedule invariant + * which states that during context switches: + * + * preempt_count() == 2*PREEMPT_DISABLE_OFFSET + * + * Note: PREEMPT_DISABLE_OFFSET is 0 for !PREEMPT_COUNT kernels. + * Note: See finish_task_switch(). + */ +#define FORK_PREEMPT_COUNT (2*PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED) + /** * struct thread_group_cputimer - thread group interval timer counts * @cputime_atomic: atomic thread group interval timers. --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2504,6 +2504,18 @@ static struct rq *finish_task_switch(str struct mm_struct *mm = rq->prev_mm; long prev_state; + /* + * The previous task will have left us with a preempt_count of 2 + * because it left us after: + * + * schedule() + * preempt_disable(); // 1 + * __schedule() + * raw_spin_lock_irq(&rq->lock) // 2 + * + * Also, see FORK_PREEMPT_COUNT. + */ + rq->prev_mm = NULL; /* @@ -2588,8 +2600,15 @@ asmlinkage __visible void schedule_tail( { struct rq *rq; - /* finish_task_switch() drops rq->lock and enables preemtion */ - preempt_disable(); + /* + * New tasks start with FORK_PREEMPT_COUNT, see there and + * finish_task_switch() for details. + * + * finish_task_switch() will drop rq->lock() and lower preempt_count + * and the preempt_enable() will end up enabling preemption (on + * PREEMPT_COUNT kernels). + */ + rq = finish_task_switch(prev); balance_callback(rq); preempt_enable();