From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-x243.google.com (mail-pg0-x243.google.com [IPv6:2607:f8b0:400e:c05::243]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3wk8fJ1RSTzDqNQ for ; Fri, 9 Jun 2017 01:36:24 +1000 (AEST) Received: by mail-pg0-x243.google.com with SMTP id v18so4951049pgb.3 for ; Thu, 08 Jun 2017 08:36:24 -0700 (PDT) From: Nicholas Piggin To: linuxppc-dev@lists.ozlabs.org Cc: Nicholas Piggin Subject: [PATCH 2/4] powerpc/64: context switch avoid reservation-clearing instruction Date: Fri, 9 Jun 2017 01:36:07 +1000 Message-Id: <20170608153609.19217-2-npiggin@gmail.com> In-Reply-To: <20170608153609.19217-1-npiggin@gmail.com> References: <20170608153609.19217-1-npiggin@gmail.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , There is no need to break reservation in _switch, because we are guranteed that context switch path will include a larx/stcx. Comment the guarantee and remove the reservation clear from _switch. This is worth 1-2% in context switch performance. Signed-off-by: Nicholas Piggin --- arch/powerpc/kernel/entry_64.S | 11 +++-------- kernel/sched/core.c | 6 ++++++ 2 files changed, 9 insertions(+), 8 deletions(-) diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S index 91f9fdc2d027..273a35926534 100644 --- a/arch/powerpc/kernel/entry_64.S +++ b/arch/powerpc/kernel/entry_64.S @@ -521,15 +521,10 @@ _GLOBAL(_switch) #endif /* CONFIG_SMP */ /* - * If we optimise away the clear of the reservation in system - * calls because we know the CPU tracks the address of the - * reservation, then we need to clear it here to cover the - * case that the kernel context switch path has no larx - * instructions. + * The kernel context switch path must contain a spin_lock, + * which contains larx/stcx, which will clear any reservation + * of the task being switched. */ -BEGIN_FTR_SECTION - ldarx r6,0,r1 -END_FTR_SECTION_IFSET(CPU_FTR_STCX_CHECKS_ADDRESS) BEGIN_FTR_SECTION /* diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 803c3bc274c4..1f0688ad09d7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2875,6 +2875,12 @@ context_switch(struct rq *rq, struct task_struct *prev, rq_unpin_lock(rq, rf); spin_release(&rq->lock.dep_map, 1, _THIS_IP_); + /* + * Some architectures require that a spin lock is taken before + * _switch. The rq_lock satisfies this condition. See powerpc + * _switch for details. + */ + /* Here we just switch the register state and the stack. */ switch_to(prev, next, prev); barrier(); -- 2.11.0