From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S263167AbUEWQoO (ORCPT ); Sun, 23 May 2004 12:44:14 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S263169AbUEWQoN (ORCPT ); Sun, 23 May 2004 12:44:13 -0400 Received: from caramon.arm.linux.org.uk ([212.18.232.186]:12813 "EHLO caramon.arm.linux.org.uk") by vger.kernel.org with ESMTP id S263167AbUEWQoC (ORCPT ); Sun, 23 May 2004 12:44:02 -0400 Date: Sun, 23 May 2004 17:43:59 +0100 From: Russell King To: Linux Kernel List Subject: scheduler: IRQs disabled over context switches Message-ID: <20040523174359.A21153@flint.arm.linux.org.uk> Mail-Followup-To: Linux Kernel List Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.2.5.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hi, The 2.6.6 scheduler disables IRQs across context switches, which is bad news for IRQ latency on ARM - to the point where 16550A FIFO UARTs to overrun. I'm considering defining prepare_arch_switch & co as follows on ARM, so that we release IRQs over the call to context_switch(). #define prepare_arch_switch(rq,next) \ do { \ spin_lock(&(next)->switch_lock); \ spin_unlock_irq(&(rq)->lock); \ } while (0) #define finish_arch_switch(rq,prev) \ spin_unlock(&(prev)->switch_lock) #define task_running(rq,p) \ ((rq)->curr == (p) || spin_is_locked(&(p)->switch_lock)) The question is... why are we keeping IRQs disabled over context_switch() in the first case? Looking at the code, the only thing which is touched outside of the two tasks is rq->prev_mm. Since runqueues are CPU- specific and we're holding at least one spinlock, I think the above is preempt safe and SMP safe. However, I'd like to find out from someone who knows this code why IRQs are disabled by default here. -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 PCMCIA - http://pcmcia.arm.linux.org.uk/ 2.6 Serial core