Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965121Ab3DHOm1 (ORCPT ); Mon, 8 Apr 2013 10:42:27 -0400 Received: from wolverine01.qualcomm.com ([199.106.114.254]:45439 "EHLO wolverine01.qualcomm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759462Ab3DHOmZ (ORCPT ); Mon, 8 Apr 2013 10:42:25 -0400 X-IronPort-AV: E=Sophos;i="4.87,432,1363158000"; d="scan'208";a="36553279" Message-ID: <5162D750.9050706@codeaurora.org> Date: Mon, 08 Apr 2013 10:42:24 -0400 From: Christopher Covington User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120714 Thunderbird/14.0 MIME-Version: 1.0 To: Will Deacon CC: Catalin Marinas , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "linux-arm-msm@vger.kernel.org" Subject: Re: [PATCH] arm64: Fix task tracing References: <1365012061-31660-1-git-send-email-cov@codeaurora.org> <20130403180443.GB21167@mudshark.cambridge.arm.com> In-Reply-To: <20130403180443.GB21167@mudshark.cambridge.arm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2052 Lines: 59 Hi Will, On 04/03/2013 02:04 PM, Will Deacon wrote: > Hi Christopher, > > On Wed, Apr 03, 2013 at 07:01:01PM +0100, Christopher Covington wrote: >> For accurate accounting call contextidr_thread_switch before a >> task is scheduled, rather than after. >> >> Signed-off-by: Christopher Covington >> --- >> arch/arm64/kernel/process.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c >> index 0337cdb..c2cc249 100644 >> --- a/arch/arm64/kernel/process.c >> +++ b/arch/arm64/kernel/process.c >> @@ -311,11 +311,11 @@ struct task_struct *__switch_to(struct task_struct *prev, >> fpsimd_thread_switch(next); >> tls_thread_switch(next); >> hw_breakpoint_thread_switch(next); >> + contextidr_thread_switch(next); >> >> /* the actual thread switch */ >> last = cpu_switch_to(prev, next); >> >> - contextidr_thread_switch(next); >> return last; >> } > > Catalin and I wondered about this and decided to go with the current > approach in case a debugger, in response to the contextidr write, decided to > go off and mine information about the *new* task using the sp. The problem with the existing implementation is that it doesn't seem to compensate for how cpu_switch_to changes the stack pointer. Consider the following sequence. cpu_switch_to(prev=A, next=B) cpu_switch_to(prev=B, next=C) cpu_switch_to(prev=C, next=A) After the third call, using A's stack, next will be B, and its thread ID will be written to CONTEXTIDR. An easy way to see this in a simulator is to just instrument the code with some printk's. Thanks, Christopher -- Employee of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/