Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965076Ab3DHPbz (ORCPT ); Mon, 8 Apr 2013 11:31:55 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:53998 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965002Ab3DHPby (ORCPT ); Mon, 8 Apr 2013 11:31:54 -0400 Date: Mon, 8 Apr 2013 16:31:32 +0100 From: Will Deacon To: Christopher Covington 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 Message-ID: <20130408153132.GQ17476@mudshark.cambridge.arm.com> References: <1365012061-31660-1-git-send-email-cov@codeaurora.org> <20130403180443.GB21167@mudshark.cambridge.arm.com> <5162D750.9050706@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5162D750.9050706@codeaurora.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2191 Lines: 56 On Mon, Apr 08, 2013 at 03:42:24PM +0100, Christopher Covington wrote: > 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. Yes, but moving the call still isn't the right thing to do if we can avoid it. How about making that contextidr_thread_switch take prev instead of next? Will -- 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/