linux-kernel - Re: [EDT][PATCH 1/1] hw_breakpoint.c :cpu hotplug handling

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Message-ID: <20150520165025.GF11498@arm.com>
Date:	Wed, 20 May 2015 17:50:25 +0100
From:	Will Deacon <will.deacon@....com>
To:	Vaneet Narang <v.narang@...sung.com>
Cc:	Maninder Singh <maninder1.s@...sung.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Amit Arora <amit.arora@...sung.com>,
	AJEET YADAV <ajeet.y@...sung.com>,
	AKHILESH KUMAR <akhilesh.k@...sung.com>
Subject: Re: [EDT][PATCH 1/1] hw_breakpoint.c :cpu hotplug handling

On Mon, May 18, 2015 at 05:46:31AM +0100, Vaneet Narang wrote:
> EP-2DAD0AFA905A4ACB804C4F82A001242F
> 
> On Wed, May 13, 2015 at 06:24:06AM +0100, Maninder Singh wrote:
> >> EP-2DAD0AFA905A4ACB804C4F82A001242F
> >> 
> >> Subject: [PATCH 1/1] hw_breakpoint.c :cpu hotplug handling
> >> 
> >> This patch adds support for CPU hotplug, It re-installl all installed watchpoints and breakpoints
> >> back on H/W in case of cpu-hot plug.
> 
> >Not sure why this is needed -- the scheduler should reinstall the
> >breakpoints when the debugged task gets scheduled in via
> >arch_install_hw_breakpoint.
> >
> >Will
> 
> I agree with you this reinstalling has to be either take care by scheduler
> or Debug tool. 

Which debug tool? If it's a ptrace user, then the scheduler should still
manage this, as ptrace will call into perf to create the breakpoints.

> In current implementation we clear H/W registers but we don't clear slots
> (wp_on_reg / bp_on_reg) for both watchpoint or breakpoint.

Ok, so are you saying that when the CPU is hotplugged off, the active
breakpoints aren't being uninstalled by perf? I thought this would have
happened as a result of the scheduler migrating the current task away
from the dying CPU.

Will
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