Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT

From: Milind Chabbi <chabbi.milind@gmail.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Milind Chabbi <chabbi.milind@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Namhyung Kim <namhyung@kernel.org>,
	linux-kernel@vger.kernel.org,
	Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
	linux-man@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>,
	Andi Kleen <ak@linux.intel.com>, Kan Liang <kan.liang@intel.com>,
	Hari Bathini <hbathini@linux.vnet.ibm.com>,
	Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>,
	Jin Yao <yao.jin@linux.intel.com>
Subject: Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT
Date: Wed, 8 Nov 2017 07:51:10 -0800	[thread overview]
Message-ID: <CAMmz+Y=PJ2kFf9mqoQDJY32VjpCQBCCuWOiHNXR3mSEzotSS_w@mail.gmail.com> (raw)
In-Reply-To: <20171108151218.GA11018@krava>

On Wed, Nov 8, 2017 at 7:12 AM, Jiri Olsa <jolsa@redhat.com> wrote:

> > I am not able to fully understand your concern.
> > Can you point to a code file and line related to your observation?
> > The patch is modeled after the existing modify_user_hw_breakpoint() function
> > present in events/hw_breakpoint.c; don't you see this problem in that code?
>
> the reserve_bp_slot/release_bp_slot functions manage
> counts for current breakpoints based on its type
>
> those counts are cumulated in here:
>   static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
>
> you allow to change the breakpoint type, so I'd expect
> to see some code that release slot count for old type
> and take new one (if it's available)
>
> jirka

Why is this not a concern for modify_user_hw_breakpoint() function?