linux-kernel - Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY

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Date:   Wed, 8 Nov 2017 07:51:10 -0800
From:   Milind Chabbi <chabbi.milind@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Milind Chabbi <chabbi.milind@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        linux-man@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        Jin Yao <yao.jin@...ux.intel.com>
Subject: Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT

On Wed, Nov 8, 2017 at 7:12 AM, Jiri Olsa <jolsa@...hat.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?