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

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Date:   Mon, 13 Nov 2017 00:02:56 -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

SNIP

On Sun, Nov 12, 2017 at 11:46 PM, Jiri Olsa <jolsa@...hat.com> wrote:

> but you closed fd4 before openning fd5..?

Yes, that is correct. I closed fd4. The reason is by closing fd4, we
are having a total of 3 hardware breakpoints active, but we are making
the software counting in the kernel think that four TYPE_DATA
breakpoints active. The counting should have disallowed us from
creating fd5 as per the following logic in the kernel:

static int __reserve_bp_slot(struct perf_event *bp)

{
 ....

        /* Flexible counters need to keep at least one slot */
        if (slots.pinned + (!!slots.flexible) > nr_slots[type])
                return -ENOSPC;
....
}