netdev - Re: [PATCH v3 1/6] perf: prepare perf_event.h for new types perf_kprobe and perf

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Date:   Mon, 4 Dec 2017 18:24:59 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v3 1/6] perf: prepare perf_event.h for new types
 perf_kprobe and perf_uprobe


> On Dec 3, 2017, at 9:03 AM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> On Thu, Nov 30, 2017 at 03:50:18PM -0800, Song Liu wrote:
>> Two new perf types, perf_kprobe and perf_uprobe, will be added to allow
>> creating [k,u]probe with perf_event_open. These [k,u]probe are associated
>> with the file decriptor created by perf_event_open, thus are easy to
>> clean when the file descriptor is destroyed.
>> 
>> kprobe_func and uprobe_path are added to union config1 for pointers to
>> function name for kprobe or binary path for uprobe.
>> 
>> kprobe_addr and probe_offset are added to union config2 for kernel
>> address (when kprobe_func is NULL), or [k,u]probe offset.
>> 
>> Signed-off-by: Song Liu <songliubraving@...com>
>> Reviewed-by: Yonghong Song <yhs@...com>
>> Reviewed-by: Josef Bacik <jbacik@...com>
>> Acked-by: Alexei Starovoitov <ast@...nel.org>
>> ---
>> include/uapi/linux/perf_event.h | 6 ++++++
>> 1 file changed, 6 insertions(+)
>> 
>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>> index 362493a..247c6cb 100644
>> --- a/include/uapi/linux/perf_event.h
>> +++ b/include/uapi/linux/perf_event.h
>> @@ -299,6 +299,8 @@ enum perf_event_read_format {
>> #define PERF_ATTR_SIZE_VER4	104	/* add: sample_regs_intr */
>> #define PERF_ATTR_SIZE_VER5	112	/* add: aux_watermark */
>> 
>> +#define MAX_PROBE_FUNC_NAME_LEN 64
> 
> I think we have to remove this restriction.
> There are already functions with names longer than 64 characters
> in the current vmlinux:
> trace_event_define_fields_ext4_ext_convert_to_initialized_fastpath
> trace_event_define_fields_mm_vmscan_direct_reclaim_begin_template
> 
> How about we drop this restriction and use NAME_MAX internally
> without adding new uapi defines ?

Yeah, I agree that we should drop this uapi define. How about we use

#define KSYM_NAME_LEN 128

If a function name is longer than KSYM_NAME_LEN, we get warning like:

Symbol long_long_name_abcdefghijklmnopqrstuvwxyz_abcdefghijklmnopqrstuvwxyz_abcdefghijklmnopqrstuvwxyz_abcdefghijklmnopqrstuvwxyz_abcdefghijklmnopqrstuvwxyz_abcdefghijklmnopqrstuvwxyz_abcdefghijklmnopqrstuvwxyz_ too long for kallsyms (204 vs 128).
Please increase KSYM_NAME_LEN both in kernel and kallsyms.c

Thanks,
Song