Re: [PATCH] perf report: Fix invalid memory accessing

["Wangnan (F)" <wangnan0@huawei.com>]

On 2015/9/7 20:51, Wang Nan wrote:


[SNIP]

> Although theoretically CPU_TOPOLOGY feature should always be selected by
> 'perf record', I did generate a perf.data without that feature. It has
> header like this:
>
>   # perf report -i ./bad.perf.data  --header-only
>   # ========
>   # captured on: Thu Jan  8 09:30:15 2009
>   # hostname : localhost
>   # os release : 3.10.49-gd672fc4
>   # perf version : 4.2.gc9df
>   # arch : aarch64
>   # nrcpus online : 8
>   # nrcpus avail : 8
>   # total memory : 1850768 kB
>   # cmdline : /system/bin/perf record -e sync:sync_timeline -e kgsl:kgsl_register_event -g -a sleep 5
>   # event : name = sync:sync_timeline, , id = { 1107, 1108, 1109, 1110, 1111, 1112 }, type = 2, size = 112, config = 0x3e7, { sample_period, sample_freq } = 1, sample_type = IP|TID|TIME|CALLCHAIN|ID|CPU|PERIOD|RAW, read_format = ID, disabled = 1, inherit = 1, mmap = 1, comm = 1, task = 1, sample_id_all = 1, exclude_guest = 1, mmap2 = 1, comm_exec = 1
>   # event : name = kgsl:kgsl_register_event, , id = { 1113, 1114, 1115, 1116, 1117, 1118 }, type = 2, size = 112, config = 0x350, { sample_period, sample_freq } = 1, sample_type = IP|TID|TIME|CALLCHAIN|ID|CPU|PERIOD|RAW, read_format = ID, disabled = 1, inherit = 1, sample_id_all = 1, exclude_guest = 1
>   # pmu mappings: cpu = 4, software = 1, tracepoint = 2
>   # ========
>   #
>
> It should be:
>
>   # ========
>   # captured on: Thu Jan  8 11:26:41 2009
>   ...
>   # HEADER_CPU_TOPOLOGY info available, use -I to display
>   # pmu mappings: cpu = 4, software = 1, tracepoint = 2
>   # ========
>
> However, bad perf.data appears randomly. I can't stably reproduce it, so I
> guess there might have another invalid memory accessing.
>
>

I found the problem.

perf relies on build_cpu_topology() to fetch CPU_TOPOLOGY from sysfs. It 
depend on
the existance of

/sys/devices/system/cpu/cpu%d/topology/core_siblings_list

However, CPU can be canceled by hotcpu subsystem. After that the 
directory of
/sys/devices/system/cpu/cpu%d/topology is gone, which causes perf's
write_cpu_topology() --> uild_cpu_topology() to fail, result in the 
above perf.data.

So I think my patch is required.

Thank you.