Re: [PATCH] perf cs-etm: Remove duplicate and incorrect aux size checks

[Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>]

On December 10, 2021 1:54:36 PM GMT-03:00, Mathieu Poirier <mathieu.poirier@linaro.org> wrote:
>On Thu, Dec 09, 2021 at 02:16:43PM +0000, James Clark wrote:
>> 
>> 
>> On 09/12/2021 13:44, Leo Yan wrote:
>> > On Wed, Dec 08, 2021 at 02:08:04PM +0000, James Clark wrote:
>> >> On 08/12/2021 13:17, Leo Yan wrote:
>> >>> Hi James,
>> >>>
>> >>> On Wed, Dec 08, 2021 at 11:54:35AM +0000, James Clark wrote:
>> >>>> There are two checks, one is for size when running without admin, but
>> >>>> this one is covered by the driver and reported on in more detail here
>> >>>> (builtin-record.c):
>> >>>>
>> >>>>   pr_err("Permission error mapping pages.\n"
>> >>>>          "Consider increasing "
>> >>>>          "/proc/sys/kernel/perf_event_mlock_kb,\n"
>> >>>>          "or try again with a smaller value of -m/--mmap_pages.\n"
>> >>>>          "(current value: %u,%u)\n",
>> >>>
>> >>> I looked into the kernel code and found:
>> >>>
>> >>>   sysctl_perf_event_mlock = 512 + (PAGE_SIZE / 1024);  // 512KB + 1 page
>> >>>
>> >>> If the system have multiple cores, let's say 8 cores, then kernel even
>> >>> can relax the limitaion with:
>> >>>
>> >>>   user_lock_limit *= num_online_cpus();
>> >>>
>> >>> So means the memory lock limitation is:
>> >>>
>> >>>   (512KB + 1 page) * 8 = 4MB + 8 pages.
>> >>>
>> >>> Seems to me, it's much relax than the user space's limitaion 128KB.
>> >>> And let's imagine for Arm server, the permitted buffer size can be a
>> >>> huge value (e.g. for a system with 128 cores).
>> >>>
>> >>> Could you confirm if this is right?
>> >>
>> >> Yes that seems to be the case. And the commit message for that addition
>> >> states the reasoning:
>> >>
>> >>   perf_counter: Increase mmap limit
>> >>   
>> >>   In a default 'perf top' run the tool will create a counter for
>> >>   each online CPU. With enough CPUs this will eventually exhaust
>> >>   the default limit.
>> >>
>> >>   So scale it up with the number of online CPUs.
>> >>
>> >> To me that makes sense. Normally the memory installed also scales with the
>> >> number of cores.
>> >>
>> >> Are you saying that we should look into modifying that scaling factor in
>> >> perf_mmap()? Or that we should still add something to userspace for
>> >> coresight to limit user supplied buffer sizes?
>> > 
>> > I don't think we should modify the scaling factor in perf_mmap(), the
>> > logic is not only used by AUX buffer, it's shared by normal event
>> > ring buffer.
>> > 
>> >> I think it makes sense to allow the user to specify any value that will work,
>> >> it's up to them.
>> > 
>> > Understand, I verified this patch with below steps:
>> > 
>> > root@debian:~# echo 0 > /proc/sys/kernel/perf_event_paranoid
>> > 
>> > leoy@debian:~$ perf record -e cs_etm// -m 4M,8M -o perf_test.data -- sleep 1
>> > Permission error mapping pages.
>> > Consider increasing /proc/sys/kernel/perf_event_mlock_kb,
>> > or try again with a smaller value of -m/--mmap_pages.
>> > (current value: 1024,2048)
>> > 
>> > leoy@debian:~$ perf record -e cs_etm// -m 4M,4M -o perf_test.data -- sleep 1
>> > Couldn't synthesize bpf events.
>> > [ perf record: Woken up 1 times to write data ]
>> > [ perf record: Captured and wrote 0.607 MB perf_test.data ]
>> > 
>> > So this patch looks good for me:
>> > 
>> > Reviewed-by: Leo Yan <leo.yan@linaro.org>
>> > 
>> Thanks Leo!
>
>Arnaldo is not on the recipient list and as such he won't see this patch...
>

I saw it now, can I take this as an acked-by: Matthieu too?

- Arnaldo