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

[James Clark <james.clark@arm.com>]

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!