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

[Mathieu Poirier <mathieu.poirier@linaro.org>]

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...