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

[Leo Yan]

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@xxxxxxxxxx>