Messages in this thread | | | Date | Thu, 9 Dec 2021 21:44:13 +0800 | From | Leo Yan <> | Subject | Re: [PATCH] perf cs-etm: Remove duplicate and incorrect aux size checks |
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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>
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