Messages in this thread | | | Subject | Re: [PATCH] perf cs-etm: Remove duplicate and incorrect aux size checks | From | James Clark <> | Date | Thu, 9 Dec 2021 14:16:43 +0000 |
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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!
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