Re: Why the need to do a perf_event_open syscall for each cpu on the system?

[William Cohen <wcohen@redhat.com>]

On 03/15/2015 01:15 AM, Elazar Leibovich wrote:
> Hi,
> 
> Not an expert, but my understanding is that it's just technical
> difficulty. Performance metrics are being saved in per-cpu buffer.
> Having pid==-1 and cpu==-1 means that something would aggregate all
> buffers in multiple CPUs to a single buffer. That code must exist,
> either in userspace or in the kernel.
> 
> The kernel preferred that this code would be in userspace.

Hi Elazar,

I suspected the reasoning was something along those lines.  I was hoping that someone could point to archived email threads with earlier discussions showing the complications that would arise by having system-wide setup perf event setup and reading handled in the kernel. Looking through the earlier versions of perf see that pid==-1 and cpu=-1 were not allowed in the very early proposed patches (http://thread.gmane.org/gmane.linux.kernel.cross-arch/2578).  However, not much in the way explanation in the design tradeoffs in there.

Making user-space set up performance events for each cpu certainly simplifies the kernel code for system-wide monitoring. The cgroup support is essentially like system-wide monitoring with additional filtering on the cgroup and things get more complicated using the perf cgroup support when the cgroups are not pinned to a particular processor, O(cgroups*cpus) opens and reads.  If the cgroups is scaled up at the same rate as cpus, this would be O(cpus^2).  I am wondering if handling the system-wide case (pid==-1 and cpu==-1) in the kernel would make cgroup and system-wide monitoring more efficient or if the complications in the kernel are just too much.

-Will
>
> On Fri, Mar 13, 2015 at 8:49 PM, William Cohen <wcohen@redhat.com> wrote:
>> Hi All,
>>
>> I have a design question about the linux kernel perf support. A number of /proc statistics aggregate data across all the cpus in the system.  Why the does perf require the user-space application to enumerate all the processors and do a perf_event_open syscall for each of the processors?  Why not have a perf_event_open with pid=-1 and cpu=-1 mean system-wide event and aggregate it in the kernel when the value is read?  The line below from design.txt specifically say it is invalid.
>>
>> (Note: the combination of 'pid == -1' and 'cpu == -1' is not valid.)
>>
>> -Will
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