Re: [RFC] Adding support for setting the affinity of the recording process

[Yang Jihong <yangjihong1@huawei.com>]

Hello,

On 2023/6/12 23:27, James Clark wrote:
> 
> 
> On 12/06/2023 11:26, Yang Jihong wrote:
>> Hello everyone,
>>
>> Currently, perf-record supports profiling an existing process, thread,
>> or a specified command.
>>
>> Sometimes we may need to set CPU affinity of the target process before
>> recording:
>>
>>    # taskset -pc <cpus> <pid>
>>    # perf record -p <pid> -- sleep 10
>>
>> or:
>>
>>    # perf record -- `taskset -c <cpus> COMMAND`
>>
>> I'm thinking about getting perf to support setting the affinity of the
>> recording process, for example:
>>
>> 1. set the CPU affinity of the <pid1> process to <cpus1>, <pid2> process
>> to <cpus2>,  and record:
>>
>>    # perf record -p <pid1>/<cpus1>:<pid2>/<cpus2> -- sleep 10
>>
> 
> I'm not sure if this is necessary. You can already do this with taskset
> when you launch the processes or for existing ones.

Yes, that's what we're doing now, and I'm thinking about whether perf 
can support this "taskset" feature.

> 
>> and
>>
>> 2. set CPU affinity of the COMMAND and record:
>>
>>    # perf record --taskset-command <cpus> COMMAND
>>
>> In doing so, perf, as an observer, actually changes some of the
>> properties of the target process, which may be contrary to the purpose
>> of perf tool.
>>
>>
>> Will we consider accepting this approach?
>>
> 
> For #2 I do this sometimes, but I prefix the perf command with taskset
> because otherwise there is a small time between when taskset does its
> thing and launching the child process that it runs in the wrong place.
> 
> Then one issue with the above method is that perf itself gets pinned to
> those CPUs as well. I suppose that could influence your application but
> I've never had an issue with it.
> 
> If you really can't live with perf also being pinned to those CPUs I
> would say it makes sense to add options for #2. Otherwise I would just
> run everything under taskset and no changes are needed.

If "perf" process and the target process are pinned to the same CPU, 
and the CPU usage of the target process is high, the perf data 
collection may be affected. Therefore, in this case, we may need to pin 
the target process and "perf" to different CPUs.

> 
> I think you would still need to have perf itself pinned to the CPUs just
> before it does the fork and exec, and then after that it can undo its
> pinning. Otherwise you'd still get that small time running on the wrong
> cores.
>

Thanks for your advice, or we can support setting different affinities 
for the "perf" process and the target process.


Thanks,
Yang.