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

[Namhyung Kim]

Hello,

On Mon, Jun 12, 2023 at 7:28 PM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
>
> 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.

I agree with James that it looks out of scope of perf tools.
You can always use `taskset` for external processes.

>
> >
> >> 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.

When it comes to controlling `perf`, you can use --threads=<spec>
option which supports fairly complex control for parallelism and
affinity.

Thanks,
Namhyung