On Thu, 14 Mar 2019 17:06:11 +0100
Claudio <claudio.fontana@xxxxxxxxx> wrote:
> Hi Steve,
>
> I have experimented with mainline, and then with mainline plus a patch to add the info for fork as well like this:
>
> diff --git a/kernel/trace/trace_sched_switch.c b/kernel/trace/trace_sched_switch.c
> index e288168..c5339ed 100644
> --- a/kernel/trace/trace_sched_switch.c
> +++ b/kernel/trace/trace_sched_switch.c
> @@ -47,6 +47,20 @@ probe_sched_wakeup(void *ignore, struct task_struct *wakee)
> tracing_record_taskinfo(current, flags);
> }
>
> +static void
> +probe_sched_fork(void *ignore,
> + struct task_struct *parent, struct task_struct *child)
> +{
> + int flags;
> +
> + flags = (RECORD_TGID * !!sched_tgid_ref) +
> + (RECORD_CMDLINE * !!sched_cmdline_ref);
> +
> + if (!flags)
> + return;
> + tracing_record_taskinfo(child, flags);
> +}
> +
> static int tracing_sched_register(void)
> {
> int ret;
> @@ -72,7 +86,16 @@ static int tracing_sched_register(void)
> goto fail_deprobe_wake_new;
> }
>
> + ret = register_trace_sched_process_fork(probe_sched_fork, NULL);
> + if (ret) {
> + pr_info("sched trace: Couldn't activate tracepoint"
> + " probe to kernel_sched_process_fork\n");
> + goto fail_deprobe_switch;
> + }
> +
> return ret;
> +fail_deprobe_switch:
> + unregister_trace_sched_switch(probe_sched_switch, NULL);
> fail_deprobe_wake_new:
> unregister_trace_sched_wakeup_new(probe_sched_wakeup, NULL);
> fail_deprobe:
> @@ -82,6 +105,7 @@ static int tracing_sched_register(void)
>
> static void tracing_sched_unregister(void)
> {
> + unregister_trace_sched_process_fork(probe_sched_fork, NULL);
> unregister_trace_sched_switch(probe_sched_switch, NULL);
> unregister_trace_sched_wakeup_new(probe_sched_wakeup, NULL);
> unregister_trace_sched_wakeup(probe_sched_wakeup, NULL);
>
>
> --
>
> It seems to work, but the model seems not to apply very well to my use case.
>
> saved_tgids cannot be reset, not even by disabling tracing completely
>
> echo 0 > tracing_on
>
> or
>
> echo 0 > options/record-tgids
>
> and the list just keeps growing, no entries are ever removed when tasks are destroyed etc,
> so my lookups become more and more expensive as the list grows.
> I don't think this is suited to be looked up frequently for my tracing scenario.
>
It shouldn't grow when you disable tracing though.
Hmm, we could add a reset if one writes to the saved_* files.
> Next I will experiment with:
>
> 1) kprobes, trying to generate an event before every sched class event carrying the tid to tgid mapping.
Just plop a kprobe on top of __schedule()
-- Steve
>
> 2) if everything fails, revert to use /proc/PID/stat, which however can be slow and racy if the process does not exist anymore when I read back the ftrace buffers.
>
> Thanks & ciao,
>
> Claudio
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