linux-kernel - Re: sched_waking vs. set_event_pid crash (Re: Tracing busy processes/threads freezes/stalls the whole machine)

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Message-ID: <20210723072906.4f4e7bd5@gandalf.local.home>
Date:   Fri, 23 Jul 2021 07:29:06 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Stefan Metzmacher <metze@...ba.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-trace-devel@...r.kernel.org,
        io-uring <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: sched_waking vs. set_event_pid crash (Re: Tracing busy
 processes/threads freezes/stalls the whole machine)

On Fri, 23 Jul 2021 12:35:09 +0200
Stefan Metzmacher <metze@...ba.org> wrote:

> > Assuming this does fix your issue, I sent out a real patch with the
> > explanation of what happened in the change log, so that you can see why
> > that change was your issue.  
> 
> Yes, it does the trick, thanks very much!

Can I get a "Tested-by" from you?

> Now I can finally use:
> 
> trace-cmd record -e all -P $(pidof io_uring-cp-forever)
> 
> But that doesn't include the iou-wrk-* threads
> and the '-c' option seems to only work with forking.

Have you tried it? It should work for threads as well. It hooks to the
sched_process_fork tracepoint, which should be triggered even when a new
thread is created.

Or do you mean that you want that process and all its threads too that are
already running? I could probably have it try to add it via the /proc file
system in that case.

Can you start the task via trace-cmd?

  trace-cmd record -e all -F -c io_uring-cp-forever ...

> 
> Is there a way to specify "trace *all* threads of the given pid"?
> (Note the threads are comming and going, so it's not possible to
> specifiy -P more than once)

Right, although, you could append tasks manually to the set_event_pid file
from another terminal after starting trace-cmd. Once a pid is added to that
file, all children it makes will also be added. That could be a work around
until we have trace-cmd do it.

Care to write a bugzilla report for this feature?

  https://bugzilla.kernel.org/buglist.cgi?component=Trace-cmd%2FKernelshark&list_id=1088173

-- Steve