linux-kernel - Re: [PATCH 2/2] fgraph: Free ret_stack when task is done with it

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Message-Id: <20241025123149.30347839a4701de276638187@kernel.org>
Date: Fri, 25 Oct 2024 12:31:49 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Mark
 Rutland <mark.rutland@....com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, Peter
 Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] fgraph: Free ret_stack when task is done with it

On Thu, 24 Oct 2024 21:05:15 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 25 Oct 2024 00:21:21 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> 
> > > +static void fgraph_ret_stack_work_func(struct work_struct *work)
> > > +{
> > > +	mutex_lock(&ftrace_lock);
> > > +	if (!ftrace_graph_active)
> > > +		free_ret_stacks();
> > > +	mutex_unlock(&ftrace_lock);
> > > +}  
> > 
> > Hmm, will you scan all tasks everytime? Shouldn't we have another global
> > list of skipped tasks in remove_ret_stack(), like below?
> > 
> > static void remove_ret_stack(struct task_struct *t, struct list_head *freelist, struct list_head *skiplist, int list_index)
> > {
> > 	struct ret_stack_free_data *free_data;
> > 	struct list_head *head;
> > 
> > 	/* If the ret_stack is still in use, skip this */
> > 	if (t->curr_ret_depth >= 0)
> > 		head = skiplist;
> > 	else
> > 		head = freelist;
> > 
> > 	free_data = (struct ret_stack_free_data*)(t->ret_stack + list_index);
> > 	list_add(&free_data->list, head);
> > 	free_data->task = t;
> > }
> > 
> > Then we can scan only skiplist in free_ret_stacks() in fgraph_ret_stack_work_func().
> > 
> > Of course this will need to decouple preparing freelist/skiplist and
> > actual free function.
> 
> I thought about doing it this way, but I felt that it made the code
> more complex with little benefit. Yeah, we scan all tasks, but it only
> happens in a work queue that is grabbing the ftrace_lock mutex. If
> anything, I rather keep it this way and if it ends up being an issue we
> can change it later.

OK, then let it goes with this in this version.

> 
> One thing Thomas always says is "correctness first, optimize later".
> This is much easier to get correct. Adding a skip list will add
> complexity. Like I said, nothing prevents us from adding that feature
> later, and if it ends up buggy, we can know which change caused the bug.

It is not buggy as far as I reviewed, just concerned about the
performance overhead. So,

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thank you,

> 
> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>