linux-kernel - Re: [PATCH] ring-buffer: Allow for rescheduling when removing pages

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Date:   Fri, 7 Sep 2018 14:30:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Vaibhav Nagarnaik <vnagarnaik@...gle.com>
Cc:     linux-kernel@...r.kernel.org, jbehmer@...gle.com
Subject: Re: [PATCH] ring-buffer: Allow for rescheduling when removing pages

On Fri,  7 Sep 2018 11:21:31 -0700
Vaibhav Nagarnaik <vnagarnaik@...gle.com> wrote:

> When reducing ring buffer size, pages are removed by scheduling a work
> item on each CPU for the corresponding CPU ring buffer. After the pages
> are removed from ring buffer linked list, the pages are free()d in a
> tight loop. The loop does not give up CPU until all pages are removed.
> In a worst case behavior, when lot of pages are to be freed, it can
> cause system stall.
> 
> After the pages are removed from the list, the free() can happen while
> the work is rescheduled. Add a check for need_sched() within the loop
> to prevent the system hangup.
> 
> Reported-by: Jason Behmer <jbehmer@...gle.com>
> Signed-off-by: Vaibhav Nagarnaik <vnagarnaik@...gle.com>
> ---
>  kernel/trace/ring_buffer.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 1d92d4a982fd..bc1789df7c53 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1546,6 +1546,9 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages)
>  	tmp_iter_page = first_page;
>  
>  	do {
> +		if (need_resched())
> +			schedule();
> +

Hi, thanks for the patch, but the proper way to do this is to stick in:

		cond_resched();

And that should solve it for you. Want to send in another patch?

-- Steve

>  		to_remove_page = tmp_iter_page;
>  		rb_inc_page(cpu_buffer, &tmp_iter_page);
>