Re: [PATCH] tracing: Mitigate possible softlockup in __tracing_open()

[Zheng Yejian]

On 2024/8/25 23:05, Masami Hiramatsu (Google) wrote:
> On Sat, 24 Aug 2024 11:03:43 +0800
> Zheng Yejian <zhengyejian@xxxxxxxxxxxxxxx> wrote:
>
> > In __tracing_open(), when max latency tracers took place on the cpu,
> > the time start of its buffer would be updated, then event entries with
> > timestamps being earlier than start of the buffer would be skipped
> > (see tracing_iter_reset()).
> >
> > Softlockup will occur if the kernel is non-preemptible and too many
> > entries were skipped in the loop that reset every cpu buffer, so add
> > cond_resched() to avoid it.
> >
> > Signed-off-by: Zheng Yejian <zhengyejian@xxxxxxxxxxxxxxx>
> > ---
> >   kernel/trace/trace.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index ebe7ce2f5f4a..88faa95b457b 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -4706,6 +4706,15 @@ __tracing_open(struct inode *inode, struct file *file, bool snapshot)
> >   		for_each_tracing_cpu(cpu) {
> >   			ring_buffer_read_start(iter->buffer_iter[cpu]);
> >   			tracing_iter_reset(iter, cpu);
> > +			/*
> > +			 * When max latency tracers took place on the cpu, the time start
> > +			 * of its buffer would be updated, then event entries with timestamps
> > +			 * being earlier than start of the buffer would be skipped
> > +			 * (see tracing_iter_reset()). Softlockup will occur if the kernel
> > +			 * is non-preemptible and too many entries were skipped in the loop,
> > +			 * so add cond_resched() to mitigate it.
> > +			 */
> > +			cond_resched();
>
> OK, but why we cond_resched() only here? Another tracing_iter_reset() in
> s_start() does not cause the soft lockups in the same situation?

Sorry for the late reply.

__tracing_open() is called when trace file is being opened,
s_start() is called when trace file is being read. Normally,
we read immediately after open at which tracing_iter_reset() was
just called, softlockup may be not that easily triggered in
s_start(), do we need cond_resched() there?



> Thank you,
>
>
> >   		}
> >   	} else {
> >   		cpu = iter->cpu_file;
> > --
> > 2.25.1

--
Thanks,
Zheng Yejian