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Message-ID: <d3c2ecfd-6590-dc17-4ea5-d80872d01016@huaweicloud.com>
Date: Tue, 27 Aug 2024 09:29:55 +0800
From: Zheng Yejian <zhengyejian@...weicloud.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: Mitigate possible softlockup in __tracing_open()
On 2024/8/26 22:35, Steven Rostedt wrote:
> On Sat, 24 Aug 2024 11:03:43 +0800
> Zheng Yejian <zhengyejian@...weicloud.com> 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@...weicloud.com>
>> ---
>> 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();
>
> This is the wrong place to put this. If the problem is with
> tracing_iter_reset(), then add it there.
>
> while (ring_buffer_iter_peek(buf_iter, &ts)) {
> if (ts >= iter->array_buffer->time_start)
> break;
> entries++;
> ring_buffer_iter_advance(buf_iter);
> /* This could be a big loop */
> cond_resched();
Do resched check every event peek, I'm afraid there will
be too many 're-schedule'.
> }
>
> -- Steve
>
>
>
>> }
>> } else {
>> cpu = iter->cpu_file;
>
--
Thanks,
Zheng Yejian
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