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

[Zheng Yejian <zhengyejian@huaweicloud.com>]

On 2024/8/26 22:35, Steven Rostedt wrote:
> On Sat, 24 Aug 2024 11:03:43 +0800
> Zheng Yejian <zhengyejian@huaweicloud.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@huaweicloud.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