On 2019/4/3 下午11:43, David Sterba wrote:
> On Tue, Apr 02, 2019 at 05:45:54PM +0800, Qu Wenruo wrote:
>> There are two tree lock events which can sleep:
>> - btrfs_tree_read_lock()
>> - btrfs_tree_lock()
>>
>> Sometimes we may need to look into the concurrency picture of the fs.
>> For that case, we need the execution time of above two functions and the
>> owner of @eb.
>>
>> Here we introduce a trace events for user space tools like bcc, to get
>> the execution time of above two functions, and get detailed owner info
>> where eBPF code can't.
>>
>> All the overhead is hidden behind the trace events, so if events are not
>> enabled, there is no overhead.
>>
>> Also, since this patch and later user space tool only cares about the
>> execution time and owner, other info like bytenr is ignored in this
>> events.
> 
> The time is definetelly the interesting part here, I'm thinking if the
> block number could also provide some useful hints. The gathered trace
> data can be used to group by blocks to compare the times, or see the
> histogram of times.
> 
> The block at minimum would be useful and I don't want to add more
> information there, like level or nritems. In case we do want that
> information, a heavier tracepoint version should be added with all the
> other data.

The block number is not that useful in current form.
As a block number can be the same while contains data for different
trees between different transactions.

That's the reason why owner makes more sense in this situation.

To make block number makes sense, we need to pair
trace_btrfs_tree_lock() with trace_btrfs_tree_unlock().


> 
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
[snip]
>> +	),
>> +
>> +	TP_printk_btrfs("start_ns=%llu end_ns=%llu owner=%llu is_log_tree=%d",
>> +		__entry->start_ns, __entry->end_ns, __entry->owner,
>> +		__entry->is_log_tree)
> 
> I think the delta between start_ns and end_ns should be printed too, so
> it's obvious without further processing:

My idea is to put that processing part into user space.
So we can reduce the space usage of the event buffer.

But it will do no harm anyway, so I'll add the output.

Thanks,
Qu

> 
> 	TP_printk_btrfs("start_ns=%llu end_ns=%llu diff=%llu owner=%llu is_log_tree=%d",
>