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SubjectRe: [PATCH] writeback: call writeback tracepoints withoud holding list_lock in wb_writeback()
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On 2/25/2016 3:31 PM, Steven Rostedt wrote:
> On Thu, 25 Feb 2016 15:16:54 -0800
> "Shi, Yang" <yang.shi@linaro.org> wrote:
>
>
>> Actually, regardless whether this is the right fix for the splat, it
>> makes me be wondering if the spin lock which protects the whole for loop
>> is really necessary. It sounds feasible to move it into the for loop and
>> just protect the necessary area.
>
> That's a separate issue, which may have its own merits that should be
> decided by the writeback folks.

Yes, definitely. I will rework my commit log for this part.

>
>>
>>>
>>>>
>>>>>
>>>>>
>>>>>> INFO: lockdep is turned off.
>>>>>> Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830
>>>
>>> Can you disassemble the vmlinux file to see exactly where that call is.
>>> I use gdb to find the right locations.
>>>
>>> gdb> li *0xffffffc000374a5c
>>> gdb> disass 0xffffffc000374a5c
>>
>> I use gdb to get the code too.
>>
>> It does point to the spin_lock.
>>
>> (gdb) list *0xffffffc000374a5c
>> 0xffffffc000374a5c is in wb_writeback (fs/fs-writeback.c:1621).
>> 1616
>> 1617 oldest_jif = jiffies;
>> 1618 work->older_than_this = &oldest_jif;
>> 1619
>> 1620 blk_start_plug(&plug);
>> 1621 spin_lock(&wb->list_lock);
>> 1622 for (;;) {
>> 1623 /*
>> 1624 * Stop writeback when nr_pages has been consumed
>> 1625 */
>>
>>
>> The disassemble:
>> 0xffffffc000374a58 <+232>: bl 0xffffffc0001300b0
>
> The above is the place it recorded. But I just realized, this isn't the
> issue. I know where the problem is.
>
>
>> <migrate_disable>
>> 0xffffffc000374a5c <+236>: mov x0, x22
>> 0xffffffc000374a60 <+240>: bl 0xffffffc000d5d518 <rt_spin_lock>
>>
>>>
>
>
>
>>>> DECLARE_EVENT_CLASS(writeback_work_class,
>>>> TP_PROTO(struct bdi_writeback *wb, struct wb_writeback_work *work),
>>>> TP_ARGS(wb, work),
>>>> TP_STRUCT__entry(
>>>> __array(char, name, 32)
>>>> __field(long, nr_pages)
>>>> __field(dev_t, sb_dev)
>>>> __field(int, sync_mode)
>>>> __field(int, for_kupdate)
>>>> __field(int, range_cyclic)
>>>> __field(int, for_background)
>>>> __field(int, reason)
>>>> __dynamic_array(char, cgroup, __trace_wb_cgroup_size(wb))
>>>>
>>>
>>> Ah, thanks for pointing that out. I missed that.
>>
>> It sounds not correct if tracepoint doesn't allow sleep.
>>
>> I considered to change sleeping lock to raw lock in kernfs_* functions,
>> but it sounds not reasonable since they are used heavily by cgroup.
>
> It is the kernfs_* that can't sleep. Tracepoints use
> rcu_read_lock_sched_notrace(), which disables preemption, and not only
> that, hides itself from lockdep as the last place to disable preemption.

Ah, thanks for pointing out this.

>
> Is there a way to not use the kernfs_* function? At least for -rt?

I'm not quite sure if there is straightforward replacement. However, I'm
wondering if lock free version could be used by tracing.

For example, create __kernfs_path_len which doesn't acquire any lock for
writeback tracing as long as there is not any race condition.

At least we could rule out preemption.

Thanks,
Yang

>
> -- Steve
>

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