linux-kernel - Re: [PATCH] writeback: call writeback tracepoints withoud holding list_lock in wb

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Message-ID: <56CF9425.20106@linaro.org>
Date:	Thu, 25 Feb 2016 15:54:13 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	tj@...nel.org, jack@...e.cz, axboe@...com, fengguang.wu@...el.com,
	tglx@...utronix.de, bigeasy@...utronix.de,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-rt-users@...r.kernel.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH] writeback: call writeback tracepoints withoud holding
 list_lock in wb_writeback()

On 2/25/2016 3:47 PM, Shi, Yang wrote:
> On 2/25/2016 3:31 PM, Steven Rostedt wrote:
>> On Thu, 25 Feb 2016 15:16:54 -0800
>> "Shi, Yang" <yang.shi@...aro.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.

Can we disable irqs in tracepoints since spin_lock_irqsave is used by 
kernfs_* functions.

Thanks,
Yang

>
> Thanks,
> Yang
>
>>
>> -- Steve
>>
>