Re: [PATCH] writeback: call writeback tracepoints withoud holding list_lock in wb_writeback()

[Shi, Yang]

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