Re: [PATCH] writeback: fix BDI_WRITTEN accounting disturbing bdi->completions

[Wu Fengguang <fengguang.wu@intel.com>]

On Fri, Sep 02, 2011 at 12:29:50AM +0800, Jan Kara wrote:
> On Thu 01-09-11 22:40:41, Wu Fengguang wrote:
> > Peter,
> > 
> > This is an amazing bug. I'm not sure how the accounting goes wrong in
> > some tricky way. But you can compare the exact bdi proportions pattern
> > before/after patch. The gray "bdi setpoint" lines are vastly different.
> > 
> > ---
> > When increasing BDI_WRITTEN together with bdi->completions inside the
> > same local irq disabling block, bdi_thresh is found to go wild in the
> > 1 disk + 1 usb stick writeback test case. Fix it by moving BDI_WRITTEN
> > accounting out.
>   I don't understand this - the patch is just NOP. The change in
> test_clear_page_writeback() does absolutely nothing and the change in
> bdi_writeout_inc() just changes:
>   	local_irq_save(flags);
>   	__inc_bdi_stat(bdi);
>   	__prop_inc_percpu_max(&vm_completions, &bdi->completions,
>   			      bdi->max_prop_frac);
>   	local_irq_restore(flags);
> 
> to:
>   	local_irq_save(flags);
>   	__inc_bdi_stat(bdi);
>   	local_irq_restore(flags);
>   	local_irq_save(flags);
>   	__prop_inc_percpu_max(&vm_completions, &bdi->completions,
>   			      bdi->max_prop_frac);
>   	local_irq_restore(flags);
> 
> So the difference must be in something else...

Yes you are right -- this patch is basically a NOP. I retried without
this patch and it's now working fine again. Something mysterious to
catch..

Thanks,
Fengguang