[dm-devel] dm bufio: Reduce dm_bufio_lock contention

[Mikulas Patocka]

On Fri, 15 Jun 2018, Michal Hocko wrote:

> On Thu 14-06-18 14:34:06, Mikulas Patocka wrote:
> > 
> > 
> > On Thu, 14 Jun 2018, Michal Hocko wrote:
> > 
> > > On Thu 14-06-18 15:18:58, jing xia wrote:
> > > [...]
> > > > PID: 22920  TASK: ffffffc0120f1a00  CPU: 1   COMMAND: "kworker/u8:2"
> > > >  #0 [ffffffc0282af3d0] __switch_to at ffffff8008085e48
> > > >  #1 [ffffffc0282af3f0] __schedule at ffffff8008850cc8
> > > >  #2 [ffffffc0282af450] schedule at ffffff8008850f4c
> > > >  #3 [ffffffc0282af470] schedule_timeout at ffffff8008853a0c
> > > >  #4 [ffffffc0282af520] schedule_timeout_uninterruptible at ffffff8008853aa8
> > > >  #5 [ffffffc0282af530] wait_iff_congested at ffffff8008181b40
> > > 
> > > This trace doesn't provide the full picture unfortunately. Waiting in
> > > the direct reclaim means that the underlying bdi is congested. The real
> > > question is why it doesn't flush IO in time.
> > 
> > I pointed this out two years ago and you just refused to fix it:
> > http://lkml.iu.edu/hypermail/linux/kernel/1608.1/04507.html
> 
> Let me be evil again and let me quote the old discussion:
> : > I agree that mempool_alloc should _primarily_ sleep on their own
> : > throttling mechanism. I am not questioning that. I am just saying that
> : > the page allocator has its own throttling which it relies on and that
> : > cannot be just ignored because that might have other undesirable side
> : > effects. So if the right approach is really to never throttle certain
> : > requests then we have to bail out from a congested nodes/zones as soon
> : > as the congestion is detected.
> : >
> : > Now, I would like to see that something like that is _really_ necessary.
> :
> : Currently, it is not a problem - device mapper reports the device as
> : congested only if the underlying physical disks are congested.
> :
> : But once we change it so that device mapper reports congested state on its
> : own (when it has too many bios in progress), this starts being a problem.
> 
> So has this changed since then? If yes then we can think of a proper
> solution but that would require to actually describe why we see the
> congestion, why it does help to wait on the caller rather than the
> allocator etc...

Device mapper doesn't report congested state - but something else does 
(perhaps the user inserted a cheap slow usb stick or sdcard?). And device 
mapper is just a victim of that.

Why should device mapper sleep because some other random block device is 
congested?

> Throwing statements like ...
> 
> > I'm sure you'll come up with another creative excuse why GFP_NORETRY 
> > allocations need incur deliberate 100ms delays in block device drivers.
> 
> ... is not really productive. I've tried to explain why I am not _sure_ what
> possible side effects such a change might have and your hand waving
> didn't really convince me. MD is not the only user of the page
> allocator...
> 
> E.g. why has 41c73a49df31 ("dm bufio: drop the lock when doing GFP_NOIO
> allocation") even added GFP_NOIO request in the first place when you
> keep retrying and sleep yourself?

Because mempool uses it. Mempool uses allocations with "GFP_NOIO | 
__GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN". An so dm-bufio uses 
these flags too. dm-bufio is just a big mempool.

If you argue that these flags are incorrect - then fix mempool_alloc.

> The changelog only describes what but
> doesn't explain why. Or did I misread the code and this is not the
> allocation which is stalling due to congestion?

Mikulas