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
