On Fri 15-06-18 07:35:07, Mikulas Patocka wrote: > > > 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. Maybe yes and that would require some more debugging to find out, analyze and think of a proper solution. > Why should device mapper sleep because some other random block device is > congested? Well, the direct reclaim is a way to throttle memory allocations. There is no real concept on who is asking for the memory. If you do not want to get blocked then use GFP_NOWAIT. > > 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. This doesn't answer my question though. Somebody else is doing it is not an explanation. Prior to your 41c73a49df31 there was no GFP_NOIO allocation AFAICS. So why do you really need it now? Why cannot you simply keep retrying GFP_NOWAIT with your own throttling? Note that I am not trying to say that 41c73a49df31, I am merely trying to understand why this blocking allocation is done in the first place. > If you argue that these flags are incorrect - then fix mempool_alloc. AFAICS there is no report about mempool_alloc stalling here. Maybe this is the same class of problem, honestly, I dunno. And I've already said that stalling __GFP_NORETRY might be a good way around that but that needs much more consideration and existing users examination. I am not aware anybody has done that. Doing changes like that based on a single user is certainly risky. -- Michal Hocko SUSE Labs -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
