Michal Hocko wrote: > On Mon 10-07-17 22:54:37, Tetsuo Handa wrote: > > Michal Hocko wrote: > > > On Sat 08-07-17 13:59:54, Tetsuo Handa wrote: > > > [...] > > > > Quoting from http://lkml.kernel.org/r/20170705081956.GA14538@xxxxxxxxxxxxxx : > > > > Michal Hocko wrote: > > > > > On Sat 01-07-17 20:43:56, Tetsuo Handa wrote: > > > > > > You are rejecting serialization under OOM without giving a chance to test > > > > > > side effects of serialization under OOM at linux-next.git. I call such attitude > > > > > > "speculation" which you never accept. > > > > > > > > > > No I am rejecting abusing the lock for purpose it is not aimed for. > > > > > > > > Then, why adding a new lock (not oom_lock but warn_alloc_lock) is not acceptable? > > > > Since warn_alloc_lock is aimed for avoiding messages by warn_alloc() getting > > > > jumbled, there should be no reason you reject this lock. > > > > > > > > If you don't like locks, can you instead accept below one? > > > > > > No, seriously! Just think about what you are proposing. You are stalling > > > and now you will stall _random_ tasks even more. Some of them for > > > unbound amount of time because of inherent unfairness of cmpxchg. > > > > The cause of stall when oom_lock is already held is that threads which failed to > > hold oom_lock continue almost busy looping; schedule_timeout_uninterruptible(1) is > > not sufficient when there are multiple threads doing the same thing, for direct > > reclaim/compaction consumes a lot of CPU time. > > > > What makes this situation worse is, since warn_alloc() periodically appends to > > printk() buffer, the thread inside the OOM killer with oom_lock held can stall > > forever due to cond_resched() from console_unlock() from printk(). > > warn_alloc is just yet-another-user of printk. We might have many > others... warn_alloc() is different from other users of printk() that printk() is called as long as oom_lock is already held by somebody else processing console_unlock(). > > > Below change significantly reduces possibility of falling into printk() v.s. oom_lock > > lockup problem, for the thread inside the OOM killer with oom_lock held no longer > > blocks inside printk(). Though there still remains possibility of sleeping for > > unexpectedly long at schedule_timeout_killable(1) with the oom_lock held. > > This just papers over the real problem. > > > --- a/mm/oom_kill.c > > +++ b/mm/oom_kill.c > > @@ -1051,8 +1051,10 @@ bool out_of_memory(struct oom_control *oc) > > panic("Out of memory and no killable processes...\n"); > > } > > if (oc->chosen && oc->chosen != (void *)-1UL) { > > + preempt_disable(); > > oom_kill_process(oc, !is_memcg_oom(oc) ? "Out of memory" : > > "Memory cgroup out of memory"); > > + preempt_enable_no_resched(); > > /* > > * Give the killed process a good chance to exit before trying > > * to allocate memory again. > > > > I wish we could agree with applying this patch until printk-kthread can > > work reliably... > > And now you have introduced soft lockups most probably because > oom_kill_process can take some time... Or maybe even sleeping while > atomic warnings if some code path needs to sleep for whatever reason. > The real fix is make sure that printk doesn't take an arbitrary amount of > time. The OOM killer is not permitted to wait for __GFP_DIRECT_RECLAIM allocations directly/indirectly (because it will cause recursion deadlock). Thus, even if some code path needs to sleep for some reason, that code path is not permitted to wait for __GFP_DIRECT_RECLAIM allocations directly/indirectly. Anyway, I can propose scattering preempt_disable()/preempt_enable_no_resched() around printk() rather than whole oom_kill_process(). You will just reject it as you have rejected in the past. Using a reproducer and a patch to disable warn_alloc() at http://lkml.kernel.org/r/201707082230.ECB51545.JtFFFVHOOSMLOQ@xxxxxxxxxxxxxxxxxxx , you will find that calling cond_resched() (from console_unlock() from printk()) can cause a delay of nearly one minute, and it can cause a delay of nearly 5 minutes to complete one out_of_memory() call. Notice that the reproducer is using not so insane number of threads (only 10 children). (It will become a DoS if 100 children or 1024 children.) ---------------------------------------- [ 589.570344] idle-priority invoked oom-killer: gfp_mask=0x14280ca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO), nodemask=(null), order=0, oom_score_adj=0 [ 602.792064] idle-priority cpuset=/ mems_allowed=0 [ 602.794185] CPU: 0 PID: 9833 Comm: idle-priority Not tainted 4.12.0-next-20170711+ #628 [ 602.796870] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/02/2015 [ 602.800102] Call Trace: [ 602.801685] dump_stack+0x67/0x9e [ 602.803444] dump_header+0x9d/0x3fa [ 602.805227] ? trace_hardirqs_on+0xd/0x10 [ 602.807106] oom_kill_process+0x226/0x650 [ 602.809002] out_of_memory+0x136/0x560 [ 602.810822] ? out_of_memory+0x206/0x560 [ 602.812688] __alloc_pages_nodemask+0xcd2/0xe50 [ 602.814676] alloc_pages_vma+0x76/0x1a0 [ 602.816519] __handle_mm_fault+0xdff/0x1180 [ 602.818416] ? native_sched_clock+0x36/0xa0 [ 602.820322] handle_mm_fault+0x186/0x360 [ 602.822170] ? handle_mm_fault+0x44/0x360 [ 602.824009] __do_page_fault+0x1da/0x510 [ 602.825808] do_page_fault+0x21/0x70 [ 602.827501] page_fault+0x22/0x30 [ 602.829121] RIP: 0033:0x4008b8 [ 602.830657] RSP: 002b:00007ffc7f1b2070 EFLAGS: 00010206 [ 602.832602] RAX: 00000000d2d29000 RBX: 0000000100000000 RCX: 00007f2a8e4debd0 [ 602.834911] RDX: 0000000000000000 RSI: 0000000000400ae0 RDI: 0000000000000004 [ 602.837212] RBP: 00007f288e5ea010 R08: 0000000000000000 R09: 0000000000021000 [ 602.839481] R10: 00007ffc7f1b1df0 R11: 0000000000000246 R12: 0000000000000006 [ 602.841712] R13: 00007f288e5ea010 R14: 0000000000000000 R15: 0000000000000000 [ 602.843964] Mem-Info: [ 660.641313] active_anon:871821 inactive_anon:4422 isolated_anon:0 [ 660.641313] active_file:0 inactive_file:1 isolated_file:0 [ 660.641313] unevictable:0 dirty:0 writeback:0 unstable:0 [ 660.641313] slab_reclaimable:0 slab_unreclaimable:0 [ 660.641313] mapped:555 shmem:6257 pagetables:3184 bounce:0 [ 660.641313] free:21377 free_pcp:188 free_cma:0 [ 660.735954] Node 0 active_anon:3487284kB inactive_anon:17688kB active_file:0kB inactive_file:4kB unevictable:0kB isolated(anon):0kB isolated(file):0kB mapped:2220kB dirty:0kB writeback:0kB shmem:25028kB shmem_thp: 0kB shmem_pmdmapped: 0kB anon_thp: 3198976kB writeback_tmp:0kB unstable:0kB all_unreclaimable? yes [ 681.835092] Node 0 DMA free:14780kB min:288kB low:360kB high:432kB active_anon:1092kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:15988kB managed:15904kB mlocked:0kB kernel_stack:0kB pagetables:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB [ 702.374011] lowmem_reserve[]: 0 2688 3624 3624 [ 707.583447] Node 0 DMA32 free:53556kB min:49908kB low:62384kB high:74860kB active_anon:2698264kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:3129216kB managed:2752884kB mlocked:0kB kernel_stack:0kB pagetables:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB [ 729.520428] lowmem_reserve[]: 0 0 936 936 [ 734.700444] Node 0 Normal free:17204kB min:17384kB low:21728kB high:26072kB active_anon:787928kB inactive_anon:17688kB active_file:0kB inactive_file:4kB unevictable:0kB writepending:0kB present:1048576kB managed:958868kB mlocked:0kB kernel_stack:3072kB pagetables:12736kB bounce:0kB free_pcp:752kB local_pcp:120kB free_cma:0kB [ 760.567618] lowmem_reserve[]: 0 0 0 0 [ 765.855466] Node 0 DMA: 1*4kB (M) 1*8kB (M) 1*16kB (M) 1*32kB (M) 2*64kB (U) 2*128kB (UM) 2*256kB (UM) 1*512kB (M) 1*1024kB (U) 0*2048kB 3*4096kB (M) = 14780kB [ 780.375573] Node 0 DMA32: 7*4kB (UM) 5*8kB (UM) 7*16kB (UM) 6*32kB (U) 5*64kB (UM) 1*128kB (M) 4*256kB (M) 5*512kB (UM) 2*1024kB (M) 1*2048kB (U) 11*4096kB (UM) = 53556kB [ 795.846001] Node 0 Normal: 103*4kB (UM) 21*8kB (UM) 90*16kB (UME) 101*32kB (ME) 43*64kB (UME) 16*128kB (UE) 6*256kB (E) 1*512kB (U) 5*1024kB (UM) 0*2048kB 0*4096kB = 17220kB [ 812.472579] Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=1048576kB [ 821.375279] Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=2048kB [ 830.344875] 6258 total pagecache pages [ 836.012842] 0 pages in swap cache [ 841.786993] Swap cache stats: add 0, delete 0, find 0/0 [ 848.324422] Free swap = 0kB [ 853.617092] Total swap = 0kB [ 858.757287] 1048445 pages RAM [ 864.079999] 0 pages HighMem/MovableOnly [ 869.764325] 116531 pages reserved [ 875.162601] 0 pages hwpoisoned [ 880.350179] Out of memory: Kill process 9833 (idle-priority) score 928 or sacrifice child [ 888.751868] Killed process 9835 (normal-priority) total-vm:4360kB, anon-rss:96kB, file-rss:0kB, shmem-rss:0kB ---------------------------------------- > > You are trying to hammer this particular path but you should realize > that as long as printk can take an unbound amount of time then there are > many other land mines which need fixing. It is simply not feasible to go > after each and ever one of them and try to tweak them around. So please > stop proposing these random hacks and rather try to work with prink guys > to find solution for this long term printk limitation. OOM killer is a > good usecase to give this a priority. Whatever approach we use for printk() not to take unbound amount of time (e.g. just enqueue to log_buf using per a thread flag), we might still take unbound amount of time if we allow cond_sched() (or whatever sleep some code path might need to use) with the oom_lock held. After all, the OOM killer is ignoring scheduling priority problem regardless of printk() lockup problem. I don't have objection about making sure that printk() doesn't take an arbitrary amount of time. But the real fix is make sure that out_of_memory() doesn't take an arbitrary amount of time (i.e. don't allow cond_resched() etc. at all) unless there is cooperation from other allocating threads which failed to hold oom_lock. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
