Messages in this thread | | | Subject | Re: [PATCH] mm/vmscan: don't scan adjust too much if current is not kswapd | From | Hongchen Zhang <> | Date | Tue, 20 Sep 2022 10:23:05 +0800 |
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Hi Mel,
The scan adjust algorithm was originally introduced by you from commmit e82e0561dae9 ("mm: vmscan: obey proportional scanning requirements for kswapd"), any suggestion about this fix patch? In short, proportional scanning is not friendly to processes other than kswapd.
Thanks Hongchen Zhang
On 2022/9/16 pm 6:19, Hongchen Zhang wrote: > Hi Andrew and Matthew, > > On 2022/9/16 pm 4:40, Matthew Wilcox wrote: >> On Fri, Sep 16, 2022 at 08:57:50AM +0800, Hongchen Zhang wrote: >>> Hi Andrew , >>> >>> On 2022/9/15 pm 5:00, Matthew Wilcox wrote: >>>> On Thu, Sep 15, 2022 at 04:02:41PM +0800, Hongchen Zhang wrote: >>>>> Hi Matthew, >>>>> On 2022/9/15 pm 3:28, Matthew Wilcox wrote: >>>>>> On Thu, Sep 15, 2022 at 09:19:48AM +0800, Hongchen Zhang wrote: >>>>>>> [ 3748.453561] INFO: task float_bessel:77920 blocked for more >>>>>>> than 120 >>>>>>> seconds. >>>>>>> [ 3748.460839] Not tainted 5.15.0-46-generic #49-Ubuntu >>>>>>> [ 3748.466490] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" >>>>>>> disables >>>>>>> this message. >>>>>>> [ 3748.474618] task:float_bessel state:D stack: 0 pid:77920 >>>>>>> ppid: >>>>>>> 77327 flags:0x00004002 >>>>>>> [ 3748.483358] Call Trace: >>>>>>> [ 3748.485964] <TASK> >>>>>>> [ 3748.488150] __schedule+0x23d/0x590 >>>>>>> [ 3748.491804] schedule+0x4e/0xc0 >>>>>>> [ 3748.495038] rwsem_down_read_slowpath+0x336/0x390 >>>>>>> [ 3748.499886] ? copy_user_enhanced_fast_string+0xe/0x40 >>>>>>> [ 3748.505181] down_read+0x43/0xa0 >>>>>>> [ 3748.508518] do_user_addr_fault+0x41c/0x670 >>>>>>> [ 3748.512799] exc_page_fault+0x77/0x170 >>>>>>> [ 3748.516673] asm_exc_page_fault+0x26/0x30 >>>>>>> [ 3748.520824] RIP: 0010:copy_user_enhanced_fast_string+0xe/0x40 >>>>>>> [ 3748.526764] Code: 89 d1 c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 >>>>>>> 31 c0 0f >>>>>>> 01 ca c3 cc cc cc cc 0f 1f 00 0f 01 cb 83 fa 40 0f 82 70 ff ff ff >>>>>>> 89 d1 <f3> >>>>>>> a4 31 c0 0f 01 ca c3 cc cc cc cc 66 08 >>>>>>> [ 3748.546120] RSP: 0018:ffffaa9248fffb90 EFLAGS: 00050206 >>>>>>> [ 3748.551495] RAX: 00007f99faa1a010 RBX: ffffaa9248fffd88 RCX: >>>>>>> 0000000000000010 >>>>>>> [ 3748.558828] RDX: 0000000000001000 RSI: ffff9db397ab8ff0 RDI: >>>>>>> 00007f99faa1a000 >>>>>>> [ 3748.566160] RBP: ffffaa9248fffbf0 R08: ffffcc2fc2965d80 R09: >>>>>>> 0000000000000014 >>>>>>> [ 3748.573492] R10: 0000000000000000 R11: 0000000000000014 R12: >>>>>>> 0000000000001000 >>>>>>> [ 3748.580858] R13: 0000000000001000 R14: 0000000000000000 R15: >>>>>>> ffffaa9248fffd98 >>>>>>> [ 3748.588196] ? copy_page_to_iter+0x10e/0x400 >>>>>>> [ 3748.592614] filemap_read+0x174/0x3e0 >>>>>> >>>>>> Interesting; it wasn't the process itself which triggered the page >>>>>> fault; the process called read() and the kernel took the page >>>>>> fault to >>>>>> satisfy the read() call. >>>>>> >>>>>>> [ 3748.596354] ? ima_file_check+0x6a/0xa0 >>>>>>> [ 3748.600301] generic_file_read_iter+0xe5/0x150 >>>>>>> [ 3748.604884] ext4_file_read_iter+0x5b/0x190 >>>>>>> [ 3748.609164] ? aa_file_perm+0x102/0x250 >>>>>>> [ 3748.613125] new_sync_read+0x10d/0x1a0 >>>>>>> [ 3748.617009] vfs_read+0x103/0x1a0 >>>>>>> [ 3748.620423] ksys_read+0x67/0xf0 >>>>>>> [ 3748.623743] __x64_sys_read+0x19/0x20 >>>>>>> [ 3748.627511] do_syscall_64+0x59/0xc0 >>>>>>> [ 3748.631203] ? syscall_exit_to_user_mode+0x27/0x50 >>>>>>> [ 3748.636144] ? do_syscall_64+0x69/0xc0 >>>>>>> [ 3748.639992] ? exit_to_user_mode_prepare+0x96/0xb0 >>>>>>> [ 3748.644931] ? irqentry_exit_to_user_mode+0x9/0x20 >>>>>>> [ 3748.649872] ? irqentry_exit+0x1d/0x30 >>>>>>> [ 3748.653737] ? exc_page_fault+0x89/0x170 >>>>>>> [ 3748.657795] entry_SYSCALL_64_after_hwframe+0x61/0xcb >>>>>>> [ 3748.663030] RIP: 0033:0x7f9a852989cc >>>>>>> [ 3748.666713] RSP: 002b:00007f9a8497dc90 EFLAGS: 00000246 ORIG_RAX: >>>>>>> 0000000000000000 >>>>>>> [ 3748.674487] RAX: ffffffffffffffda RBX: 00007f9a8497f5c0 RCX: >>>>>>> 00007f9a852989cc >>>>>>> [ 3748.681817] RDX: 0000000000027100 RSI: 00007f99faa18010 RDI: >>>>>>> 0000000000000061 >>>>>>> [ 3748.689150] RBP: 00007f9a8497dd60 R08: 0000000000000000 R09: >>>>>>> 00007f99faa18010 >>>>>>> [ 3748.696493] R10: 0000000000000000 R11: 0000000000000246 R12: >>>>>>> 00007f99faa18010 >>>>>>> [ 3748.703841] R13: 00005605e11c406f R14: 0000000000000001 R15: >>>>>>> 0000000000027100 >>>>>> >>>>>> ORIG_RAX is 0, which matches sys_read. >>>>>> RDI is file descriptor 0x61 >>>>>> RSI is plausibly a userspace pointer, 0x7f99faa18010 >>>>>> RDX is the length, 0x27100 or 160kB. >>>>>> >>>>>> That all seems reasonable. >>>>>> >>>>>> What I really want to know is who is _holding_ the lock. We stash >>>>>> a pointer to the task_struct in 'owner', so we could clearly find >>>>>> this >>>>>> out in the 'blocked for too long' report, and print their stack >>>>>> trace. >>>>>> >>>>> As described in the comment for __rwsem_set_reader_owned,it is hard >>>>> to track >>>>> read owners.So we could not clearly find out who blocked the >>>>> process,it was >>>>> caused by multiple tasks. >>>> >>>> Readers don't block readers. You have a reader here, so it's being >>>> blocked by a writer. And that writer's task_struct is stashed in >>>> rwsem->owner. It would be nice if we dumped that information >>>> automatically ... but we don't do that today. Perhaps you could >>>> grab that information from a crash dump if you have one. >>>> >>>>>> You must have done something like this already in order to deduce >>>>>> that >>>>>> it was the direct reclaim path that was the problem? >>>>>> >>>>> The method we used is to track the direct reclaim using the >>>>> trace_mm_vmscan_direct_reclaim_{begin,end} interface.When the problem >>>>> occurred,we could get a very large "nr_reclaimed" which is not a >>>>> desirable >>>>> value for process except kswapd. >>>> >>>> I disagree. If a process needs to allocate memory then it should be >>>> paying the cost of reclaiming that memory itself. kswapd is a last >>>> resort to reclaim memory when we have a workload (eg a network router) >>>> that does its memory allocation primarily in interrupt context. >>>> >>> What's your opinion about this scan adjust issue? Is there a better >>> way to >>> fix this issue? >> >> Yes, but we need you to gather more information about what's causing >> the issue before we can suggest what that is. >> > I think the following scenery triggers the scan adjust issue: > In function shrink_lruvec, we call get_scan_count and get the following > values: > targets[LRU_INACTIVE_ANON]=50000 > targets[LRU_ACTIVE_ANON]=50000 > targets[LRU_INACTIVE_FILE]=128 > targets[LRU_ACTIVE_FILE]=129 > > After the first scan, we get more than nr_to_reclaim pages, but the > percentage of scanning nr[LRU_INACTIVE_FILE+LRU_ACTIVE_FILE] is 256/257, > Then when we scan adjust, we must scan(possibly reclaim) > 256*(50000+50000)/257-256=99354 pages, which is too large and would > waste too many time. > If it is not kswapd, it is unacceptable to reclaim so many pages. > So we should limit the number of pages of scan adjust. > > Thanks > Hongchen Zhang
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