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Date:   Wed, 1 Apr 2020 17:45:11 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Yiqian Wei <yiwei@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Shile Zhang <shile.zhang@linux.alibaba.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Alexander Duyck <alexander.duyck@gmail.com>,
        Baoquan He <bhe@redhat.com>, Oscar Salvador <osalvador@suse.de>
Subject: Re: [PATCH v1 1/2] mm/page_alloc: fix RCU stalls during deferred
 page initialization
Message-ID: <20200401154511.GR22681@dhcp22.suse.cz>
References: <20200401104156.11564-1-david@redhat.com>
 <20200401104156.11564-2-david@redhat.com>
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On Wed 01-04-20 12:41:55, David Hildenbrand wrote:
> With CONFIG_DEFERRED_STRUCT_PAGE_INIT and without CONFIG_PREEMPT, it can
> happen that we get RCU stalls detected when booting up.
> 
> [   60.474005] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> [   60.475000] rcu:  1-...0: (0 ticks this GP) idle=02a/1/0x4000000000000000 softirq=1/1 fqs=15000
> [   60.475000] rcu:  (detected by 0, t=60002 jiffies, g=-1199, q=1)
> [   60.475000] Sending NMI from CPU 0 to CPUs 1:
> [    1.760091] NMI backtrace for cpu 1
> [    1.760091] CPU: 1 PID: 20 Comm: pgdatinit0 Not tainted 4.18.0-147.9.1.el8_1.x86_64 #1
> [    1.760091] Hardware name: Red Hat KVM, BIOS 1.13.0-1.module+el8.2.0+5520+4e5817f3 04/01/2014
> [    1.760091] RIP: 0010:__init_single_page.isra.65+0x10/0x4f
> [    1.760091] Code: 48 83 cf 63 48 89 f8 0f 1f 40 00 48 89 c6 48 89 d7 e8 6b 18 80 ff 66 90 5b c3 31 c0 b9 10 00 00 00 49 89 f8 48 c1 e6 33 f3 ab <b8> 07 00 00 00 48 c1 e2 36 41 c7 40 34 01 00 00 00 48 c1 e0 33 41
> [    1.760091] RSP: 0000:ffffba783123be40 EFLAGS: 00000006
> [    1.760091] RAX: 0000000000000000 RBX: fffffad34405e300 RCX: 0000000000000000
> [    1.760091] RDX: 0000000000000000 RSI: 0010000000000000 RDI: fffffad34405e340
> [    1.760091] RBP: 0000000033f3177e R08: fffffad34405e300 R09: 0000000000000002
> [    1.760091] R10: 000000000000002b R11: ffff98afb691a500 R12: 0000000000000002
> [    1.760091] R13: 0000000000000000 R14: 000000003f03ea00 R15: 000000003e10178c
> [    1.760091] FS:  0000000000000000(0000) GS:ffff9c9ebeb00000(0000) knlGS:0000000000000000
> [    1.760091] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    1.760091] CR2: 00000000ffffffff CR3: 000000a1cf20a001 CR4: 00000000003606e0
> [    1.760091] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [    1.760091] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [    1.760091] Call Trace:
> [    1.760091]  deferred_init_pages+0x8f/0xbf
> [    1.760091]  deferred_init_memmap+0x184/0x29d
> [    1.760091]  ? deferred_free_pages.isra.97+0xba/0xba
> [    1.760091]  kthread+0x112/0x130
> [    1.760091]  ? kthread_flush_work_fn+0x10/0x10
> [    1.760091]  ret_from_fork+0x35/0x40
> [   89.123011] node 0 initialised, 1055935372 pages in 88650ms
> 
> The issue becomes visible when having a lot of memory (e.g., 4TB)
> assigned to a single NUMA node - a system that can easily be created
> using QEMU. Inside VMs on a hypervisor with quite some memory
> overcommit, this is fairly easy to trigger.
> 
> Adding the cond_resched() makes RCU happy.

I believe the patch you depend on is a wrong way to go so please let's
wait until that settles down. But your cond_resched makes a perfect
sense. Just have it called $FOO pages - e.g. hotplug is once per
section. This is not bound to SPARSEMEM so you would have to use
a differen't constant but something along those lines would work.

> 
> Reported-by: Yiqian Wei <yiwei@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
> Cc: Shile Zhang <shile.zhang@linux.alibaba.com>
> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
> Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Alexander Duyck <alexander.duyck@gmail.com>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/page_alloc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ca1453204e66..084cabffc90d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1877,6 +1877,7 @@ static int __init deferred_init_memmap(void *data)
>  			prev_nr_pages = nr_pages;
>  			pgdat->first_deferred_pfn = spfn;
>  			pgdat_resize_unlock(pgdat, &flags);
> +			cond_resched();
>  			goto again;
>  		}
>  	}
> -- 
> 2.25.1

-- 
Michal Hocko
SUSE Labs