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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Fri, 25 Jun 2021 18:00:11 +0200
To:     Michal Hocko <mhocko@suse.com>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        Rafael Aquini <aquini@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: vmalloc: add cond_resched() in __vunmap()
Message-ID: <20210625160011.GA50201@pc638.lan>
References: <20210622225030.478384-1-aquini@redhat.com>
 <YNR4ZkwF+Bh11XMC@dhcp22.suse.cz>
 <20210624142339.GA2267@pc638.lan>
 <YNWY/IY+ftszkjM5@dhcp22.suse.cz>
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Precedence: bulk

On Fri, Jun 25, 2021 at 10:51:08AM +0200, Michal Hocko wrote:
> On Thu 24-06-21 16:23:39, Uladzislau Rezki wrote:
> > On Thu, Jun 24, 2021 at 02:21:21PM +0200, Michal Hocko wrote:
> > > On Tue 22-06-21 18:50:30, Rafael Aquini wrote:
> > > > On non-preemptible kernel builds the watchdog can complain
> > > > about soft lockups when vfree() is called against large
> > > > vmalloc areas:
> > > > 
> > > > [  210.851798] kvmalloc-test: vmalloc(2199023255552) succeeded
> > > > [  238.654842] watchdog: BUG: soft lockup - CPU#181 stuck for 26s! [rmmod:5203]
> > > > [  238.662716] Modules linked in: kvmalloc_test(OE-) ...
> > > > [  238.772671] CPU: 181 PID: 5203 Comm: rmmod Tainted: G S         OE     5.13.0-rc7+ #1
> > > > [  238.781413] Hardware name: Intel Corporation PURLEY/PURLEY, BIOS PLYXCRB1.86B.0553.D01.1809190614 09/19/2018
> > > > [  238.792383] RIP: 0010:free_unref_page+0x52/0x60
> > > > [  238.797447] Code: 48 c1 fd 06 48 89 ee e8 9c d0 ff ff 84 c0 74 19 9c 41 5c fa 48 89 ee 48 89 df e8 b9 ea ff ff 41 f7 c4 00 02 00 00 74 01 fb 5b <5d> 41 5c c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 f0 29 77
> > > > [  238.818406] RSP: 0018:ffffb4d87868fe98 EFLAGS: 00000206
> > > > [  238.824236] RAX: 0000000000000000 RBX: 000000001da0c945 RCX: ffffb4d87868fe40
> > > > [  238.832200] RDX: ffffd79d3beed108 RSI: ffffd7998501dc08 RDI: ffff9c6fbffd7010
> > > > [  238.840166] RBP: 000000000d518cbd R08: ffffd7998501dc08 R09: 0000000000000001
> > > > [  238.848131] R10: 0000000000000000 R11: ffffd79d3beee088 R12: 0000000000000202
> > > > [  238.856095] R13: ffff9e5be3eceec0 R14: 0000000000000000 R15: 0000000000000000
> > > > [  238.864059] FS:  00007fe082c2d740(0000) GS:ffff9f4c69b40000(0000) knlGS:0000000000000000
> > > > [  238.873089] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > [  238.879503] CR2: 000055a000611128 CR3: 000000f6094f6006 CR4: 00000000007706e0
> > > > [  238.887467] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > [  238.895433] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > [  238.903397] PKRU: 55555554
> > > > [  238.906417] Call Trace:
> > > > [  238.909149]  __vunmap+0x17c/0x220
> > > > [  238.912851]  __x64_sys_delete_module+0x13a/0x250
> > > > [  238.918008]  ? syscall_trace_enter.isra.20+0x13c/0x1b0
> > > > [  238.923746]  do_syscall_64+0x39/0x80
> > > > [  238.927740]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > > > 
> > > > Like in other range zapping routines that iterate over
> > > > a large list, lets just add cond_resched() within __vunmap()'s
> > > > page-releasing loop in order to avoid the watchdog splats.
> > > 
> > > cond_resched makes a lot of sense. We do not want vmalloc to be visible
> > > the userspace (e.g. by stalling it) so all time consuming operations
> > > should yield regularly whenever possible. I would expect that any
> > > susbsystem which needs huge vmalloc areas would have it for the whole
> > > boot life time so such large vfrees should be really rare.
> > > 
> > There is at least one more place with potentially similar issue. I see that
> > the bulk allocator disables irqs during obtaining pages and filling an array.
> > 
> > So i suspect if we request a huge size to allocate over vmalloc same soft
> > lockup should occur. For example 10G alloactions simultaneously on different
> > CPUs.
> 
> I haven't payed a close attention to the changes regarding the bulk
> allocator but my high level understanding is that it only allocates from
> from pcp lists so the amount of allocatable pages is quite limited.

I am able to trigger it. To simulate it i run 10 threads to allocate and vfree
~1GB(262144 pages) of vmalloced memory at the same time: 

<snip>
[   62.512621] RIP: 0010:__alloc_pages_bulk+0xa9f/0xbb0
[   62.512628] Code: ff 8b 44 24 48 44 29 f8 83 f8 01 0f 84 ea fe ff ff e9 07 f6 ff ff 48 8b 44 24 60 48 89 28 e9 00 f9 ff ff fb 66 0f 1f 44 00 00 <e9> e8 fd ff ff 65 48 01 51 10 e9 3e fe ff ff 48 8b 44 24 78 4d 89
[   62.512629] RSP: 0018:ffffa7bfc29ffd20 EFLAGS: 00000206
[   62.512631] RAX: 0000000000000200 RBX: ffffcd5405421888 RCX: ffff8c36ffdeb928
[   62.512632] RDX: 0000000000040000 RSI: ffffa896f06b2ff8 RDI: ffffcd5405421880
[   62.512633] RBP: ffffcd5405421880 R08: 000000000000007d R09: ffffffffffffffff
[   62.512634] R10: ffffffff9d63c084 R11: 00000000ffffffff R12: ffff8c373ffaeb80
[   62.512635] R13: ffff8c36ffdf65f8 R14: ffff8c373ffaeb80 R15: 0000000000040000
[   62.512637] FS:  0000000000000000(0000) GS:ffff8c36ffdc0000(0000) knlGS:0000000000000000
[   62.512638] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   62.512639] CR2: 000055c8e2fe8610 CR3: 0000000c13e10000 CR4: 00000000000006e0
[   62.512641] Call Trace:
[   62.512646]  __vmalloc_node_range+0x11c/0x2d0
[   62.512649]  ? full_fit_alloc_test+0x140/0x140 [test_vmalloc]
[   62.512654]  __vmalloc_node+0x4b/0x70
[   62.512656]  ? fix_size_alloc_test+0x44/0x60 [test_vmalloc]
[   62.512659]  fix_size_alloc_test+0x44/0x60 [test_vmalloc]
[   62.512662]  test_func+0xe7/0x1f0 [test_vmalloc]
[   62.512666]  ? fix_align_alloc_test+0x50/0x50 [test_vmalloc]
[   62.512668]  kthread+0x11a/0x140
[   62.512671]  ? set_kthread_struct+0x40/0x40
[   62.512672]  ret_from_fork+0x22/0x30
<snip>

As for how much a bulk allocator can allocate, it is quite a lot. In my case i see
that 262144 pages can be obtained per one bulk call, if pcp-list is empty it is
refilled.

From the other hand allocating 1GB on 10 CPUs simultaneously is not common test
case in real world.

Not sure if we can do something with it and if it is worth to fix. At least we can
invoke a bulk allocator several times doing it per specific batch, for example 50
pages.

Any thoughts about it?

--
Vlad Rezki