linux-kernel - Re: [PATCH] mm: thp: use down_read_trylock in khugepaged to avoid long block

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Date:   Sat, 16 Dec 2017 12:45:25 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.s@...baba-inc.com>
Cc:     kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        aarcange@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: thp: use down_read_trylock in khugepaged to avoid
 long block

On Sat 16-12-17 04:04:10, Yang Shi wrote:
> Hi Kirill & Michal,
> 
> Since both of you raised the same question about who holds the semaphore for
> that long time, I just reply here to both of you.
> 
> The backtrace shows vm-scalability is running with 300G memory and it is
> doing munmap as below:
> 
> [188995.241865] CPU: 15 PID: 8063 Comm: usemem Tainted: G            E
> 4.9.65-006.ali3000.alios7.x86_64 #1
> [188995.242252] Hardware name: Huawei Technologies Co., Ltd. Tecal RH2288H
> V2-12L/BC11SRSG1, BIOS RMIBV368 11/01/2013
> [188995.242637] task: ffff883f610a5b00 task.stack: ffffc90037280000
> [188995.242838] RIP: 0010:[<ffffffff811e2319>] .c [<ffffffff811e2319>]
> unmap_page_range+0x619/0x940
> [188995.243231] RSP: 0018:ffffc90037283c98  EFLAGS: 00000282
> [188995.243429] RAX: 00002b760ac57000 RBX: 00002b760ac56000 RCX:
> 0000000003eb13ca
> [188995.243820] RDX: ffffea003971e420 RSI: 00002b760ac56000 RDI:
> ffff8837cb832e80
> [188995.244211] RBP: ffffc90037283d78 R08: ffff883ebf8fc3c0 R09:
> 0000000000008000
> [188995.244600] R10: 00000000826b7e00 R11: 0000000000000000 R12:
> ffff8821e70f72b0
> [188995.244993] R13: ffffea00fac4f280 R14: ffffc90037283e00 R15:
> 00002b760ac57000
> [188995.245390] FS:  00002b34b4861700(0000) GS:ffff883f7d3c0000(0000)
> knlGS:0000000000000000
> [188995.245788] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [188995.245990] CR2: 00002b7092160fed CR3: 0000000977850000 CR4:
> 00000000001406e0
> [188995.246388] Stack:
> [188995.246581]  00002b92f71edfff.c 00002b7fffffffff.c 00002b92f71ee000.c
> ffff8809778502b0.c
> [188995.246981]  00002b763fffffff.c ffff8802e1895ec0.c ffffc90037283d48.c
> ffff883f610a5b00.c
> [188995.247365]  ffffc90037283d70.c 00002b8000000000.c ffffc00000000fff.c
> ffffea00879c3df0.c
> [188995.247759] Call Trace:
> [188995.247957]  [<ffffffff811e26bd>] unmap_single_vma+0x7d/0xe0
> [188995.248161]  [<ffffffff811e2a11>] unmap_vmas+0x51/0xa0
> [188995.248367]  [<ffffffff811e98ed>] unmap_region+0xbd/0x130
> [188995.248571]  [<ffffffff8170b04c>] ?
> rwsem_down_write_failed_killable+0x31c/0x3f0
> [188995.248961]  [<ffffffff811eb94c>] do_munmap+0x26c/0x420
> [188995.249162]  [<ffffffff811ebbc0>] SyS_munmap+0x50/0x70
> [188995.249361]  [<ffffffff8170cab7>] entry_SYSCALL_64_fastpath+0x1a/0xa9
> 
> By analyzing vmcore, khugepaged is waiting for vm-scalability process's
> mmap_sem.

OK, I see.
 
> unmap_vmas will unmap every vma in the memory space, it sounds the test
> generated huge amount of vmas.

I would expect that it just takes some time to munmap 300G address
range.

> Shall we add "cond_resched()" in unmap_vmas(), i.e for every 100 vmas? It
> may improve the responsiveness a little bit for non-preempt kernel, although
> it still can't release the semaphore.

We already do, once per pmd (see zap_pmd_range).
-- 
Michal Hocko
SUSE Labs