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

[Yang Shi]

On 12/16/17 12:09 PM, Kirill A. Shutemov wrote:
> On Sat, Dec 16, 2017 at 12:45:25PM +0100, Michal Hocko wrote:
> > 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).
>
> It doesn't help. We would need to find a way to drop mmap_sem, if we're
> holding it way too long. And doing it on per-vma count basis is not right
> call. It won't address issue with single huge vma.

Yes, I agree. We have to find a way to release the semaphore at some 
point instead of holding it for the whole unmap.

> Do we have any instrumentation that would help detect starvation on a
> rw_semaphore?

ebpf might help?

Thanks,
Yang

>