On Mon, Jul 02, 2018 at 10:19:32AM -0700, Yang Shi wrote: > > > On 7/2/18 5:33 AM, Kirill A. Shutemov wrote: > > On Sat, Jun 30, 2018 at 06:39:44AM +0800, Yang Shi wrote: > > > When running some mmap/munmap scalability tests with large memory (i.e. > > > > 300GB), the below hung task issue may happen occasionally. > > > INFO: task ps:14018 blocked for more than 120 seconds. > > > Tainted: G E 4.9.79-009.ali3000.alios7.x86_64 #1 > > > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this > > > message. > > > ps D 0 14018 1 0x00000004 > > > ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0 > > > ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040 > > > 00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000 > > > Call Trace: > > > [<ffffffff817154d0>] ? __schedule+0x250/0x730 > > > [<ffffffff817159e6>] schedule+0x36/0x80 > > > [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150 > > > [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30 > > > [<ffffffff81717db0>] down_read+0x20/0x40 > > > [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0 > > > [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100 > > > [<ffffffff81241d87>] __vfs_read+0x37/0x150 > > > [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0 > > > [<ffffffff81242266>] vfs_read+0x96/0x130 > > > [<ffffffff812437b5>] SyS_read+0x55/0xc0 > > > [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5 > > > > > > It is because munmap holds mmap_sem from very beginning to all the way > > > down to the end, and doesn't release it in the middle. When unmapping > > > large mapping, it may take long time (take ~18 seconds to unmap 320GB > > > mapping with every single page mapped on an idle machine). > > > > > > It is because munmap holds mmap_sem from very beginning to all the way > > > down to the end, and doesn't release it in the middle. When unmapping > > > large mapping, it may take long time (take ~18 seconds to unmap 320GB > > > mapping with every single page mapped on an idle machine). > > > > > > Zapping pages is the most time consuming part, according to the > > > suggestion from Michal Hock [1], zapping pages can be done with holding > > > read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write > > > mmap_sem to cleanup vmas. All zapped vmas will have VM_DEAD flag set, > > > the page fault to VM_DEAD vma will trigger SIGSEGV. > > > > > > Define large mapping size thresh as PUD size or 1GB, just zap pages with > > > read mmap_sem for mappings which are >= thresh value. > > > > > > If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, then just > > > fallback to regular path since unmapping those mappings need acquire > > > write mmap_sem. > > > > > > For the time being, just do this in munmap syscall path. Other > > > vm_munmap() or do_munmap() call sites remain intact for stability > > > reason. > > > > > > The below is some regression and performance data collected on a machine > > > with 32 cores of E5-2680 @ 2.70GHz and 384GB memory. > > > > > > With the patched kernel, write mmap_sem hold time is dropped to us level > > > from second. > > > > > > [1] https://lwn.net/Articles/753269/ > > > > > > Cc: Michal Hocko <mhocko@xxxxxxxxxx> > > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> > > > Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx> > > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> > > > --- > > > mm/mmap.c | 136 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++- > > > 1 file changed, 134 insertions(+), 2 deletions(-) > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > index 87dcf83..d61e08b 100644 > > > --- a/mm/mmap.c > > > +++ b/mm/mmap.c > > > @@ -2763,6 +2763,128 @@ static int munmap_lookup_vma(struct mm_struct *mm, struct vm_area_struct **vma, > > > return 1; > > > } > > > +/* Consider PUD size or 1GB mapping as large mapping */ > > > +#ifdef HPAGE_PUD_SIZE > > > +#define LARGE_MAP_THRESH HPAGE_PUD_SIZE > > > +#else > > > +#define LARGE_MAP_THRESH (1 * 1024 * 1024 * 1024) > > > +#endif > > PUD_SIZE is defined everywhere. > > If THP is defined, otherwise it is: > > #define HPAGE_PUD_SIZE ({ BUILD_BUG(); 0; }) I'm talking about PUD_SIZE, not HPAGE_PUD_SIZE. -- Kirill A. Shutemov
