> -----Original Message----- > From: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> > Sent: Thursday, July 6, 2023 3:06 AM > To: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>; Ma, Yu > <yu.ma@xxxxxxxxx>; akpm@xxxxxxxxxxxxxxxxxxxx; Chen, Tim C > <tim.c.chen@xxxxxxxxx>; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > Hansen, Dave <dave.hansen@xxxxxxxxx>; Williams, Dan J > <dan.j.williams@xxxxxxxxx>; shakeelb@xxxxxxxxxx; Deng, Pan > <pan.deng@xxxxxxxxx>; Li, Tianyou <tianyou.li@xxxxxxxxx>; Zhu, Lipeng > <lipeng.zhu@xxxxxxxxx>; tim.c.chen@xxxxxxxxxxxxxxx > Subject: Re: [PATCH] mm/mmap: move vma operations to mm_struct out of > the critical section of file mapping lock > > On Wed, Jul 05, 2023 at 01:33:48PM -0400, Liam R. Howlett wrote: > > * Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> [230705 12:54]: > > > On Tue, Jun 06, 2023 at 03:20:13PM -0400, Liam R. Howlett wrote: > > > > * Yu Ma <yu.ma@xxxxxxxxx> [230606 08:23]: > > > > > UnixBench/Execl represents a class of workload where bash > > > > > scripts are spawned frequently to do some short jobs. When > > > > > running multiple parallel tasks, hot osq_lock is observed from > > > > > do_mmap and exit_mmap. Both of them come from load_elf_binary > > > > > through the call chain > > > > > "execl->do_execveat_common->bprm_execve->load_elf_binary". In > > > > > do_mmap,it will call mmap_region to create vma node, initialize > > > > > it and insert it to vma maintain structure in mm_struct and > > > > > i_mmap tree of the mapping file, then increase map_count to > > > > > record the number of vma nodes used. The hot osq_lock is to > > > > > protect operations on file’s i_mmap tree. For the mm_struct > > > > > member change like vma insertion and map_count update, they do > not affect i_mmap tree. Move those operations out of the lock's critical > section, to reduce hold time on the lock. > > > > > > > > > > With this change, on Intel Sapphire Rapids 112C/224T platform, > > > > > based on v6.0-rc6, the 160 parallel score improves by 12%. The > > > > > patch has no obvious performance gain on v6.4-rc4 due to > > > > > regression of this benchmark from this commit > > > > > f1a7941243c102a44e8847e3b94ff4ff3ec56f25 (mm: convert mm's rss > stats into percpu_counter). > > > > > > > > I didn't think it was safe to insert a VMA into the VMA tree > > > > without holding this write lock? We now have a window of time > > > > where a file mapping doesn't exist for a vma that's in the tree? > > > > Is this always safe? Does the locking order in mm/rmap.c need to > change? > > > > > > We hold mmap lock on write here, right? > > > > Yes. > > > > >Who can observe the VMA until the > > > lock is released? > > > > With CONFIG_PER_VMA_LOCK we can have the VMA read under the rcu > read > > lock for page faults from the tree. I am not sure if the vma is > > initialized to avoid page fault issues - vma_start_write() should > > either be taken or initialise the vma as this is the case. > > Right, with CONFIG_PER_VMA_LOCK the vma has to be unusable until it is > fully initialized, effectively providing the same guarantees as mmap write lock. > If it is not the case, it is CONFIG_PER_VMA_LOCK bug. > > > There is also a possibility of a driver mapping a VMA and having entry > > points from other locations. It isn't accessed through the tree > > though so I don't think this change will introduce new races? > > Right. > > > > It cannot be retrieved from the VMA tree as it requires at least > > > read mmap lock. And the VMA doesn't exist anywhere else. > > > > > > I believe the change is safe. > > > > I guess insert_vm_struct(), and vma_link() callers should be checked > > and updated accordingly? > > Yep. > Thanks Kirill and Liam, I'll update patch and send out for review. Regards Yu
