On Fri, 8 Oct 2021 11:15:25 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote: > On 08.10.21 10:52, Claudio Imbrenda wrote: > > On Fri, 8 Oct 2021 10:17:50 +0200 > > David Hildenbrand <david@xxxxxxxxxx> wrote: > > > >> On 08.10.21 08:39, ultrachin@xxxxxxx wrote: > >>> From: chen xiaoguang <xiaoggchen@xxxxxxxxxxx> > >>> > >>> The exit time is long when program allocated big memory and > >>> the most time consuming part is free memory which takes 99.9% > >>> of the total exit time. By using async free we can save 25% of > >>> exit time. > >>> > >>> Signed-off-by: chen xiaoguang <xiaoggchen@xxxxxxxxxxx> > >>> Signed-off-by: zeng jingxiang <linuszeng@xxxxxxxxxxx> > >>> Signed-off-by: lu yihui <yihuilu@xxxxxxxxxxx> > >> > >> I recently discussed with Claudio if it would be possible to tear down > >> the process MM deferred, because for some use cases (secure/encrypted > >> virtualization, very large mmaps) tearing down the page tables is > >> already the much more expensive operation. > >> > >> There is mmdrop_async(), and I wondered if one could reuse that concept > >> when tearing down a process -- I didn't look into feasibility, however, > >> so it's just some very rough idea. > > > > I have done some experiments by unconditionally replacing mmdrop with > > mmdrop_async in exit.c and nothing broke, and exit time of large > > processes was almost instant (with the actual cleanup being performed in > > background) > > > > my approach is probably simpler/cleaner: > > > > diff --git a/include/asm-generic/mmu_context.h b/include/asm-generic/mmu_context.h > > index 91727065bacb..900931a6a105 100644 > > --- a/include/asm-generic/mmu_context.h > > +++ b/include/asm-generic/mmu_context.h > > @@ -73,4 +73,8 @@ static inline void deactivate_mm(struct task_struct *tsk, > > } > > #endif > > > > +#ifndef arch_exit_mm_mmput > > +#define arch_exit_mm_mmput mmput > > +#endif > > + > > #endif /* __ASM_GENERIC_MMU_CONTEXT_H */ > > diff --git a/kernel/exit.c b/kernel/exit.c > > index 9a89e7f36acb..604cb9c740fa 100644 > > --- a/kernel/exit.c > > +++ b/kernel/exit.c > > @@ -498,7 +498,7 @@ static void exit_mm(void) > > task_unlock(current); > > mmap_read_unlock(mm); > > mm_update_next_owner(mm); > > - mmput(mm); > > + arch_exit_mm_mmput(mm); > > if (test_thread_flag(TIF_MEMDIE)) > > exit_oom_victim(); > > } > > > > these are the minimal changes to common code, then each architecture can > > define their own arch_exit_mm_mmput as they see fit (for example, to free > > asynchronously only for certain classes of mm, like big ones, VMs, or so). > > > > Another option is to simply always replace mmput with mmput_async, which I > > expect will raise more eyebrows. > > Thanks Claudio. > > I guess we'd use some heuristic to keep the eyebrows down. Having > something like > > if (should_mput_async_on_exit(mm)) > mmput_async(mm); > else > mmput(mm); > > whereby the heuristic can optionally consult the arch/config-knobs/... > doesn't sound too wrong to me if it works. > yes, that is one of the possible solutions I had thought of. although probably the small patch I posted above is even less intrusive and should hopefully raise even fewer eyebrows, while also leaving the door open to arch-specific code to do more than just mmput_async, if needed. in the end I really do not have any preference, I simply want something everybody can agree on :)
