Re: [PATCH] mm: Free per cpu pages async to shorten program exit time

[David Hildenbrand]

On 08.10.21 11:22, Claudio Imbrenda wrote:
> 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.

More flexibility might raise more eyebrows. :)

--
Thanks,

David / dhildenb