Re: [PATCH v2] Revert "mm: skip CMA pages when they are not available"

[Barry Song]

On Wed, Mar 20, 2024 at 1:27 AM 刘海龙(LaoLiu) <liuhailong@xxxxxxxx> wrote:
>
> On 2024/3/19 19:09, Barry Song wrote:
> > On Tue, Mar 19, 2024 at 4:56 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >>
> >> On Fri 15-03-24 16:18:03, liuhailong@xxxxxxxx wrote:
> >>> From: "Hailong.Liu" <liuhailong@xxxxxxxx>
> >>>
> >>> This reverts
> >>> commit b7108d66318a ("Multi-gen LRU: skip CMA pages when they are not eligible")
> >>> commit 5da226dbfce3 ("mm: skip CMA pages when they are not available")
> >>>
> >>> skip_cma may cause system not responding. if cma pages is large in lru_list
> >>> and system is in lowmemory, many tasks would direct reclaim and waste
> >>> cpu time to isolate_lru_pages and return.
> >>>
> >>> Test this patch on android-5.15 8G device
> >>> reproducer:
> >>> - cma_declare_contiguous 3G pages
> >>> - set /proc/sys/vm/swappiness 0 to enable direct_reclaim reclaim file
> >>>   only.
> >>> - run a memleak process in userspace
> >>
> >> Does this represent a sane configuration? CMA memory is unusable for
> >> kernel allocations and memleak process is also hard to reclaim due to
> >> swap suppression. Isn't such a system doomed to struggle to reclaim any
> >> memory?
> Yes, All processes in the system are also hard to reclaim. and all processes
> enter direct reclaim. with this patch, much of process which should skip_cma
> would retry, scan, skipped in the process of isolsate_lru_pages. and system
> process will have high priority, some normal processes (like kswapd) are
> preempted.
>
>
> Btw. how does the same setup behave with the regular LRU
> >> implementation? My guess would be that it would struggle as well.
> >
> > I assume the regular LRU implementation you are talking about is the LRU
> > without skip_cma()?
> >
> > I remember Hailong mentioned something like " it also trigger memory psi
> > event to allow admin do something to release memory" and " without
> > patch the devices would kill camera process".  So it seems the difference
> > is if a killing will occur.
> >
> > Hailong, would you like to provide more detail?
>
> psi_event triggered after psi_memstall_leave. much system processes
> perform_reclaim scan and skipped and leave without reclaim any pages.
> the process is fast, so lmkd (userspace lowmemory killer) could not work
> as before.

Can you provide more details?

Does PSI still give accurate reports when processes are stuck in an idle loop
because most pages in the LRU list are in CMA? Also, in such a case, will
the LMKD not receive CPU resources to execute?

>
> >
> >> --
> >> Michal Hocko
> >> SUSE Labs
> >>
>