Re: [External] Re: [PATCH v2] mm/vmscan: fix infinite loop in drop_slab_node

[Michal Hocko]

On Mon 14-09-20 23:02:15, Chunxin Zang wrote:
> On Mon, Sep 14, 2020 at 9:47 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> 
> > On Mon 14-09-20 21:25:59, Chunxin Zang wrote:
> > > On Mon, Sep 14, 2020 at 5:30 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > > The subject is misleading because this patch doesn't fix an infinite
> > > > loop, right? It just allows the userspace to interrupt the operation.
> > > >
> > > >
> > > Yes,  so we are making a separate patch follow Vlastimil's
> > recommendations.
> > > Use double of threshold to end the loop.
> >
> > That still means the changelog needs an update
> >
> 
> The patch is already merged in Linux-next branch.  Can I update the
> changelog now?

Yes. Andrew will refresh it. He doesn't have a git tree which would
prevent rewriting the patch.

> This is my first patch, please forgive me :)

No worries. The mm patch workflow is rather different from others.

> > > On Thu, Sep 10, 2020 at 1:59 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
> > > > > From: Chunxin Zang <zangchunxin@xxxxxxxxxxxxx>
> > > > >
> > > > ...
> > > > - IMHO it's still worth to bail out in your scenario even without a
> > > > signal, e.g.
> > > > by the doubling of threshold. But it can be a separate patch.
> > > > Thanks!
> > > > ...
> > >
> > >
> > >
> > > On Wed 09-09-20 23:20:47, zangchunxin@xxxxxxxxxxxxx wrote:
> > > > > From: Chunxin Zang <zangchunxin@xxxxxxxxxxxxx>
> > > > >
> > > > > On our server, there are about 10k memcg in one machine. They use
> > memory
> > > > > very frequently. When I tigger drop caches，the process will infinite
> > loop
> > > > > in drop_slab_node.
> > > >
> > > > Is this really an infinite loop, or it just takes a lot of time to
> > > > process all the metadata in that setup? If this is really an infinite
> > > > loop then we should look at it. My current understanding is that the
> > > > operation would finish at some time it just takes painfully long to get
> > > > there.
> > > >
> > >
> > > Yes,  it's really an infinite loop.  Every loop spends a lot of time. In
> > > this time,
> > > memcg will alloc/free memory,  so the next loop, the total of  'freed'
> > > always bigger than 10.
> >
> > I am still not sure I follow. Do you mean that there is somebody
> > constantly generating more objects to reclaim?
> >
> 
> Yes, this is my meaning. :)
> 
> 
> > Maybe we are just not agreeing on the definition of an infinite loop but
> > in my book that means that the final condition can never be met. While a
> > busy adding new object might indeed cause drop caches to loop for a long
> > time this is to be expected from that interface as it is supposed to
> > drop all the cache and that can grow during the operation.
> > --
> >
> 
> Because I have 10k memcg , all of them are heavy users of memory.
> During each loop, there are always more than 10 reclaimable objects
> generating, so the
> condition is never met.

10k or any number of memcgs shouldn't really make much of a difference.
Except for the time the scan adds. Fundamentally we are talking about
freed objects and whether they are on the global or per memcg lists
should result in a similar behavior.

> The drop cache process has no chance to exit the
> loop.
> Although the purpose of the 'drop cache' interface is to release all
> caches, we still need a
> way to terminate it, e.g. in this case, the process took too long to run .

Yes, this is perfectly understandable. Having a bail out on fatal signal
is a completely reasonable thing to do. I am mostly confused by your
infinite loop claims and what the relation of this patch to it. I would
propose this wording instead

"
We have observed that drop_caches can take a considerable amount of
time (<put data here>). Especially when there are many memcgs involved
because they are adding an additional overhead.

It is quite unfortunate that the operation cannot be interrupted by a
signal currently. Add a check for fatal signals into the main loop
so that userspace can control early bailout.
"

or something along those lines.

> 
>   root  357956 ... R    Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches

-- 
Michal Hocko
SUSE Labs