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?
This is my first patch, please forgive me :)
> 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.
--
During each loop, there are always more than 10 reclaimable objects generating, so the
condition is never met. 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 .
root 357956 ... R Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches
