Re: Memory CG and 5.1 to 5.6 uprade slows backup

["Bruno Prémont" <bonbons-ud5FBsm0p/xEiooADzr8i9i2O/JbrIOy@public.gmane.org>]

On Wed, 15 Apr 2020 12:24:42 Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
> On Wed 15-04-20 12:17:53, Bruno Prémont wrote:
> [...]
> > > Anyway the following simply tracing patch should give a better clue.
> > > The output will appear in the trace buffer (mount tracefs and read
> > > trace_pipe file).  
> > 
> > This is the output I get on 5.6.4 with simple tar -zc call (max=high+4096):
> >   tar-16943 [000] ....  1098.796955: mem_cgroup_handle_over_high: memcg_nr_pages_over_high:1 penalty_jiffies:200 current:262122 high:262144
> >   tar-16943 [000] ....  1100.876794: mem_cgroup_handle_over_high: memcg_nr_pages_over_high:1 penalty_jiffies:200 current:262122 high:262144
> >   tar-16943 [000] ....  1102.956636: mem_cgroup_handle_over_high: memcg_nr_pages_over_high:1 penalty_jiffies:200 current:262120 high:262144
> >   tar-16943 [000] ....  1105.037388: mem_cgroup_handle_over_high: memcg_nr_pages_over_high:1 penalty_jiffies:200 current:262121 high:262144
> >   tar-16943 [000] ....  1107.117246: mem_cgroup_handle_over_high: memcg_nr_pages_over_high:1 penalty_jiffies:200 current:262122 high:262144  
> 
> OK, that points to the underflow fix.
> 
> > 
> > With 5.7-rc1 it runs just fine, pressure remains zero and no output in trace_pipe or throttling.
> > 
> > So the fixes that went in there do fix it.
> > Now matter of cherry-picking the right ones... e26733e0d0ec and its fixe's fix,
> > maybe some others (will start with those tagged for stable).  
> 
> I have seen Greg picking up this for stable trees so it should show up
> there soon.

Applying just 9b8b17541f13809d06f6f873325305ddbb760e3e which went to
stable-rc for 5.6.5 gets things running fine where.
(e26733e0d0ec seems to have gone in shortly prior to 5.6 release, need
to improve my git-foo to locate commits between tags!)

So yes it's the fix.

Thanks,
Bruno

> Thanks!