Re: [RFC PATCH] mm: have kswapd only reclaiming use min protection on memcg

[Zhaoyang Huang]

On Wed, Oct 27, 2021 at 8:31 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 27-10-21 20:05:30, Zhaoyang Huang wrote:
> > On Wed, Oct 27, 2021 at 7:52 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Wed 27-10-21 17:19:56, Zhaoyang Huang wrote:
> > > > On Wed, Oct 27, 2021 at 4:26 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > >
> > > > > On Wed 27-10-21 15:46:19, Zhaoyang Huang wrote:
> > > > > > On Wed, Oct 27, 2021 at 3:20 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Wed 27-10-21 15:01:50, Huangzhaoyang wrote:
> > > > > > > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > > > > > > >
> > > > > > > > For the kswapd only reclaiming, there is no chance to try again on
> > > > > > > > this group while direct reclaim has. fix it by judging gfp flag.
> > > > > > >
> > > > > > > There is no problem description (same as in your last submissions. Have
> > > > > > > you looked at the patch submission documentation as recommended
> > > > > > > previously?).
> > > > > > >
> > > > > > > Also this patch doesn't make any sense. Both direct reclaim and kswapd
> > > > > > > use a gfp mask which contains __GFP_DIRECT_RECLAIM (see balance_pgdat
> > > > > > > for the kswapd part)..
> > > > > > ok, but how does the reclaiming try with memcg's min protection on the
> > > > > > alloc without __GFP_DIRECT_RECLAIM?
> > > > >
> > > > > I do not follow. There is no need to protect memcg if the allocation
> > > > > request doesn't have __GFP_DIRECT_RECLAIM because that would fail the
> > > > > charge if a hard limit is reached, see try_charge_memcg and
> > > > > gfpflags_allow_blocking check.
> > > > >
> > > > > Background reclaim, on the other hand never breaches reclaim protection.
> > > > >
> > > > > What is the actual problem you want to solve?
> > > > Imagine there is an allocation with gfp_mask & ~GFP_DIRECT_RECLAIM and
> > > > all processes are under cgroups. Kswapd is the only hope here which
> > > > however has a low efficiency of get_scan_count. I would like to have
> > > > kswapd work as direct reclaim in 2nd round which will have
> > > > protection=memory.min.
> > >
> > > Do you have an example where this would be a practical problem? Atomic
> > > allocations should be rather rare.
> > Please find below for the search result of '~__GFP_DIRECT_RECLAIM'
> > which shows some drivers and net prefer to behave like that.
> > Furthermore, the allocations are always together with high order.
>
> And what is the _practical_ problem you are seeing or trying to solve?
We do have out of tree code behave like this and want to make the
mechanics more robust
>
> --
> Michal Hocko
> SUSE Labs