Re: [PATCH RFC] delayacct: add memory reclaim delay in get_page_from_freelist

[Education Directorate <bsingharora@gmail.com>]

On Tue, Sep 05, 2023 at 05:32:15AM +0000, liwenyu01@bilibili.com wrote:
> >> reclaim of the task in do_try_to_free_pages(). In systems with NUMA
> >> open, some tasks occasionally experience slower response times, but the
> >> total count of reclaim does not increase, using ftrace can show that
> >> node_reclaim has occurred.
> >>
> >> The memory reclaim occurring in get_page_from_freelist() is also due to
> >> heavy memory load. To get the impact of tasks in memory reclaim, this
> >> patch adds the statistics of the memory reclaim delay statistics for
> >> __node_reclaim().
> >>
> >> ...
> >>
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -8010,6 +8010,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> >>
> >>         cond_resched();
> >>         psi_memstall_enter(&pflags);
> >> +       delayacct_freepages_start();
> >>         fs_reclaim_acquire(sc.gfp_mask);
> >>         /*
> >>          * We need to be able to allocate from the reserves for RECLAIM_UNMAP
> >> @@ -8032,6 +8033,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> >>         memalloc_noreclaim_restore(noreclaim_flag);
> >>         fs_reclaim_release(sc.gfp_mask);
> >>         psi_memstall_leave(&pflags);
> >> +       delayacct_freepages_end();
> >>
> >>         trace_mm_vmscan_node_reclaim_end(sc.nr_reclaimed);
> >
> > __node_reclaim() calls shrink_node() which at some point will call
> > do_try_to_free_pages() (yes?), which calls delayacct_freepages_start().
> >
> > So we're effectively nesting calls to delayacct_freepages_start(),
> > which isn't designed for that?
> >
> sorry, the last reply was a mistake.
> 
> It seems that no point in shrink_node() will call do_try_to_free_pages().
> And do_try_to_free_pages() will call shrink_node() through shrink_zones(),
> if shrink_node() also has some point will call do_try_to_free_pages,then
> delayacct_freepages_start() is nested now?

That's because shrink_node() goes through shrink_list() via
shrink_lruvec()? do_try_to_free_pages() will call shrink_node(). Ideally
we should have some counters around __node_reclaim() and balance_pgdat()
like psi_memstall_* does. Do you want to mimic what psi_memstall_* does?
This would change the definition of delayacct free pages, but I don't think
it will make it worse.

Balbir Singh