Re: [PATCH v4 4/5] page_idle: Drain all LRU pagevec before idle tracking

[Michal Hocko <mhocko@kernel.org>]

On Tue 06-08-19 06:45:54, Joel Fernandes wrote:
> On Tue, Aug 06, 2019 at 10:43:57AM +0200, Michal Hocko wrote:
> > On Mon 05-08-19 13:04:50, Joel Fernandes (Google) wrote:
> > > During idle tracking, we see that sometimes faulted anon pages are in
> > > pagevec but are not drained to LRU. Idle tracking considers pages only
> > > on LRU. Drain all CPU's LRU before starting idle tracking.
> > 
> > Please expand on why does this matter enough to introduce a potentially
> > expensinve draining which has to schedule a work on each CPU and wait
> > for them to finish.
> 
> Sure, I can expand. I am able to find multiple issues involving this. One
> issue looks like idle tracking is completely broken. It shows up in my
> testing as if a page that is marked as idle is always "accessed" -- because
> it was never marked as idle (due to not draining of pagevec).
> 
> The other issue shows up as a failure in my "swap test", with the following
> sequence:
> 1. Allocate some pages
> 2. Write to them
> 3. Mark them as idle                                    <--- fails
> 4. Introduce some memory pressure to induce swapping.
> 5. Check the swap bit I introduced in this series.      <--- fails to set idle
>                                                              bit in swap PTE.
> 
> Draining the pagevec in advance fixes both of these issues.

This belongs to the changelog.

> This operation even if expensive is only done once during the access of the
> page_idle file. Did you have a better fix in mind?

Can we set the idle bit also for non-lru pages as long as they are
reachable via pte?
-- 
Michal Hocko
SUSE Labs