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

[Michal Hocko]

On Tue 06-08-19 07:19:21, Joel Fernandes wrote:
> On Tue, Aug 06, 2019 at 12:51:49PM +0200, Michal Hocko wrote:
> > 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.
> 
> Sure, will add.
> 
> 
> > > 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?
> 
> Not at the moment with the current page idle tracking code. PageLRU(page)
> flag is checked in page_idle_get_page().

yes, I am aware of the current code. I strongly suspect that the PageLRU
check was there to not mark arbitrary page looked up by pfn with the
idle bit because that would be unexpected. But I might be easily wrong
here.

> Even if we could set it for non-LRU, the idle bit (page flag) would not be
> cleared if page is not on LRU because page-reclaim code (page_referenced() I
> believe) would not clear it.

Yes, it is either reclaim when checking references as you say but also
mark_page_accessed. I believe the later might still have the page on the
pcp LRU add cache. Maybe I am missing something something but it seems
that there is nothing fundamentally requiring the user mapped page to be
on the LRU list when seting the idle bit.

That being said, your big hammer approach will work more reliable but if
you do not feel like changing the underlying PageLRU assumption then
document that draining should be removed longterm.
-- 
Michal Hocko
SUSE Labs