Messages in this thread | | | Subject | Re: [v2 PATCH] mm: vmscan: correct nr_reclaimed for THP | From | Yang Shi <> | Date | Tue, 21 May 2019 11:16:50 +0800 |
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On 5/20/19 5:43 PM, Yang Shi wrote: > > > On 5/16/19 11:10 PM, Johannes Weiner wrote: >> On Tue, May 14, 2019 at 01:44:35PM -0700, Yang Shi wrote: >>> On 5/13/19 11:20 PM, Michal Hocko wrote: >>>> On Mon 13-05-19 21:36:59, Yang Shi wrote: >>>>> On Mon, May 13, 2019 at 2:45 PM Michal Hocko <mhocko@kernel.org> >>>>> wrote: >>>>>> On Mon 13-05-19 14:09:59, Yang Shi wrote: >>>>>> [...] >>>>>>> I think we can just account 512 base pages for nr_scanned for >>>>>>> isolate_lru_pages() to make the counters sane since >>>>>>> PGSCAN_KSWAPD/DIRECT >>>>>>> just use it. >>>>>>> >>>>>>> And, sc->nr_scanned should be accounted as 512 base pages too >>>>>>> otherwise we >>>>>>> may have nr_scanned < nr_to_reclaim all the time to result in >>>>>>> false-negative >>>>>>> for priority raise and something else wrong (e.g. wrong >>>>>>> vmpressure). >>>>>> Be careful. nr_scanned is used as a pressure indicator to slab >>>>>> shrinking >>>>>> AFAIR. Maybe this is ok but it really begs for much more explaining >>>>> I don't know why my company mailbox didn't receive this email, so I >>>>> replied with my personal email. >>>>> >>>>> It is not used to double slab pressure any more since commit >>>>> 9092c71bb724 ("mm: use sc->priority for slab shrink targets"). It >>>>> uses >>>>> sc->priority to determine the pressure for slab shrinking now. >>>>> >>>>> So, I think we can just remove that "double slab pressure" code. >>>>> It is >>>>> not used actually and looks confusing now. Actually, the "double slab >>>>> pressure" does something opposite. The extra inc to sc->nr_scanned >>>>> just prevents from raising sc->priority. >>>> I have to get in sync with the recent changes. I am aware there were >>>> some patches floating around but I didn't get to review them. I was >>>> trying to point out that nr_scanned used to have a side effect to be >>>> careful about. If it doesn't have anymore then this is getting much >>>> more >>>> easier of course. Please document everything in the changelog. >>> Thanks for reminding. Yes, I remembered nr_scanned would double slab >>> pressure. But, when I inspected into the code yesterday, it turns >>> out it is >>> not true anymore. I will run some test to make sure it doesn't >>> introduce >>> regression. >> Yeah, sc->nr_scanned is used for three things right now: >> >> 1. vmpressure - this looks at the scanned/reclaimed ratio so it won't >> change semantics as long as scanned & reclaimed are fixed in parallel >> >> 2. compaction/reclaim - this is broken. Compaction wants a certain >> number of physical pages freed up before going back to compacting. >> Without Yang Shi's fix, we can overreclaim by a factor of 512. >> >> 3. kswapd priority raising - this is broken. kswapd raises priority if >> we scan fewer pages than the reclaim target (which itself is obviously >> expressed in order-0 pages). As a result, kswapd can falsely raise its >> aggressiveness even when it's making great progress. >> >> Both sc->nr_scanned & sc->nr_reclaimed should be fixed. > > Yes, v3 patch (sit in my local repo now) did fix both. > >> >>> BTW, I noticed the counter of memory reclaim is not correct with THP >>> swap on >>> vanilla kernel, please see the below: >>> >>> pgsteal_kswapd 21435 >>> pgsteal_direct 26573329 >>> pgscan_kswapd 3514 >>> pgscan_direct 14417775 >>> >>> pgsteal is always greater than pgscan, my patch could fix the problem. >> Ouch, how is that possible with the current code? >> >> I think it happens when isolate_lru_pages() counts 1 nr_scanned for a >> THP, then shrink_page_list() splits the THP and we reclaim tail pages >> one by one. This goes all the way back to the initial THP patch! > > I think so. It does make sense. But, the weird thing is I just see > this with synchronous swap device (some THPs got swapped out in a > whole, some got split), but I've never seen this with rotate swap > device (all THPs got split). > > I haven't figured out why. > >> >> isolate_lru_pages() needs to be fixed. Its return value, nr_taken, is >> correct, but its *nr_scanned parameter is wrong, which causes issues: >> >> 1. The trace point, as Yang Shi pointed out, will underreport the >> number of pages scanned, as it reports it along with nr_to_scan (base >> pages) and nr_taken (base pages) >> >> 2. vmstat and memory.stat count 'struct page' operations rather than >> base pages, which makes zero sense to neither user nor kernel >> developers (I routinely multiply these counters by 4096 to get a sense >> of work performed). >> >> All of isolate_lru_pages()'s accounting should be in base pages, which >> includes nr_scanned and PGSCAN_SKIPPED. >> >> That should also simplify the code; e.g.: >> >> for (total_scan = 0; >> scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src); >> total_scan++) { >> >> scan < nr_to_scan && nr_taken >= nr_to_scan is a weird condition that >> does not make sense in page reclaim imo. Reclaim cares about physical >> memory - freeing one THP is as much progress for reclaim as freeing >> 512 order-0 pages. > > Yes, I do agree. The v3 patch did this. > >> >> IMO *all* '++' in vmscan.c are suspicious and should be reviewed: >> nr_scanned, nr_reclaimed, nr_dirty, nr_unqueued_dirty, nr_congested, >> nr_immediate, nr_writeback, nr_ref_keep, nr_unmap_fail, pgactivate, >> total_scan & scan, nr_skipped. > > Some of them should be fine but I'm not sure the side effect. IMHO, > let's fix the most obvious problem first.
A quick review shows we should correct nr_scanned, nr_reclaimed, pgactivate, nr_skipped, nr_ref_keep and nr_unmap_fail since they are user visible (via cgroup, /proc/vmstat or trace point) and the wrong number may confuse and mislead the users.
nr_dirty, nr_unqueued_dirty, nr_congested and nr_writeback are used by file cache, so they are not impacted by THP swap.
> >> >> Yang Shi, it would be nice if you could convert all of these to base >> page accounting in one patch, as it's a single logical fix for the >> initial introduction of THP that had huge pages show up on the LRUs.\ > > Yes, sure. > >> >> [ check_move_unevictable_pages() seems weird. It gets a pagevec from >> find_get_entries(), which, if I understand the THP page cache code >> correctly, might contain the same compound page over and over. It'll >> be !unevictable after the first iteration, so will only run once. So >> it produces incorrect numbers now, but it is probably best to ignore >> it until we figure out THP cache. Maybe add an XXX comment. ] >
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