Hi, Gregory, Thanks for working on this! Gregory Price <gourry@xxxxxxxxxx> writes: > Unmapped page cache pages can be demoted to low-tier memory, but > they can presently only be promoted in two conditions: > 1) The page is fully swapped out and re-faulted > 2) The page becomes mapped (and exposed to NUMA hint faults) > > This RFC proposes promoting unmapped page cache pages by using > folio_mark_accessed as a hotness hint for unmapped pages. > > Patches 1-3 > allow NULL as valid input to migration prep interfaces > for vmf/vma - which is not present in unmapped folios. > Patch 4 > adds NUMA_HINT_PAGE_CACHE to vmstat > Patch 5 > adds the promotion mechanism, along with a sysfs > extension which defaults the behavior to off. > /sys/kernel/mm/numa/pagecache_promotion_enabled > > Functional test showed that we are able to reclaim some performance > in canned scenarios (a file gets demoted and becomes hot with > relatively little contention). See test/overhead section below. > > v2 > - cleanup first commit to be accurate and take Ying's feedback > - cleanup NUMA_HINT_ define usage > - add NUMA_HINT_ type selection macro to keep code clean > - mild comment updates > > Open Questions: > ====== > 1) Should we also add a limit to how much can be forced onto > a single task's promotion list at any one time? This might > piggy-back on the existing TPP promotion limit (256MB?) and > would simply add something like task->promo_count. > > Technically we are limited by the batch read-rate before a > TASK_RESUME occurs. > > 2) Should we exempt certain forms of folios, or add additional > knobs/levers in to deal with things like large folios? > > 3) We added NUMA_HINT_PAGE_CACHE to differentiate hint faults > so we could validate the behavior works as intended. Should > we just call this a NUMA_HINT_FAULT and not add a new hint? > > 4) Benchmark suggestions that can pressure 1TB memory. This is > not my typical wheelhouse, so if folks know of a useful > benchmark that can pressure my 1TB (768 DRAM / 256 CXL) setup, > I'd like to add additional measurements here. > > Development Notes > ================= > > During development, we explored the following proposals: > > 1) directly promoting within folio_mark_accessed (FMA) > Originally suggested by Johannes Weiner > https://lore.kernel.org/all/20240803094715.23900-1-gourry@xxxxxxxxxx/ > > This caused deadlocks due to the fact that the PTL was held > in a variety of cases - but in particular during task exit. > It also is incredibly inflexible and causes promotion-on-fault. > It was discussed that a deferral mechanism was preferred. > > > 2) promoting in filemap.c locations (calls of FMA) > Originally proposed by Feng Tang and Ying Huang > https://git.kernel.org/pub/scm/linux/kernel/git/vishal/tiering.git/patch/?id=5f2e64ce75c0322602c2ec8c70b64bb69b1f1329 > > First, we saw this as less problematic than directly hooking FMA, > but we realized this has the potential to miss data in a variety of > locations: swap.c, memory.c, gup.c, ksm.c, paddr.c - etc. > > Second, we discovered that the lock state of pages is very subtle, > and that these locations in filemap.c can be called in an atomic > context. Prototypes lead to a variety of stalls and lockups. > > > 3) a new LRU - originally proposed by Keith Busch > https://git.kernel.org/pub/scm/linux/kernel/git/kbusch/linux.git/patch/?id=6616afe9a722f6ebedbb27ade3848cf07b9a3af7 > > There are two issues with this approach: PG_promotable and reclaim. > > First - PG_promotable has generally be discouraged. > > Second - Attach this mechanism to an LRU is both backwards and > counter-intutive. A promotable list is better served by a MOST > recently used list, and since LRUs are generally only shrank when > exposed to pressure it would require implementing a new promotion > list shrinker that runs separate from the existing reclaim logic. > > > 4) Adding a separate kthread - suggested by many > > This is - to an extent - a more general version of the LRU proposal. > We still have to track the folios - which likely requires the > addition of a page flag. Additionally, this method would actually > contend pretty heavily with LRU behavior - i.e. we'd want to > throttle addition to the promotion candidate list in some scenarios. > > > 5) Doing it in task work > > This seemed to be the most realistic after considering the above. > > We observe the following: > - FMA is an ideal hook for this and isolation is safe here > - the new promotion_candidate function is an ideal hook for new > filter logic (throttling, fairness, etc). > - isolated folios are either promoted or putback on task resume, > there are no additional concurrency mechanics to worry about > - The mechanic can be made optional via a sysfs hook to avoid > overhead in degenerate scenarios (thrashing). > > We also piggy-backed on the numa_hint_fault_latency timestamp to > further throttle promotions to help avoid promotions on one or > two time accesses to a particular page. > > > Test: > ====== > > Environment: > 1.5-3.7GHz CPU, ~4000 BogoMIPS, > 1TB Machine with 768GB DRAM and 256GB CXL > A 64GB file being linearly read by 6-7 Python processes > > Goal: > Generate promotions. Demonstrate stability and measure overhead. > > System Settings: > echo 1 > /sys/kernel/mm/numa/demotion_enabled > echo 1 > /sys/kernel/mm/numa/pagecache_promotion_enabled > echo 2 > /proc/sys/kernel/numa_balancing > > Each process took up ~128GB each, with anonymous memory growing and > shrinking as python filled and released buffers with the 64GB data. > This causes DRAM pressure to generate demotions, and file pages to > "become hot" - and therefore be selected for promotion. > > First we ran with promotion disabled to show consistent overhead as > a result of forcing a file out to CXL memory. We first ran a single > reader to see uncontended performance, launched many readers to force > demotions, then droppedb back to a single reader to observe. > > Single-reader DRAM: ~16.0-16.4s > Single-reader CXL (after demotion): ~16.8-17s The difference is trivial. This makes me thought that why we need this patchset? > Next we turned promotion on with only a single reader running. > > Before promotions: > Node 0 MemFree: 636478112 kB > Node 0 FilePages: 59009156 kB > Node 1 MemFree: 250336004 kB > Node 1 FilePages: 14979628 kB Why are there some many file pages on node 1 even if there're a lot of free pages on node 0? You moved some file pages from node 0 to node 1? > After promotions: > Node 0 MemFree: 632267268 kB > Node 0 FilePages: 72204968 kB > Node 1 MemFree: 262567056 kB > Node 1 FilePages: 2918768 kB > > Single-reader (after_promotion): ~16.5s > > Turning the promotion mechanism on when nothing had been demoted > produced no appreciable overhead (memory allocation noise overpowers it) > > Read time did not change after turning promotion off after promotion > occurred, which implies that the additional overhead is not coming from > the promotion system itself - but likely other pages still trapped on > the low tier. Either way, this at least demonstrates the mechanism is > not particularly harmful when there are no pages to promote - and the > mechanism is valuable when a file actually is quite hot. > > Notability, it takes some time for the average read loop to come back > down, and there still remains unpromoted file pages trapped in pagecache. > This isn't entirely unexpected, there are many files which may have been > demoted, and they may not be very hot. > > > Overhead > ====== > When promotion was tured on we saw a loop-runtime increate temporarily > > before: 16.8s > during: > 17.606216192245483 > 17.375206470489502 > 17.722095489501953 > 18.230552434921265 > 18.20712447166443 > 18.008254528045654 > 17.008427381515503 > 16.851454257965088 > 16.715774059295654 > stable: ~16.5s > > We measured overhead with a separate patch that simply measured the > rdtsc value before/after calls in promotion_candidate and task work. > > e.g.: > + start = rdtsc(); > list_for_each_entry_safe(folio, tmp, promo_list, lru) { > list_del_init(&folio->lru); > migrate_misplaced_folio(folio, NULL, nid); > + count++; > } > + atomic_long_add(rdtsc()-start, &promo_time); > + atomic_long_add(count, &promo_count); > > numa_migrate_prep: 93 - time(3969867917) count(42576860) > migrate_misplaced_folio_prepare: 491 - time(3433174319) count(6985523) > migrate_misplaced_folio: 1635 - time(11426529980) count(6985523) > > Thoughts on a good throttling heuristic would be appreciated here. We do have a throttle mechanism already, for example, you can used $ echo 100 > /proc/sys/kernel/numa_balancing_promote_rate_limit_MBps to rate limit the promotion throughput under 100 MB/s for each DRAM node. > Suggested-by: Huang Ying <ying.huang@xxxxxxxxxxxxxxxxx> > Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx> > Suggested-by: Keith Busch <kbusch@xxxxxxxx> > Suggested-by: Feng Tang <feng.tang@xxxxxxxxx> > Signed-off-by: Gregory Price <gourry@xxxxxxxxxx> > > Gregory Price (5): > migrate: Allow migrate_misplaced_folio_prepare() to accept a NULL VMA. > memory: move conditionally defined enums use inside ifdef tags > memory: allow non-fault migration in numa_migrate_check path > vmstat: add page-cache numa hints > migrate,sysfs: add pagecache promotion > > .../ABI/testing/sysfs-kernel-mm-numa | 20 ++++++ > include/linux/memory-tiers.h | 2 + > include/linux/migrate.h | 2 + > include/linux/sched.h | 3 + > include/linux/sched/numa_balancing.h | 5 ++ > include/linux/vm_event_item.h | 8 +++ > init/init_task.c | 1 + > kernel/sched/fair.c | 26 +++++++- > mm/memory-tiers.c | 27 ++++++++ > mm/memory.c | 32 +++++----- > mm/mempolicy.c | 25 +++++--- > mm/migrate.c | 61 ++++++++++++++++++- > mm/swap.c | 3 + > mm/vmstat.c | 2 + > 14 files changed, 193 insertions(+), 24 deletions(-) --- Best Regards, Huang, Ying
