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charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Queue-Id: 6BD1EC001C X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: dzcxh6xmpcmrmbtmy9zhqryrqydjdn4p X-HE-Tag: 1710275395-586672 X-HE-Meta: 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 pxz2RQjT N/mboXRqSfQsCFD2FrcmZb0N6wM77AQ+aweA2PDtQ3eV5iPZb7jcIaLj3VNPhfzyx9d7sGbEFySO4/9ttisxtQzB+7mVUAOHu6S8PbOJxM8yzP5UDBmd3dYxUccJ8MJ5CIJtJfhTKierblJLsdeHtUuvX7A== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Mar 08, 2024 at 04:57:08PM +0800, Yafang Shao wrote: > On Fri, Mar 8, 2024 at 1:06 AM Andrew Morton wrote: > > > > On Thu, 7 Mar 2024 11:19:52 +0800 Yafang Shao wrote: > > > > > After we enabled mglru on our 384C1536GB production servers, we > > > encountered frequent soft lockups attributed to scanning folios. > > > > > > The soft lockup as follows, > > > > > > ... > > > > > > There were a total of 22 tasks waiting for this spinlock > > > (RDI: ffff99d2b6ff9050): > > > > > > crash> foreach RU bt | grep -B 8 queued_spin_lock_slowpath | grep "RDI: ffff99d2b6ff9050" | wc -l > > > 22 > > > > If we're holding the lock for this long then there's a possibility of > > getting hit by the NMI watchdog also. > > The NMI watchdog is disabled as these servers are KVM guest. > > kernel.nmi_watchdog = 0 > kernel.soft_watchdog = 1 > > > > > > Additionally, two other threads were also engaged in scanning folios, one > > > with 19 waiters and the other with 15 waiters. > > > > > > To address this issue under heavy reclaim conditions, we introduced a > > > hotfix version of the fix, incorporating cond_resched() in scan_folios(). > > > Following the application of this hotfix to our servers, the soft lockup > > > issue ceased. > > > > > > ... > > > > > > --- a/mm/vmscan.c > > > +++ b/mm/vmscan.c > > > @@ -4367,6 +4367,10 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc, > > > > > > if (!--remaining || max(isolated, skipped_zone) >= MIN_LRU_BATCH) > > > break; > > > + > > > + spin_unlock_irq(&lruvec->lru_lock); > > > + cond_resched(); > > > + spin_lock_irq(&lruvec->lru_lock); > > > } > > > > Presumably wrapping this with `if (need_resched())' will save some work. > > good suggestion. > > > > > This lock is held for a reason. I'd like to see an analysis of why > > this change is safe. > > I believe the key point here is whether we can reduce the scope of > this lock from: > > evict_folios > spin_lock_irq(&lruvec->lru_lock); > scanned = isolate_folios(lruvec, sc, swappiness, &type, &list); > scanned += try_to_inc_min_seq(lruvec, swappiness); > if (get_nr_gens(lruvec, !swappiness) == MIN_NR_GENS) > scanned = 0; > spin_unlock_irq(&lruvec->lru_lock); > > to: > > evict_folios > spin_lock_irq(&lruvec->lru_lock); > scanned = isolate_folios(lruvec, sc, swappiness, &type, &list); > spin_unlock_irq(&lruvec->lru_lock); > > spin_lock_irq(&lruvec->lru_lock); > scanned += try_to_inc_min_seq(lruvec, swappiness); > if (get_nr_gens(lruvec, !swappiness) == MIN_NR_GENS) > scanned = 0; > spin_unlock_irq(&lruvec->lru_lock); > > In isolate_folios(), it merely utilizes the min_seq to retrieve the > generation without modifying it. If multiple tasks are running > evict_folios() concurrently, it seems inconsequential whether min_seq > is incremented by one task or another. I'd appreciate Yu's > confirmation on this matter. Hi Yafang, Thanks for the patch! Yes, your second analysis is correct -- we can't just drop the lock as the original patch does because min_seq can be updated in the mean time. If this happens, the gen value becomes invalid, since it's based on the expired min_seq: sort_folio() { .. gen = lru_gen_from_seq(lrugen->min_seq[type]); .. } The following might be a better approach (untested): diff --git a/mm/vmscan.c b/mm/vmscan.c index 4255619a1a31..6fe53cfa8ef8 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -4365,7 +4365,8 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc, skipped_zone += delta; } - if (!--remaining || max(isolated, skipped_zone) >= MIN_LRU_BATCH) + if (!--remaining || max(isolated, skipped_zone) >= MIN_LRU_BATCH || + spin_is_contended(&lruvec->lru_lock)) break; } @@ -4375,7 +4376,8 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc, skipped += skipped_zone; } - if (!remaining || isolated >= MIN_LRU_BATCH) + if (!remaining || isolated >= MIN_LRU_BATCH || + (scanned && spin_is_contended(&lruvec->lru_lock))) break; }