From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta1.migadu.com (out-171.mta1.migadu.com [95.215.58.171]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 732C9238C3B for ; Thu, 23 Apr 2026 16:43:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776962610; cv=none; b=GI4vZuPHq9WU7EJDCWwW+dy5vRapqz6E93iCmpOgoaqlo40XKFiDhrldFxSXRPzjl1j1YHq707wphcyWQbBChzcDrslD4aCv/AjX2fnu3Idi9ALyJHWXL/5PjEnvHivfVU3YxRNLJl7GKEiRjbsQ+xfohd+zRsctcxYin9zCk3k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776962610; c=relaxed/simple; bh=WL9leTlja1c8/ThhV8yGvn8fzDMDijlZJbYf6Fi/ooM=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=tOOwFPlT+YZjga92U80ROv+VTHwpPunxHFAJc1GMC5vOIUs0aC3fOPDxhZc05NuaBMgy2fhsVzJhRbhRu604n5Nu9nEPPbkfiCrojGlWKBPGWNL+j+w/dyp4fiMdbufliVnDmUCsDiW9BWxgmbmbRb8prgIRWbIjYbWDh6BMeP4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=uiOc+nuX; arc=none smtp.client-ip=95.215.58.171 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="uiOc+nuX" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1776962606; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=YIEKJZNSJ9OeiPhU0FRTZ02fkQczRvgGzar+b8miLXs=; b=uiOc+nuXzSkMncpve1zOJ6r496DTBRy1/LeDDBTNkjhQpJo2WLQS+blMYN8XGaT3LahYpP hWqco+oyHmjT0KloyqDD2j/TW7JejM/aS+RULQ+E1dvoy822/ISmZPAg6AtWQBSsHYD7QI nssqrwS/t1gyYa2StGfPWMMWSBvuKWg= From: "JP Kobryn (Meta)" To: linux-mm@kvack.org, akpm@linux-foundation.org, vbabka@kernel.org, mhocko@suse.com, willy@infradead.org, hannes@cmpxchg.org, shakeel.butt@linux.dev, riel@surriel.com, chrisl@kernel.org, kasong@tencent.com, shikemeng@huaweicloud.com, nphamcs@gmail.com, bhe@redhat.com, baohua@kernel.org, youngjun.park@lge.com, qi.zheng@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: [PATCH] mm/lruvec: preemptively free dead folios during lru_add drain Date: Thu, 23 Apr 2026 09:43:07 -0700 Message-ID: <20260423164307.29805-1-jp.kobryn@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT Of all observable lruvec lock contention in our fleet, we find that ~24% occurs when dead folios are present in lru_add batches at drain time. This is wasteful in the sense that the folio is added to the LRU just to be immediately removed via folios_put_refs(), incurring two unnecessary lock acquisitions. Eliminate this overhead by preemptively cleaning up dead folios before they make it into the LRU. Use folio_ref_freeze() to filter folios whose only remaining refcount is the batch ref. When dead folios are found, move them off the add batch and onto a temporary batch to be freed. During A/B testing on one of our prod instagram workloads (high-frequency short-lived requests), the patch intercepted almost all dead folios before they entered the LRU. Data collected using the mm_lru_insertion tracepoint shows the effectiveness of the patch: Per-host LRU add averages at 95% CPU load (60 hosts each side, 3 x 60s intervals) dead folios/min total folios/min dead % unpatched: 1,297,785 19,341,986 6.7097% patched: 14 19,039,996 0.0001% Within this workload, we save ~2.6M lock acquisitions per minute per host as a result. System-wide memory stats improved on the patched side also at 95% CPU load: - direct reclaim scanning reduced 7% - allocation stalls reduced 5.2% - compaction stalls reduced 12.3% - page frees reduced 4.9% No regressions were observed in requests served per second or request tail latency (p99). Both metrics showed directional improvement at higher CPU utilization (comparing 85% to 95%). Signed-off-by: JP Kobryn (Meta) --- mm/swap.c | 36 +++++++++++++++++++++++++++++++++++- 1 file changed, 35 insertions(+), 1 deletion(-) diff --git a/mm/swap.c b/mm/swap.c index 5cc44f0de9877..71607b0ce3d18 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -160,13 +160,36 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn) int i; struct lruvec *lruvec = NULL; unsigned long flags = 0; + struct folio_batch free_fbatch; + bool is_lru_add = (move_fn == lru_add); + + /* + * If we're adding to the LRU, preemptively filter dead folios. Use + * this dedicated folio batch for temp storage and deferred cleanup. + */ + if (is_lru_add) + folio_batch_init(&free_fbatch); for (i = 0; i < folio_batch_count(fbatch); i++) { struct folio *folio = fbatch->folios[i]; /* block memcg migration while the folio moves between lru */ - if (move_fn != lru_add && !folio_test_clear_lru(folio)) + if (!is_lru_add && !folio_test_clear_lru(folio)) + continue; + + /* + * Filter dead folios by moving them from the add batch to the temp + * batch for freeing after this loop. + * + * Since the folio may be part of a huge page, unqueue from + * deferred split list to avoid a dangling list entry. + */ + if (is_lru_add && folio_ref_freeze(folio, 1)) { + folio_unqueue_deferred_split(folio); + fbatch->folios[i] = NULL; + folio_batch_add(&free_fbatch, folio); continue; + } folio_lruvec_relock_irqsave(folio, &lruvec, &flags); move_fn(lruvec, folio); @@ -176,6 +199,13 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn) if (lruvec) lruvec_unlock_irqrestore(lruvec, flags); + + /* Cleanup filtered dead folios. */ + if (is_lru_add) { + mem_cgroup_uncharge_folios(&free_fbatch); + free_unref_folios(&free_fbatch); + } + folios_put(fbatch); } @@ -964,6 +994,10 @@ void folios_put_refs(struct folio_batch *folios, unsigned int *refs) struct folio *folio = folios->folios[i]; unsigned int nr_refs = refs ? refs[i] : 1; + /* Folio batch entry may have been preemptively removed during drain. */ + if (!folio) + continue; + if (is_huge_zero_folio(folio)) continue; -- 2.52.0