From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 B25633537FA for ; Thu, 23 Apr 2026 17:15:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776964536; cv=none; b=DUa5pXQXfEApnGja9vrjcUiwa10y0Y2iDLDBpE9TZhSRe+OjKqyKWhtfYb+Da5ldvv3QTTfCgQv8eqM/OrdK2dIkjU7grXJgdCYvzcJIS5Fm2eturPU0C0S9HfasFZyLyPxM6abGiKFnATfcjVp0Bbi1i1Pg+uaxNaAz1xG0ldQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776964536; c=relaxed/simple; bh=1TolvHI6ohyJM33w3hwS73V5/Cry5gQ828AxNdigw/A=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=UdJy9gQ09dRSnM8VRDypndzvcteqgJHseCsN4kuz1uDqKwNy8XZWiCdOeX98CWzXKvmQYngIa68sOAy02DBqjvqo1GX2uyqamKt7qmMRCKCsqOqIriT1IOGDOMq/Ha7a6zjnCVDKG/kX+oF9jxdfIEslFvlMpYOd0pRkpMDVePw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=IWyrGBjX; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="IWyrGBjX" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=DFftlaJQns74Njz4lBEQZzjlwXSymXeQGxv3fxsHDy0=; b=IWyrGBjX3QQb/kuGhdQ9SF9tUA UyiHzbtFTvSQcC4vmEx12CvFU7wYB5/d2YoBfTA2IXn5cX2dvBk9OXbEvqTgmysugmKVjqYGs/wpE f6Co9AV1AU9Su2qSHiBaie7loJt6lH9SaPVJUqZztwhSVwCS5Z8ZBJZjUqOUlkwqs+a+bx6svuMoi oOfarXxZNTAQfanvLbA/Po+tGSK8kL7/UMmIEpUNa8XCDXrdlE6ZtFxzFaIqrg/jWQnzMMEYCJNRp MV5xwSgLfwZ8bpffbhj/ygmw352fOh1zV0wPe8w8O6QaLx9myZlQvIaJu1HsTIgOVz+Ks1L93myMN DBCtp+cA==; Received: from willy by casper.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1wFxds-0000000Dmh7-1YQV; Thu, 23 Apr 2026 17:15:08 +0000 Date: Thu, 23 Apr 2026 18:15:08 +0100 From: Matthew Wilcox To: "JP Kobryn (Meta)" Cc: linux-mm@kvack.org, akpm@linux-foundation.org, vbabka@kernel.org, mhocko@suse.com, 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, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH] mm/lruvec: preemptively free dead folios during lru_add drain Message-ID: References: <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-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260423164307.29805-1-jp.kobryn@linux.dev> On Thu, Apr 23, 2026 at 09:43:07AM -0700, JP Kobryn (Meta) wrote: > 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. Well, this is a lovely patch with no obvious downsides. Nicely done. > 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); Would it be better to do this outside the lru lock; it's just that we don't have a convenient batched version to do it? It seems like there are a few places that could use a batched version in vmscan.c and swap.c. Not that I think we should hold up this patch to investigate that micro-optimisation! Just something you couldlook at as a follow-up. Reviewed-by: Matthew Wilcox (Oracle)