From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794513362; b=Ppya1c/lIq+wVJlZO7blk57RaxZgABjcx1ayr1mQJ6/fmP9LImtuG/AALwony+uzaGtv bzLOS1cWPgYvaC0spgbIsGKpWrhbMfMzTvl0aFJuLx9ssq8YaX+8niyP/+GRS0pYKvq0c ecb7xzwI8EPcjp2EcCz/BH30LC1i4thnH/8rpy0+g8XozafV8J93IsHr0X11CpjfrkMLM 7+FPt73+ODJMX4WSyM074rQ+Lpkzc+zcLPo59JE4C18B3UpX2avGCdmP49VLzTRPBe1hU 49ajVE9ttATKDb6thz5BdVAtKhadzJwamQRR4L9KNpDm25MC+yP0024aT6y8OLyszpA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794513362; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=/DVz9TcMzmRhI4NAq0Bx3DDA+PaOD3fbYpa1xG77J98=; b=FMN6vQf4G3DJAquDMSmLuNOSPrrf7LqipLHJ4yG/jK4CyBQK5L6fLCii5NFGUuRGqlYP nU+IVgUz3ryzCPBl+MlzTe78rKESDm38Yk9c78jvnejBbASAExSVYpnvC2eX7nWlF4IZu XbaiFO4/BvfjvEke4hoFdXvOyxvQV7Orlecsvt57x/l5y7X2srJayzgXWItfgj28v2YxG zbg1fVsp8V7TgHxftB3QpfETyDqQloz+yOt82O0L3YIZinVdFg6vJuurI2fIxgpQGXUd8 qbn2Aa0byEZAoAv3ODEIdx+9g4weWrTG/lsOGfGlbpjwGOmi+hwLRcIlfG/8tm6TJvg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=eXTAheRB; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 38932606A31C for ; Thu, 12 Nov 2026 19:56:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=eXTAheRB Received: by smtp.kernel.org (Postfix) with ESMTPSA id 04C4D1F000C4; Thu, 12 Nov 2026 19:55:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794513362; bh=/DVz9TcMzmRhI4NAq0Bx3DDA+PaOD3fbYpa1xG77J98=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=eXTAheRBiXvSIWFMTLXBhb3k8aCTQf5sIn3X3vLWvl1WBSbLRGNP0YplzVv8SOypX DtGLthT7Qi5esc4ctzgtSrhoMOXpbVbotkhM1fjdltQjdvwgDWZ3gWbJsXmClNMsP+ cHZHoB2RkkveFBPonlCc0X23P8hmYiUX9BJw+Ybhxpkxj4ZqmxM8+47qtcuKUEmxwz dMIinoOq+ie1zHHAlNURtxmtlx5c6Jd68l4BgLCYGA4T1IKB7TB63JUWJjkXZzgOGj EtAX4DJOCTWU4tTjW5G9+Ps+vqqTQX0xqbnO6PEP0rFdsmWT7aTSwGFUxij5TpRspd HNWbvB8Y5tJRA== Date: Thu, 12 Nov 2026 19:55:56 +0000 From: "Lorenzo Stoakes (ARM)" To: Joshua Hahn Cc: Johannes Weiner , Gregory Price , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Zi Yan , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH v2] mm/page_alloc: only skip kswapd nodes below their kswapd watermark Message-ID: References: <20261110224432.2347606-1-joshua.hahnjy@gmail.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261110224432.2347606-1-joshua.hahnjy@gmail.com> On Tue, Nov 10, 2026 at 02:44:31PM -0800, Joshua Hahn wrote: > Commit 19c5fb83f2a4 ("mm: page_alloc: avoid kswapd thrashing due to > NUMA restrictions") makes get_page_from_freelist() skip any node with an > active kswapd on the first pass through the zonelist, so unrestricted > allocations don't immediately consume the memory kswapd is freeing. > > But kswapd is also woken for high-order requests. With THP defrag set to > defer, each THP fault that fails on a fragmented node wakes kswapd at > PMD order. kswapd reclaims a batch, wakes kcompactd, and sleeps until > the next failed fault, so it stays awake much of the time even on a node > with plenty of free order-0 memory. This means those order-0 allocations > get directed elsewhere. > > This was observed on some Meta production workloads running on 2-NUMA > systems, where processes with a preference for node0 allocations > continuously allocated order-0 pages on node1, despite node0 having > hundreds of GB of free memory. > > Only skip a zone with an active kswapd if it is also below kswapd's > target watermark. > > On a 2-NUMA VM with a 60G node0 and a 20G node1, with THP enabled=always > and defrag=defer, I ran a process that prefers allocating on node0. > With a fragmented node0: > - Before the patch, 68% of pages were served from node0 > - After the patch, 100% of pages were served from node0 > > One remaining gap is that since this uses kswapd's own threshold, > unrestricted allocations can take what kswapd frees above the target > watermark. The current code already lets such allocations drain from the > high to the low watermark while kswapd sleeps. > > One potential solution, as hinted by Johannes in the discussion from the > original fix [1] is to add some kind of fudge factor (for instance, > wmark_pages + high_wmark_pages - low_wmark_pages). > > But in tests that I ran in a VM where the allocation demand outpaces > kswapd, about 4% more pages landed on the busy node and kswapd reclaimed > about 15% more, but kswapd still finished and slept. So let's hold off > on the fudge factor until it becomes an observable issue in workloads. > > [1] https://lore.kernel.org/linux-mm/20251001164131.GB1597553@cmpxchg.org/ > > Fixes: 19c5fb83f2a4 ("mm: page_alloc: avoid kswapd thrashing due to NUMA restrictions") > Suggested-by: Daniel Hodges > Signed-off-by: Joshua Hahn LGTM and: Reviewed-by: Lorenzo Stoakes (ARM) > --- > v1 --> v2: > - Added a Fixes tag and separated this from the other cleanup patches > - Check zone_watermark_fast with no flag to match the call in > pgdat_balanced(). > - Calculates mark outside the zone_watermark_fast check to match the > style for the other zone checks. > - Wordsmithing commit message to explain the unaddressed gap. > - Yes, this is ugly. I have another series to clean this code up but > I wanted this to be a feature-minimal patch so that it could be > easily backported. Things will be much prettier once those are landed. > > mm/page_alloc.c | 23 ++++++++++++++++------- > 1 file changed, 16 insertions(+), 7 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 8532af052d74b..56121c3f9c4a7 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -45,6 +45,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -3799,6 +3800,9 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags, > struct zoneref *z; > struct zone *zone; > struct pglist_data *last_pgdat = NULL; > + enum zone_watermarks kswapd_watermark = > + sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING ? > + WMARK_PROMO : WMARK_HIGH; > bool last_pgdat_dirty_ok = false; > bool no_fallback; > bool skip_kswapd_nodes = nr_online_nodes > 1; > @@ -3866,16 +3870,21 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags, > } > > /* > - * If kswapd is already active on a node, keep looking > - * for other nodes that might be idle. This can happen > - * if another process has NUMA bindings and is causing > - * kswapd wakeups on only some nodes. Avoid accidental > - * "node_reclaim_mode"-like behavior in this case. > + * If kswapd is already active on a node and the zone is below > + * its kswapd watermark for this request, keep looking for > + * other nodes that might be idle. This can happen if another > + * process has NUMA bindings and is causing kswapd wakeups on > + * only some nodes. Avoid accidental "node_reclaim_mode"-like > + * behavior in this case. > */ > if (skip_kswapd_nodes && > !waitqueue_active(&zone->zone_pgdat->kswapd_wait)) { > - skipped_kswapd_nodes = true; > - continue; > + mark = wmark_pages(zone, kswapd_watermark); > + if (!zone_watermark_fast(zone, order, mark, > + ac->highest_zoneidx, 0, gfp_mask)) { > + skipped_kswapd_nodes = true; > + continue; > + } Much better thanks! > } > > cond_accept_memory(zone, order, alloc_flags); > -- > 2.53.0-Meta > -- Cheers, Lorenzo