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Tue, 19 May 2026 13:28:29 -0700 (PDT) Received: from localhost ([2603:7001:f100:500:365a:60ff:fe62:ff29]) by smtp.gmail.com with ESMTPSA id af79cd13be357-910baf2236fsm1951430885a.20.2026.05.19.13.28.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 May 2026 13:28:28 -0700 (PDT) Date: Tue, 19 May 2026 16:28:20 -0400 From: Johannes Weiner To: "JP Kobryn (Meta)" Cc: akpm@linux-foundation.org, vbabka@kernel.org, surenb@google.com, mhocko@suse.com, jackmanb@google.com, ziy@nvidia.com, linux-mm@kvack.org, usama.arif@linux.dev, kirill@shutemov.name, willy@infradead.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH] mm/page_alloc: skip high atomic reservation at or below costly order Message-ID: References: <20260519012532.272770-1-jp.kobryn@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260519012532.272770-1-jp.kobryn@linux.dev> X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 5E3A91C000B X-Rspam-User: X-Stat-Signature: uc1g91tqwjiesioupg1f94mh3dksrmis X-HE-Tag: 1779222510-263707 X-HE-Meta: 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 8HBMLpCB NBPlxg7GQKNnNhEoFTOnE2DDWHsA/lQQ6hmvpotR3URE41eoWVVYiB1pKYcznVBMvgYBBXyxlhJw/s+Ou8X2vTxoDn0nA0Jt50aGq/i3sMsqxB7QCzGmug18F4MQAPmPBAaK0d6SJROhCThJtga3qRPU2rYmLEOLx7xYdYDmZpTFx0IJeXCYybZog4lDqgLkeIJ7k4JXFlAp1hU4uzPaMoplgPVcvTef3Q2Ltu3qk1XGyGvEMTrGmtnXhFRielUbBU2I2lQ/VAaMWwP4vmIxx+7TArD2su1VXGttbj0j7UjuVg7hQXS3zXLBvhPt5rjh6jH/vSEWpNkbR72/1A07fbxj5DI5FtzpWx12qeh2ybL0CIpRde/l+aTmChsUQjtAiznxHj8MYXYuP2iHYX8ogtsFYdtnDt9g8vzna Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, May 18, 2026 at 06:25:32PM -0700, JP Kobryn (Meta) wrote: > We're seeing a pattern in production where 2MB THP order-9 allocations are > failing due to fragmentation and triggering reclaim on systems with plenty > of free memory. Over time, the success rate of these THP allocations do not > increase at all. > > Inspecting zone->vm_stat[NR_FREE_PAGES] via kprobe on compaction_suitable() > indicated the given zone had sufficient free pages for order-9 allocations, > yet they were going unused. Drilling down into the zone and inspecting > /proc/pagetypeinfo revealed why. Order-9 blocks were accumulating in the > zone's HighAtomic bucket (while zero were present in Movable). THP is > unable to draw blocks from HighAtomic since that bucket is not in the > fallback list. > > The heuristic for reserving pageblocks in HighAtomic is that any atomic > allocation greater than order-0 will result in the full pageblock being > captured. This means that an order-1 atomic allocation will over-reserve by > 256x, a full 512 pageblock. > > Gate the reservation on order. Skip for allocations at or below > PAGE_ALLOC_COSTLY_ORDER. This prevents smaller atomic allocations from > reserving entire pageblocks, and significantly helps when THP is in use on > a fragmented but otherwise healthy system. > > Testing was performed using an A/B instagram workload receiving prod > traffic. Each side had ~60 hosts with 64G memory. The patch resulted in > several gains: > > Unpatched > HighAtomic pageblocks per host: 309-312 (1% of zone or 620MB), > ...all order-9 blocks in HighAtomic > THP success rate: 1-6% > Compaction success rate: 0-2% > pgscan_kswapd (total across ~60 hosts, per minute): ~70.2M > Atomic order-4+ allocations: 0 > > Patched > HighAtomic pageblocks per host: 1 > THP success rate: 44-78% > Compaction success rate: 24-47% > pgscan_kswapd (total across ~60 hosts, per minute): ~29.9M > Atomic order-4+ allocations: 0 This is an interesting patch. A couple of thoughts: 1. You disabled the highatomic reserve for this workload and it didn't seem to matter. Presumably