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[173.79.60.52]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-535f4eee7bcsm30668321cf.30.2026.10.14.06.16.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 14 Oct 2026 06:16:49 -0700 (PDT) Date: Wed, 14 Oct 2026 09:16:46 -0400 From: Gregory Price To: Li Zhe Cc: akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, joshua.hahnjy@gmail.com, ziy@nvidia.com, mingo@redhat.com, peterz@infradead.org, mgorman@suse.de, riel@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, vschneid@redhat.com, kprateek.nayak@amd.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 4/6] sched/numa: make free-space tiering promotion bypass configurable Message-ID: References: <20261014094347.99157-1-lizhe.67@bytedance.com> <20261014094347.99157-5-lizhe.67@bytedance.com> Precedence: bulk X-Mailing-List: linux-doc@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: <20261014094347.99157-5-lizhe.67@bytedance.com> On Wed, Oct 14, 2026 at 05:43:45PM +0800, Li Zhe wrote: > Memory tiering normally promotes any recently accessed slow-tier page if > the target fast-tier node has enough free space. This preserves the > existing policy of filling available fast memory before relying on > hotness and rate-limit control. > > That is not always the desired policy. Some workloads may intentionally > place memory across CXL-attached memory and DRAM to use aggregate memory > bandwidth. With the free-space bypass, cold one-shot accesses and bursts > of hot accesses can refill the fast tier and pull the workload away from > that distribution whenever DRAM has free space. > > Add kernel.numa_balancing_promote_free_space_bypass. It defaults to 1, > preserving the existing behavior. When cleared, enough free space in the > target fast-tier node no longer bypasses the normal hotness and > rate-limit controls. > A global switch for this is a poor solution for this problem. Also, if the memory was spread out intentionally, it presumably was done so with a memory policy. In that case, if the user did not set MPOL_F_NUMA_BALANCING - that spread is already protected because the mempolicy overrides the numa balancing fault and no migration occurs. I don't think this is the right direction. ~Gregory