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([175.170.92.22]) by smtp.googlemail.com with ESMTPSA id d9443c01a7336-2a2f3d4d36esm7368135ad.63.2025.12.18.19.53.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 18 Dec 2025 19:53:51 -0800 (PST) From: Wanpeng Li To: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , Paolo Bonzini , Sean Christopherson Cc: K Prateek Nayak , Christian Borntraeger , Steven Rostedt , Vincent Guittot , Juri Lelli , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Wanpeng Li Subject: [PATCH v2 3/9] sched/fair: Add cgroup LCA finder for hierarchical yield Date: Fri, 19 Dec 2025 11:53:27 +0800 Message-ID: <20251219035334.39790-4-kernellwp@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251219035334.39790-1-kernellwp@gmail.com> References: <20251219035334.39790-1-kernellwp@gmail.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Wanpeng Li Implement yield_deboost_find_lca() to locate the lowest common ancestor (LCA) in the cgroup hierarchy for EEVDF-aware yield operations. The LCA represents the appropriate hierarchy level where vruntime adjustments should be applied to ensure fairness is maintained across cgroup boundaries. This is critical for virtualization workloads where vCPUs may be organized in nested cgroups. Key aspects: - For CONFIG_FAIR_GROUP_SCHED: Walk up both entity hierarchies by aligning depths, then ascending together until common cfs_rq found - For flat hierarchy: Simply verify both entities share the same cfs_rq - Validate that meaningful contention exists (h_nr_queued > 1) - Ensure yielding entity has non-zero slice for safe penalty calculation Function operates under rq->lock protection. Static helper integrated in subsequent patches. v1 -> v2: - Change nr_queued to h_nr_queued for accurate hierarchical task counting that includes tasks in child cgroups - Improve comments to clarify the LCA algorithm Signed-off-by: Wanpeng Li --- kernel/sched/fair.c | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 2f327882bf4d..39dbdd222687 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9102,6 +9102,36 @@ yield_deboost_validate_tasks(struct rq *rq, struct task_struct *p_target) return p_yielding; } +/* + * Find the lowest common ancestor (LCA) in the cgroup hierarchy. + * Uses find_matching_se() to locate sibling entities at the same level, + * then returns their common cfs_rq for vruntime adjustments. + * + * Returns true if a valid LCA with meaningful contention (h_nr_queued > 1) + * is found, storing the LCA entities and common cfs_rq in output parameters. + */ +static bool __maybe_unused +yield_deboost_find_lca(struct sched_entity *se_y, struct sched_entity *se_t, + struct sched_entity **se_y_lca_out, + struct sched_entity **se_t_lca_out, + struct cfs_rq **cfs_rq_out) +{ + struct sched_entity *se_y_lca = se_y; + struct sched_entity *se_t_lca = se_t; + struct cfs_rq *cfs_rq; + + find_matching_se(&se_y_lca, &se_t_lca); + + cfs_rq = cfs_rq_of(se_y_lca); + if (cfs_rq->h_nr_queued <= 1) + return false; + + *se_y_lca_out = se_y_lca; + *se_t_lca_out = se_t_lca; + *cfs_rq_out = cfs_rq; + return true; +} + /* * sched_yield() is very simple */ -- 2.43.0