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([2408:822f:1aba:84a0:651:104c:ba0c:1f4a]) by smtp.googlemail.com with ESMTPSA id 98e67ed59e1d1-37a1f07bbfdsm250713a91.5.2026.06.11.18.34.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Jun 2026 18:34:11 -0700 (PDT) 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 , Richie Buturla Subject: [PATCH v3 02/10] sched/fair: Credit a persistent, queue-depth-scaled vlag margin Date: Fri, 12 Jun 2026 09:33:47 +0800 Message-ID: <20260612013355.59231-3-kernellwp@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260612013355.59231-1-kernellwp@gmail.com> References: <20260612013355.59231-1-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 Crediting only up to vlag = 0 makes the buddy eligible for a single pick_eevdf() pass. The next update_curr() can push it below zero again before PICK_BUDDY consumes the hint. Credit to a bounded positive-vlag margin instead, so the buddy stays eligible across several scheduling decisions. Scale the margin with runqueue depth, because deeper runqueues dilute eligibility faster, and clamp it to entity_lag()'s legal positive-lag bound to preserve EEVDF fairness. The helper is not called in this change; there is no functional change. Signed-off-by: Wanpeng Li --- kernel/sched/fair.c | 58 ++++++++++++++++++++++++++++++++++++--------- 1 file changed, 47 insertions(+), 11 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index e7f5ea25fdae..c6502db62cd3 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9342,21 +9342,45 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev, struct t } /* - * eevdf_credit_entity_vlag - credit a nominated next-buddy to eligibility + * Positive-vlag target margin for a credited buddy, scaled by runqueue + * depth so it stays eligible across several picks. The caller clamps it to + * entity_lag()'s legal bound, so EEVDF fairness is preserved. + */ +static u64 __maybe_unused +eevdf_persistent_margin(struct cfs_rq *cfs_rq, struct sched_entity *se) +{ + u64 base = sysctl_sched_base_slice; + unsigned int n = cfs_rq->h_nr_queued; + u64 raw; + + if (n <= 4) + raw = base * 4; + else if (n <= 8) + raw = base * 6; + else if (n <= 16) + raw = base * 8; + else + raw = base * 12; + + return calc_delta_fair(raw, se); +} + +/* + * eevdf_credit_entity_vlag - credit bounded vlag to a nominated next-buddy * * Advance @se (already nominated by set_next_buddy(), so cfs_rq->next == se) - * just enough negative vlag to reach the eligibility boundary (vlag = 0) so - * pick_eevdf()'s PICK_BUDDY branch returns it. cfs_rq->curr is shifted in - * place (off-tree, carrying any vprot window). Queued entities are left - * unchanged. + * to a bounded positive-vlag margin so pick_eevdf()'s PICK_BUDDY branch + * keeps returning it across several picks, without exceeding entity_lag()'s + * legal bound. cfs_rq->curr is shifted in place (off-tree, carrying any + * vprot window). Queued entities are left unchanged. * - * Idempotent: a no-op once @se is already eligible. Caller must hold + * Idempotent once @se holds the margin. Caller must hold * rq_of(cfs_rq)->lock with rq_clock up to date. */ static void __maybe_unused eevdf_credit_entity_vlag(struct cfs_rq *cfs_rq, struct sched_entity *se) { - u64 avruntime, credit; + u64 avruntime, credit, want, margin, max_slice, lag_limit; s64 vlag; /* Callers gate this helper with YIELD_TO_LAG_CREDIT. */ @@ -9371,11 +9395,23 @@ eevdf_credit_entity_vlag(struct cfs_rq *cfs_rq, struct sched_entity *se) avruntime = avg_vruntime(cfs_rq); vlag = entity_lag(cfs_rq, se, avruntime); - /* Already eligible: nothing to do. */ - if (vlag >= 0) - return; + /* Clamp the margin to entity_lag()'s bound so place_entity() keeps it. */ + max_slice = cfs_rq_max_slice(cfs_rq) + TICK_NSEC; + lag_limit = calc_delta_fair(max_slice, se); + margin = eevdf_persistent_margin(cfs_rq, se); + if (lag_limit && margin > lag_limit) + margin = lag_limit; + if (vlag >= 0) { + if ((u64)vlag >= margin) + return; + want = margin - (u64)vlag; + } else { + want = margin + (u64)(-vlag); + } - credit = (u64)(-vlag); + credit = want; + if (!credit) + return; if (cfs_rq->curr == se) { /* curr is off-tree: in-place shift, carrying any vprot window. */ -- 2.43.0