From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B33E32DEA94 for ; Fri, 30 Jan 2026 09:47:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769766444; cv=none; b=URfu8+cXSdBlPcsKeQBDQfcqv6BV0+Rv/0EwwsvibZmw24TnPffdMIaMveoGEbnqVCPgYQCAbhgT26ibIisBJUoNEmF60TskfqbT2qfM40l31L39hGvYfsgVECTNTTUemZJR07/R1bWFRQYx+LkIKcJZFZyc/oQ6S+RlhQslaTU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769766444; c=relaxed/simple; bh=4sCfDrcawGQUUXxOtUKxLPfr16XQFFQaCVLSA0FI8Qk=; h=Message-ID:Date:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=LAryT8jqFvpLhh42HOQmFO60IUJBt4fGqxZUh/tjP8Nmpd3f1AYyYjTjqOMeCTHkrdIELP6T4x/1MO9YlWiOjSFOlvIHLP7iB9tx7Kokk5o9aIB18vbBNnmnUt2Z2zKYxbks8c/MvTZ0CWY37cppsExllUjHvEPtlFA7AaV/BE0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=V7et1b6a; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="V7et1b6a" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=Content-Type:MIME-Version:References: Subject:Cc:To:From:Date:Message-ID:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:In-Reply-To; bh=LZhsF/Eim8xuo3bNblOXGtXHxO3fvjWt2GLWDS/ISmM=; b=V7et1b6aidAiSEkubD++fLFVcN c1xBv+Vcn46G3+ndrhDCJnqKb1hWA4nyV0R1ZfT8+mduCwb92J1leMo8ysV+9O8t8aVkyw8YWQqHI HR4vNZZVjtRotSagNoDsuTT3jdZrq5yVzdI9FmSyqqaUMHlko2D7KiN58qtW1QQ/Hp5RjvWR1xqWt 6ZtRWQLhAGLW2EAU5+hMD2YvMAtcx3QNCewUmTtXQ5IQrTOSsJIb4GeFZwvxexPWcYqRRb4TTdoSE 4sLPwgc0WwYcbA9ac4oVhVJNRt9xXrL2wQ2kkAZgM4YsyvR4IotbzbrlBhSsUY6eXGMD1onYfaJUK 2nbKXILg==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vll5n-0000000BZrF-3ICa; Fri, 30 Jan 2026 09:47:08 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 0) id 3A0B1303329; Fri, 30 Jan 2026 10:47:06 +0100 (CET) Message-ID: <20260130094608.304041157@infradead.org> User-Agent: quilt/0.68 Date: Fri, 30 Jan 2026 10:34:43 +0100 From: Peter Zijlstra To: mingo@kernel.org Cc: peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, linux-kernel@vger.kernel.org, wangtao554@huawei.com, quzicheng@huawei.com, kprateek.nayak@amd.com, wuyun.abel@bytedance.com, dsmythies@telus.net Subject: [PATCH 4/4] sched/fair: Revert 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag") References: <20260130093439.803225718@infradead.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Zicheng Qu reported that, because avg_vruntime() always includes cfs_rq->curr, when ->on_rq, place_entity() doesn't work right. Specifically, the lag scaling in place_entity() relies on avg_vruntime() being the state *before* placement of the new entity. However in this case avg_vruntime() will actually already include the entity, which breaks things. Also, Zicheng Qu argues that avg_vruntime should be invariant under reweight. IOW commit 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag") was wrong! The issue reported in 6d71a9c61604 could possibly be explained by rounding artifacts -- notably the extreme weight '2' is outside of the range of avg_vruntime/sum_w_vruntime, since that uses scale_load_down(). By scaling vruntime by the real weight, but accounting it in vruntime with a factor 1024 more, the average moves significantly. Tested by reverting 66951e4860d3 ("sched/fair: Fix update_cfs_group() vs DELAY_DEQUEUE") and tracing vruntime and vlag figures again. Reported-by: Zicheng Qu Signed-off-by: Peter Zijlstra (Intel) --- kernel/sched/fair.c | 154 +++++++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 129 insertions(+), 25 deletions(-) --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -782,16 +782,21 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq) * * XXX could add max_slice to the augmented data to track this. */ -static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se) +static s64 entity_lag(u64 avruntime, struct sched_entity *se) { s64 vlag, limit; - WARN_ON_ONCE(!se->on_rq); - - vlag = avg_vruntime(cfs_rq) - se->vruntime; + vlag = avruntime - se->vruntime; limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se); - se->vlag = clamp(vlag, -limit, limit); + return clamp(vlag, -limit, limit); +} + +static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se) +{ + WARN_ON_ONCE(!se->on_rq); + + se->vlag = entity_lag(avg_vruntime(cfs_rq), se); } /* @@ -3839,23 +3844,135 @@ dequeue_load_avg(struct cfs_rq *cfs_rq, se_weight(se) * -se->avg.load_sum); } -static void place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags); +static void rescale_entity(struct sched_entity *se, unsigned long weight, + u64 avruntime, bool rel_vprot) +{ + unsigned long old_weight = se->load.weight; + + /* + * VRUNTIME + * -------- + * + * COROLLARY #1: The virtual runtime of the entity needs to be + * adjusted if re-weight at !0-lag point. + * + * Proof: For contradiction assume this is not true, so we can + * re-weight without changing vruntime at !0-lag point. + * + * Weight VRuntime Avg-VRuntime + * before w v V + * after w' v' V' + * + * Since lag needs to be preserved through re-weight: + * + * lag = (V - v)*w = (V'- v')*w', where v = v' + * ==> V' = (V - v)*w/w' + v (1) + * + * Let W be the total weight of the entities before reweight, + * since V' is the new weighted average of entities: + * + * V' = (WV + w'v - wv) / (W + w' - w) (2) + * + * by using (1) & (2) we obtain: + * + * (WV + w'v - wv) / (W + w' - w) = (V - v)*w/w' + v + * ==> (WV-Wv+Wv+w'v-wv)/(W+w'-w) = (V - v)*w/w' + v + * ==> (WV - Wv)/(W + w' - w) + v = (V - v)*w/w' + v + * ==> (V - v)*W/(W + w' - w) = (V - v)*w/w' (3) + * + * Since we are doing at !0-lag point which means V != v, we + * can simplify (3): + * + * ==> W / (W + w' - w) = w / w' + * ==> Ww' = Ww + ww' - ww + * ==> W * (w' - w) = w * (w' - w) + * ==> W = w (re-weight indicates w' != w) + * + * So the cfs_rq contains only one entity, hence vruntime of + * the entity @v should always equal to the cfs_rq's weighted + * average vruntime @V, which means we will always re-weight + * at 0-lag point, thus breach assumption. Proof completed. + * + * + * COROLLARY #2: Re-weight does NOT affect weighted average + * vruntime of all the entities. + * + * Proof: According to corollary #1, Eq. (1) should be: + * + * (V - v)*w = (V' - v')*w' + * ==> v' = V' - (V - v)*w/w' (4) + * + * According to the weighted average formula, we have: + * + * V' = (WV - wv + w'v') / (W - w + w') + * = (WV - wv + w'(V' - (V - v)w/w')) / (W - w + w') + * = (WV - wv + w'V' - Vw + wv) / (W - w + w') + * = (WV + w'V' - Vw) / (W - w + w') + * + * ==> V'*(W - w + w') = WV + w'V' - Vw + * ==> V' * (W - w) = (W - w) * V (5) + * + * If the entity is the only one in the cfs_rq, then reweight + * always occurs at 0-lag point, so V won't change. Or else + * there are other entities, hence W != w, then Eq. (5) turns + * into V' = V. So V won't change in either case, proof done. + * + * + * So according to corollary #1 & #2, the effect of re-weight + * on vruntime should be: + * + * v' = V' - (V - v) * w / w' (4) + * = V - (V - v) * w / w' + * = V - vl * w / w' + * = V - vl' + */ + se->vlag = div_s64(se->vlag * old_weight, weight); + if (avruntime) + se->vruntime = avruntime - se->vlag; + + /* + * DEADLINE + * -------- + * + * When the weight changes, the virtual time slope changes and + * we should adjust the relative virtual deadline accordingly. + * + * d' = v' + (d - v)*w/w' + * = V' - (V - v)*w/w' + (d - v)*w/w' + * = V - (V - v)*w/w' + (d - v)*w/w' + * = V + (d - V)*w/w' + */ + if (se->rel_deadline) { + se->deadline = div_s64(se->deadline * old_weight, weight); + if (avruntime) { + se->rel_deadline = 0; + se->deadline += avruntime; + } + } + + if (rel_vprot) { + se->vprot = div_s64(se->vprot * old_weight, weight); + if (avruntime) + se->vprot += avruntime; + } +} static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, unsigned long weight) { bool curr = cfs_rq->curr == se; bool rel_vprot = false; - u64 vprot; + u64 avruntime = 0; if (se->on_rq) { /* commit outstanding execution time */ update_curr(cfs_rq); - update_entity_lag(cfs_rq, se); - se->deadline -= se->vruntime; + avruntime = avg_vruntime(cfs_rq); + se->vlag = entity_lag(avruntime, se); + se->deadline -= avruntime; se->rel_deadline = 1; if (curr && protect_slice(se)) { - vprot = se->vprot - se->vruntime; + se->vprot -= avruntime; rel_vprot = true; } @@ -3866,30 +3983,17 @@ static void reweight_entity(struct cfs_r } dequeue_load_avg(cfs_rq, se); - /* - * Because we keep se->vlag = V - v_i, while: lag_i = w_i*(V - v_i), - * we need to scale se->vlag when w_i changes. - */ - se->vlag = div_s64(se->vlag * se->load.weight, weight); - if (se->rel_deadline) - se->deadline = div_s64(se->deadline * se->load.weight, weight); - - if (rel_vprot) - vprot = div_s64(vprot * se->load.weight, weight); + rescale_entity(se, weight, avruntime, rel_vprot); update_load_set(&se->load, weight); do { u32 divider = get_pelt_divider(&se->avg); - se->avg.load_avg = div_u64(se_weight(se) * se->avg.load_sum, divider); } while (0); enqueue_load_avg(cfs_rq, se); if (se->on_rq) { - place_entity(cfs_rq, se, 0); - if (rel_vprot) - se->vprot = se->vruntime + vprot; update_load_add(&cfs_rq->load, se->load.weight); if (!curr) __enqueue_entity(cfs_rq, se); @@ -5247,7 +5351,7 @@ place_entity(struct cfs_rq *cfs_rq, stru se->vruntime = vruntime - lag; - if (se->rel_deadline) { + if (sched_feat(PLACE_REL_DEADLINE) && se->rel_deadline) { se->deadline += se->vruntime; se->rel_deadline = 0; return;