From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 E18213BCD0B; Tue, 15 Sep 2026 15:35:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789486557; cv=none; b=Ibwb/hll7tokSlZecJeNADkMEBE9EBGpLk4PQaZu+aTww4IVwwE34ixkzplEU8G9qAXTh4WYYLpLLr7arquW3gGXhMREAJHusg1F7zStzthysMCM3CgvlUvVnfmtrC64ZRB0njf+qOoTacahrsn3VcWVvKInyBUrmDlNhq3f1cY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789486557; c=relaxed/simple; bh=619GG4pSIYQXqyXSDpjaSeEbJx7eg4HrrgiyWyiBUFc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kMjurQm3+4ubko1tc9J9aCpPK01hTxx82gwG/tD1HLma5zgmiNRGdGk4mfYtFRisk5bVI9h5zjCepvS5CVfs208n9hkxh0bx2drT8y3VYSGCzVY8+XWiwlQEuYl7dzEuzCLRcRGp8UWBU8L2yBdhxXNCslZPKrmq5wULU/Lokz8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=lyaCPt7u; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="lyaCPt7u" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 824701F000FF; Tue, 15 Sep 2026 15:35:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789486555; bh=eIE5Vq0CFBUSKsZPUEoumfbD3kBp123cMRHTTVMd3H4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=lyaCPt7uJYk7aQeXKJJunQOhN1oPhfnH1MQcZABpnm8N8BvkYMdFxkvwSRwAa3i4R FWuaACGOC7uOJ6kR7xBeIAD5xXitVzAzPh/ZIcSoDHb0hMpnlkbpkUHxl/KUpK/KN9 4UKCc79gLBXrdwdqBC0PgMYLt3OZd02b4I82+Rh7ziMXKZNaMXxs7Zf6hzTbB1TUUR SsQjFc8mzP4vcyQnUa8toMwtovn0jpumDpgHWFp3VC2/gOzcybpc9mHCj6Cuw6Z6oN soLeETzD+/RihQ2HU9bPat2OweUy2Aa4Vvc+5tCpNd7MH/FG30XIu1LoqIB+oMu+Jm MuLPDe607YiWQ== Message-ID: <599fd878-e670-4ee7-8ded-4299c19c085c@kernel.org> Date: Tue, 15 Sep 2026 17:35:50 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Beta Subject: Re: [PATCH AUTOSEL 6.18-6.1] sched/fair: Reject misfit pulls onto busy SMT siblings on asym-capacity Content-Language: fr To: Sasha Levin , patches@lists.linux.dev, stable@vger.kernel.org Cc: Andrea Righi , Felix Abecassis , "Peter Zijlstra (Intel)" , Vincent Guittot , mingo@redhat.com, juri.lelli@redhat.com, linux-kernel@vger.kernel.org References: <20260831133314.4125787-1-sashal@kernel.org> <20260831133314.4125787-171-sashal@kernel.org> From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit Hi Sasha, On 31/08/2026 15:23, Sasha Levin wrote: > From: Andrea Righi > > [ Upstream commit bf6aa722198d3c06e4236e8c5a480f30a64e1513 ] > > When SD_ASYM_CPUCAPACITY load balancing considers pulling a misfit task, > capacity_of(dst_cpu) can overstate available compute if the SMT sibling is > busy: the core does not deliver its full nominal capacity. > > If SMT is active and dst_cpu is not on a fully idle core, skip this > destination so we do not migrate a misfit expecting a capacity upgrade we > cannot actually provide. > > Reported-by: Felix Abecassis > Signed-off-by: Andrea Righi > Signed-off-by: Peter Zijlstra (Intel) > Reviewed-by: Vincent Guittot > Link: https://patch.msgid.link/20260509180955.1840064-5-arighi@nvidia.com > Signed-off-by: Sasha Levin > --- > > LLM Generated explanations, may be completely bogus: > > Connection lost, reconnecting to https://agentn.us.api5.cursor.sh > (attempt 1)... > Retry attempt 1... > The background `git log --grep` search finished without finding this > commit in the 6.18.43 tree — which matches what we saw in code > (`dst_core_idle` is absent from `kernel/sched/fair.c`). Mmh, you might need to check this. Is it why it didn't detect issues on v6.1, see below. > **Verdict for 6.18.y: YES.** It's a small, maintainer-reviewed > scheduling correctness fix for misfit task migration on asymmetric- > capacity + SMT systems. The buggy logic is present in this tree and the > patch should apply cleanly. > > kernel/sched/fair.c | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index ac5f08cd01a83..fee58d01ae0f9 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -9395,6 +9395,7 @@ struct lb_env { > > int dst_cpu; > struct rq *dst_rq; > + bool dst_core_idle; > > struct cpumask *dst_grpmask; > int new_dst_cpu; > @@ -10636,10 +10637,16 @@ static bool update_sd_pick_busiest(struct lb_env *env, > * We can use max_capacity here as reduction in capacity on some > * CPUs in the group should either be possible to resolve > * internally or be covered by avg_load imbalance (eventually). > + * > + * When SMT is active, only pull a misfit to dst_cpu if it is on a > + * fully idle core; otherwise the effective capacity of the core is > + * reduced and we may not actually provide more capacity than the > + * source. > */ > if ((env->sd->flags & SD_ASYM_CPUCAPACITY) && > (sgs->group_type == group_misfit_task) && > - (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) || > + (!env->dst_core_idle || > + !capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) || > sds->local_stat.group_type != group_has_spare)) > return false; > > @@ -11206,6 +11213,8 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd > unsigned long sum_util = 0; > bool sg_overloaded = 0, sg_overutilized = 0; > > + env->dst_core_idle = !sched_smt_active() || is_core_idle(env->dst_cpu); On v6.1, I got this: kernel/sched/fair.c: In function 'update_sd_lb_stats': kernel/sched/fair.c:9912:53: error: implicit declaration of function 'is_core_idle' [-Wimplicit-function-declaration] 9912 | env->dst_core_idle = !sched_smt_active() || is_core_idle(env->dst_cpu); | ^~~~~~~~~~~~ Cheers, Matt -- Sponsored by the NGI0 Core fund.