From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 5E7A547206A for ; Tue, 1 Sep 2026 09:09:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788253764; cv=none; b=X/KyqGdbgK+0scTBytS0x/yCQfLU00nu3NmagivkL4W5LmQQUjeKVOzVNDBBJ+REfK0zDvRW7WQ8iTM2I6oZdSW2ezs14mhdgvMNIuyHXAnPm4mxzWCB+nP2u3DeA9pj9jnisiv7N2kna2wCpCsl/6g10RsWUEVixSWO+veY5pw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788253764; c=relaxed/simple; bh=No0g8sMpQ62w8gRG+EeMNJh/VNFdqhcrMjrIZCnCe2Y=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=rjd5zE/ZJRx4/BJpfNiPnmx+OrhHrd5PK99lwWf4SS3Jk1cX0afjEQTyqb5r3SbN5dAsNmAtyvJyU0E61ikeBhfe2to3v8e8Wmt7yqnA4nbszuXmbf8QivDffP25h6/Hmb6WFZERco3uY+vwERd/kiX/L2By9SSWwgpByyLVBXo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=pass smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=jK6t+CDm; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=pass 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="jK6t+CDm" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=czh3rQAWAmJyiyiLMm64PQaNiIh2S51TPnPG1qoRn4g=; b=jK6t+CDmAUicfWHGYjsrPPugUy KUPA77dVMrLsAO3sOkKQJwbHtYgr6z7oyU/oB61XjwBANQkSUyx/TnOhLRzlj0qn+99bsDFr0bP6n Pb2pWsK8uuwWawtqnl4nEmvZFn8pfjBihFHyUHKeheg0/Qxeyx3xFOv+sebDzTJ52b65rt6oMnfp3 M51YHD8PabAHlIi8I9dEzf3ne6nArM9Jt/xsdmFryhAvVqRsnJ1NrMYQhGZNXITos8/2nFTuxpnzQ mTUH6RDHIGkoLBf2Tm4bJI3yOPBlRy0XJjFc8T8XlQ+I/10Lw9ojCFSz1roRQgBeZfEcrSCOt+O/v XSDcKkow==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1x1KU1-00000004oNT-3yiM; Tue, 01 Sep 2026 09:08:46 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 7EA6A30031E; Tue, 01 Sep 2026 11:08:43 +0200 (CEST) Date: Tue, 1 Sep 2026 11:08:43 +0200 From: Peter Zijlstra To: Jianyong Wu Cc: Ingo Molnar , Juri Lelli , Vincent Guittot , Chen Yu , Tim Chen , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , Shrikanth Hegde , Phil Auld , Andrew Morton , David Hildenbrand , linux-kernel@vger.kernel.org, linux-mm@kvack.org, jianyong.wu@outlook.com, zhongyuan@hygon.cn, huangsj@hygon.cn, wangfengyu@hygon.cn, yingzhiwei@hygon.cn, justin.he@arm.com Subject: Re: [RFC PATCH v2 11/23] sched/cache: Introduce helpers for task migration decisions Message-ID: <20260901090843.GQ4120091@noisy.programming.kicks-ass.net> References: <20260827122816.756234-1-wujianyong@hygon.cn> <20260827122816.756234-12-wujianyong@hygon.cn> 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=us-ascii Content-Disposition: inline In-Reply-To: <20260827122816.756234-12-wujianyong@hygon.cn> On Thu, Aug 27, 2026 at 08:28:04PM +0800, Jianyong Wu wrote: > +/* > + * Decide if migration should happen on a specific node. > + * The node here is an LLC or a NUMA. > + */ > +static enum llc_mig __maybe_unused can_migrate_node(int src_cpu, int dst_cpu, > + struct task_struct *p, bool to_pref) > +{ > + const struct cpumask *span; > + struct mm_struct *mm; > + unsigned long dst_util, dst_cap, tsk_util = 0; > + unsigned long src_util = 0, src_cap = 0; > + unsigned long acc_util = 0, acc_cap = 0; > + int node, target_cpu = src_cpu; > + int get_src = 0; > + > + if (!get_llc_stats(dst_cpu, &dst_util, &dst_cap)) > + return mig_unrestricted; > + > + if (!get_llc_stats(src_cpu, &src_util, &src_cap)) > + src_cap = 0; Comparing against can_migrate_llc(), that bails with mig_unrestricted when !get_llc_stats(src_cpu). > + > + if (p) { > + mm = p->mm; > + if (mm) { > + if (mm->sc_stat.cpu >= 0) > + target_cpu = mm->sc_stat.cpu; > + } > + tsk_util = task_util(p); > + } It isn't clear when !p would be valid. migration is always about a task, no? > + > + dst_util = dst_util + tsk_util; can_migrate_llc() also adjusts src_util by subtracting tsk_util (and flooring at 0). > + if (to_pref) { > + unsigned long dst_pre = dst_util - tsk_util; > + > + if (fits_llc_capacity(dst_util, dst_cap)) > + return mig_llc; > + > + /* > + * The destination is over the margin. That is a reason to > + * refuse a task while the margin can still be met, but not > + * while every LLC of the node is over it: no placement > + * satisfies the margin then, and refusing every migration > + * leaves the imbalance in place. > + * > + * Let the task through when the move still lowers the peak, > + * that is when the source is noticeably heavier than the > + * destination and carries at least two more tasks worth of > + * utilization. The second condition keeps the destination > + * from becoming the heavier side, which would bounce the > + * task straight back. > + */ > + if (src_cap && util_greater(src_util, dst_pre) && > + src_util >= dst_pre + 2 * tsk_util) > + return mig_llc; > + > + return mig_forbid; > + } > + > + for_each_sched_node(target_cpu, node) { So this iterates the nodes in the order specific to the node that contains target_cpu.. > + unsigned long u = 0, c = 0, nu, nc; > + > + /* > + * The walk starts at the anchor, so the nodes it crosses before > + * reaching the source say nothing about this migration: the task > + * does not live there and is not going there. Judging them only > + * lets an unrelated node with room refuse the move. Start at the > + * node the task actually sits on. > + */ > + if (!get_src) { > + if (!cpumask_test_cpu(src_cpu, cpumask_of_node(node))) > + continue; > + else Strictly speaking this else is superfluous. > + get_src = 1; > + } And then you skip the nodes between target and src, which are the nodes with best locality, confusing, but lets read on.. (sometimes target == src, and you thus don't skip anything, but other times this is the preferred cpu) > + if (cpumask_test_cpu(dst_cpu, cpumask_of_node(node))) { For the node that contains dst_cpu.. > + nu = 0; > + nc = 0; > + for_each_llc_node_span(node, span) { Iterate the LLCs in this node.. > + get_span_stats(span, &u, &c); Now I'm confused again, @span is the span of the llc, but get_span_stats() is a wrapper around get_llc_stats(), and since span is just a single llc, why use this rather than get_llc_stats() directly? > + nu += u; > + nc += c; > + if (cpumask_test_cpu(dst_cpu, span)) { If llc includes dst_cpu (indent is getting a little out of hand here) > + if (fits_llc_capacity(u + tsk_util, c)) > + return mig_llc; > + > + /* > + * The destination is over the margin, > + * but so may be the source. Refusing > + * then leaves the peak where it is: > + * a LLC at seven tasks stays at seven > + * while a neighbour in the same node > + * sits at four, because taking one > + * more would put that neighbour over > + * the margin as well. > + * > + * Let the task through when the move > + * still lowers the peak, guarded the > + * same way as the aggregation path: > + * the source must be noticeably > + * heavier and carry at least two more > + * tasks worth of utilization, so the > + * destination cannot end up the > + * heavier side and bounce it back. > + */ > + if (src_cap && > + util_greater(src_util, u + tsk_util) && > + src_util >= u + 2 * tsk_util) > + return mig_llc; > + > + return mig_forbid; This is an unconditional return.. > + /* > + * A nearer LLC only justifies vetoing this > + * migration if the task would actually fit But you just skipped the nodes between target and src, those are nearer, no? > + * there, so account for its utilization the > + * same way the destination branch above does. > + * Without it a LLC already holding one task > + * per core still reads as having room and > + * vetoes every migration towards a farther, > + * genuinely idle LLC. > + */ > + } else if (fits_llc_capacity(acc_util + nu + tsk_util, .. which renders this else superfluous, but that won't help with the indent because you still have the chained if :-( > + acc_cap + nc) > + && fits_llc_capacity(u + tsk_util, c) > + && !util_greater(u, dst_pre)) (logical operators go at the end of the previous line, your patch is inconsistent on this point, since that is what you do elsewhere) > + return mig_forbid; > + } > + } > + > + /* Don't migrate if this is a good place to live. */ > + for_each_llc_node_span(node, span) { > + get_span_stats(span, &u, &c); This is shared with the above loop, meaning you're now duplicating this work in case you fell through. > + if (cpumask_test_cpu(src_cpu, span)) { > + if (fits_llc_capacity(u, c)) > + return mig_forbid; > + } else { > + if (fits_llc_capacity(u + tsk_util, c)) > + return mig_forbid; > + } > + } > + } > + > + return mig_unrestricted; > +} > + > /* > * Check if task p can migrate from source LLC to > * destination LLC in terms of cache aware load balance. > -- > 2.34.1 > >