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 C5C60320A09 for ; Fri, 20 Feb 2026 09:43:44 +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=1771580626; cv=none; b=jMof83+TdyX5Tz1lhBn89LQ/12lxOvuBFdam2hexYWIx5IO60K4kjXNq6rJStVRxn+F6Y7jj5dXQMM6ISp14N5nwairOov+zdsEAqgIyBsJ3Vk3Oe9WnG3Y8hsen+btjsb7KKtvyMPQJSiop8cBrVIQv0OJ8VLbl4gG6/Q/lNnQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771580626; c=relaxed/simple; bh=8kTVNUUI1EK4f885Bep01oXDFWYk8ba31OJBBTr4w20=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=p70YLx8dkbs9Qi0k6hq1ANq9Y60slvdpbWcJMwjOj9bw32oDXKnLNu/dvw5lrZMRZlCMKqpuOnJiMIzApUyNA+4bmWm4roz3xEPxCvcVrk5JXxFV3xlVzzranRyBLTbMyQ1q89AlwzToYxrERxQFNpj0A7mTJFQoEvgBYEmh4Sk= 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=p9XLhC4v; 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=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="p9XLhC4v" 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=8jsCulHbawDPD0G+x0DPe8N/ipeRAjrc6N6dYBX7N4Q=; b=p9XLhC4vY16fR/WDWo/Lsh0sj9 QVfK1JwANF/VHG4lKPs4p0CafQ2Hbwwao5F0FIKf9LmkzA4D5Hxhbx3dpEgdXd62BdzRQ5rBvASAL pKr+cXGXxBhlIelVO5gPQVTEDWzsjBkjqMr4r5B/SknriLiEKj44ed9FAs7KntoD8o+7h6j001oFP AqtuXR+UnUaKic6KAp+9AqmVxNLj9MWGW7MnP1PRV9IrqrSq1zD+50K5+OncU6S50F5fLQGNUjU21 ogJB+iVMSPjexQ4ezxuEOPu2GtKlXpJj+pUtlb9VBib5apHf3Pj0GxV93jolU6lUAnK9otkI5GWS6 NCrWqAKQ==; 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 4.98.2 #2 (Red Hat Linux)) id 1vtN2U-00000009F3X-37hM; Fri, 20 Feb 2026 09:43:10 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 8E7013030B3; Fri, 20 Feb 2026 10:43:03 +0100 (CET) Date: Fri, 20 Feb 2026 10:43:03 +0100 From: Peter Zijlstra To: Qais Yousef Cc: Tim Chen , "Chen, Yu C" , Ingo Molnar , K Prateek Nayak , "Gautham R . Shenoy" , Vincent Guittot , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Madadi Vineeth Reddy , Hillf Danton , Shrikanth Hegde , Jianyong Wu , Yangyu Chen , Tingyin Duan , Vern Hao , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Adam Li , Aaron Lu , Tim Chen , Josh Don , Gavin Guo , Libo Chen , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 00/21] Cache Aware Scheduling Message-ID: <20260220094303.GF2995752@noisy.programming.kicks-ass.net> References: <20260219140828.a7pyzupun7lsdw34@airbuntu> <20260219144108.GI1282955@noisy.programming.kicks-ass.net> <66a23acb-973e-4987-a006-18c50e18d3aa@intel.com> <4514b6aef56d0ae144ebd56df9211c6599744633.camel@linux.intel.com> <20260220032941.cw7zdlcy7n6fxmsj@airbuntu> 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: <20260220032941.cw7zdlcy7n6fxmsj@airbuntu> On Fri, Feb 20, 2026 at 03:29:41AM +0000, Qais Yousef wrote: > What's the reason wake up doesn't have the latest info? Is this a limitation of > these large systems where stats updates are too expensive to do? Is it not > fixable at all? Scalability is indeed the main problem. The periodic load-balancer, by virtue of being 'slow' has two advantages: - the cost of aggregating the numbers is amortized by the relative low frequency of aggregation - it can work with averages; it is less concerned with immediate spikes. This obviously has the exact inverse set of problems in that it is not able to deal with immediate/short term issues. Anyway, we're already at the point where EAS wakeup path is getting far too expensive for the current set of hardware. While we started with a handful of asymmetric CPUs, we're now pushing 32 CPUs or so. (Look at Intel Nova Lake speculation online, that's supposedly going to get us 2 dies of 8P+16E with another 4 bonus weaklings on the south bridge or something, for a grand total of 52 asymmetric CPUs of 3 kinds) Then consider: - Intel Granite Rapids-SP at 8*86 cores for 688 cores / 1376 threads. - AMD Prometheus at 2*192 cores with 384 cores / 768 threads. These are silly number of CPUs. - Power10, it is something like 16 sockets, 16 cores per socket, 8 threads per core for a mere 2048 threads. Now, these are the extreme end of the spectrum systems, 'nobody' will actually have them, but in a few generations they'll seem small again. So whatever we build now, will have to deal with silly numbers of CPUs.