From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 86D9A1BC57 for ; Tue, 25 Jun 2024 02:36:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719282996; cv=none; b=fSb8sZ3vq6BK8gZm3XhZSrN7LhHR58S8L9OeVGacCJZwqNvcV1lU0k0mLso2nU0ChrzMk6bimyojwrTiPT/k+x+4uR9Kr05+7LDdeX1Si981BR8T9myeTjEyHgsnkSUwxiuT5z4bK8NoXB39ULRXFsIcN8IeQqEhrwFLHg/TpjQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719282996; c=relaxed/simple; bh=E3dRV3THCRl8+7F7g/EIdSW3iGC+lN12chAsAfvgans=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type:Content-Disposition; b=j9mQI59BaC1J4qj+kGjihyMsC0ByQ3opQyOunfypdRzy4HsAd/m4EOJeBWmjefuXiZujimWb4WkanhIDtngOtQJJKzCs3FBFdVqfQnHtNC5uiqZierY+y0I3jp9l0vO9AxLHvnlJhXXe+W8JOJeueWp+ASi6DxQj+4ZOlb6BpeM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=S6Of6d5J; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="S6Of6d5J" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1719282992; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=lgjh/ZqyuyNc2ziwefd5jIC6p/c9Pbm133iPEufxgFk=; b=S6Of6d5Jdw3yiPYHikH1KxuxNbJ2CbLTzI9FKc7ihXnOGKlc9zFTqZIGZxxQzvkz0Tvb7i Fwe6I/H4hy4MHtriKQW4O8xBYTxu1NIByWsFxB17UV8aEk/LcU22ZseM7A4hWq6kgJeD5H nVAwqOQFDr1e0Oig3y+n6hj/gEIeUj8= Received: from mail-pj1-f72.google.com (mail-pj1-f72.google.com [209.85.216.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-37-HA7UuiOeO9qmN_8WdztCrA-1; Mon, 24 Jun 2024 22:36:31 -0400 X-MC-Unique: HA7UuiOeO9qmN_8WdztCrA-1 Received: by mail-pj1-f72.google.com with SMTP id 98e67ed59e1d1-2c7c3069f37so6267722a91.0 for ; Mon, 24 Jun 2024 19:36:31 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1719282990; x=1719887790; h=content-transfer-encoding:content-disposition:mime-version :references:in-reply-to:message-id:date:subject:cc:to:from :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=lgjh/ZqyuyNc2ziwefd5jIC6p/c9Pbm133iPEufxgFk=; b=Bsa+yzxKjRjYuC0Nlzg/ai25vYk/UHVXml6XVrAJVFG1yJvLwrWDV0x7U1czdmMUXj Epof8gE0G4fnNBVLWlKKtvujhJKKzDqqafKbQ54gwAER19KBIW95H5rErej5PImb68/H oZalIbP7D9zaq0oczGwC/BJKoOpYeRUaqrkopjipvLadJM3NKbnLMY7MnNJfePN+Zz1J sK6S7DJxdJoEKoZIoaMP4BOIhTKXTN6o9cyKHySciuZNS+a4F9utzdjiUrO0YaaUApre IEOMuBfjapUmu6IcMAwQ6dv4c54/slEwwQaeYcdTRZtXSNGPlWk3RPkUjEiyDAr6N+YD scdg== X-Forwarded-Encrypted: i=1; AJvYcCU7wz5aaBBkEJ3V4WWicbt3IyC7TbfH25Qk7/LsDoFe0NTZ7PQUxfSRRbn++WfvllD2sgzpPpWUPI3zA8r+aL6YBFZBSnG5Zw== X-Gm-Message-State: AOJu0YzGPqO9v/nMY+iolPmCrVrHzsW5dkpfzQWnBSQ5eh6kbTYLE4OC rtVUMvCZkilso6pTphg3yISL1XoNIc0hjqZcLjUAk9Z3Uyxyfhussv5NaXC7tYJbyzAfmY1ehHJ n86QiiEyO588+yfG0ZMu6krRlqmePeNtMRWw+pvIehkX1O3duoFT3kGs= X-Received: by 2002:a17:902:f54c:b0:1f9:ddea:451d with SMTP id d9443c01a7336-1fa23bdf3b8mr64915025ad.3.1719282990058; Mon, 24 Jun 2024 19:36:30 -0700 (PDT) X-Google-Smtp-Source: AGHT+IE4WMoko6gcC4VFyy3g03y2NymCh8kug4N9kCWpYbiJNlDLdPwV2S623AMwW20um4Llrvh4oA== X-Received: by 2002:a17:902:f54c:b0:1f9:ddea:451d with SMTP id d9443c01a7336-1fa23bdf3b8mr64914795ad.3.1719282989651; Mon, 24 Jun 2024 19:36:29 -0700 (PDT) Received: from LeoBras.redhat.com ([2804:1b3:a801:fda9:d11e:3755:61da:97fd]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-1f9eb2f038asm69440895ad.52.2024.06.24.19.36.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 Jun 2024 19:36:29 -0700 (PDT) From: Leonardo Bras To: Vlastimil Babka Cc: Leonardo Bras , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Thomas Gleixner , Marcelo Tosatti , Peter Zijlstra , Ingo Molnar , Will Deacon , Waiman Long , Boqun Feng , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH v1 0/4] Introduce QPW for per-cpu operations Date: Mon, 24 Jun 2024 23:36:21 -0300 Message-ID: X-Mailer: git-send-email 2.45.2 In-Reply-To: <261612b9-e975-4c02-a493-7b83fa17c607@suse.cz> References: <20240622035815.569665-1-leobras@redhat.com> <261612b9-e975-4c02-a493-7b83fa17c607@suse.cz> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: 8bit On Mon, Jun 24, 2024 at 09:31:51AM +0200, Vlastimil Babka wrote: > Hi, > > you've included tglx, which is great, but there's also LOCKING PRIMITIVES > section in MAINTAINERS so I've added folks from there in my reply. > Link to full series: > https://lore.kernel.org/all/20240622035815.569665-1-leobras@redhat.com/ Thanks Vlastimil! > > On 6/22/24 5:58 AM, Leonardo Bras wrote: > > The problem: > > Some places in the kernel implement a parallel programming strategy > > consisting on local_locks() for most of the work, and some rare remote > > operations are scheduled on target cpu. This keeps cache bouncing low since > > cacheline tends to be mostly local, and avoids the cost of locks in non-RT > > kernels, even though the very few remote operations will be expensive due > > to scheduling overhead. > > > > On the other hand, for RT workloads this can represent a problem: getting > > an important workload scheduled out to deal with remote requests is > > sure to introduce unexpected deadline misses. > > > > The idea: > > Currently with PREEMPT_RT=y, local_locks() become per-cpu spinlocks. > > In this case, instead of scheduling work on a remote cpu, it should > > be safe to grab that remote cpu's per-cpu spinlock and run the required > > work locally. Tha major cost, which is un/locking in every local function, > > already happens in PREEMPT_RT. > > I've also noticed this a while ago (likely in the context of rewriting SLUB > to use local_lock) and asked about it on IRC, and IIRC tglx wasn't fond of > the idea. But I forgot the details about why, so I'll let the the locking > experts reply... > > > Also, there is no need to worry about extra cache bouncing: > > The cacheline invalidation already happens due to schedule_work_on(). > > > > This will avoid schedule_work_on(), and thus avoid scheduling-out an > > RT workload. > > > > For patches 2, 3 & 4, I noticed just grabing the lock and executing > > the function locally is much faster than just scheduling it on a > > remote cpu. > > > > Proposed solution: > > A new interface called Queue PerCPU Work (QPW), which should replace > > Work Queue in the above mentioned use case. > > > > If PREEMPT_RT=n, this interfaces just wraps the current > > local_locks + WorkQueue behavior, so no expected change in runtime. > > > > If PREEMPT_RT=y, queue_percpu_work_on(cpu,...) will lock that cpu's > > per-cpu structure and perform work on it locally. This is possible > > because on functions that can be used for performing remote work on > > remote per-cpu structures, the local_lock (which is already > > a this_cpu spinlock()), will be replaced by a qpw_spinlock(), which > > is able to get the per_cpu spinlock() for the cpu passed as parameter. > > > > Patch 1 implements QPW interface, and patches 2, 3 & 4 replaces the > > current local_lock + WorkQueue interface by the QPW interface in > > swap, memcontrol & slub interface. > > > > Please let me know what you think on that, and please suggest > > improvements. > > > > Thanks a lot! > > Leo > > > > Leonardo Bras (4): > > Introducing qpw_lock() and per-cpu queue & flush work > > swap: apply new queue_percpu_work_on() interface > > memcontrol: apply new queue_percpu_work_on() interface > > slub: apply new queue_percpu_work_on() interface > > > > include/linux/qpw.h | 88 +++++++++++++++++++++++++++++++++++++++++++++ > > mm/memcontrol.c | 20 ++++++----- > > mm/slub.c | 26 ++++++++------ > > mm/swap.c | 26 +++++++------- > > 4 files changed, 127 insertions(+), 33 deletions(-) > > create mode 100644 include/linux/qpw.h > > > > > > base-commit: 50736169ecc8387247fe6a00932852ce7b057083 >