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 AF0768BE7 for ; Wed, 11 Sep 2024 07:17:34 +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=1726039056; cv=none; b=GHfRQuXkUhxT0K7cQaYifOpq4hi+MvIKtzCsOtFoP/rPj44YxY4EdBmXXhNPBTmAfgA6aPQuxG02BqdKsDIDFy5CuL8W3J5ijYGzI+yGY7qtI8jeeWFT9+xHY8mDF7jiVS5hLyCEoIPt0E1omFc2YdB4tBwcQLhCiS+NwSr8/rg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726039056; c=relaxed/simple; bh=rAXfuwy7B2+QXySHJ14taKQMc/nkJ5KahS/kgFJm4Vc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type:Content-Disposition; b=rqdQCGoO8/5yhDpEizNRVRGSaI+8bLrfZ1yhcjpH+3PC4sQSfDTP7Z4X3KF+3jMZaH6gizGRuOQoagzyH+Z/LemP0UaqDygngzB1fog010eBUNBD7B4wCmYQT6q5EKWvQFEGFjRPSGFwBTc/VCLD7lt5XIW0BBTLJ2CLt5Ue9ew= 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=BDuvwKwH; 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="BDuvwKwH" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1726039052; 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=EWS0zdSRA6QBuHeVPu9p9QQM9sNJybBO18pUWus00es=; b=BDuvwKwHV5+2sbuZT+3eT2+2rZ/p9j0YbVifPGMjJB7q2fHEA4Gh/ddy+4XOgeml8RB6q2 z2Atpt0Fgkv972VXLyj3RL9WSuy78jI1ZXtONJDXMWUtAWxdAjAjbx1YxQclxQaam58/N9 cUgPHMvIuwcWy5xRrXY1jlq4XE7TPEU= Received: from mail-pg1-f197.google.com (mail-pg1-f197.google.com [209.85.215.197]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-43-oNaXzNZoNUqxwosi42M08Q-1; Wed, 11 Sep 2024 03:17:30 -0400 X-MC-Unique: oNaXzNZoNUqxwosi42M08Q-1 Received: by mail-pg1-f197.google.com with SMTP id 41be03b00d2f7-7d4f9974c64so5180763a12.1 for ; Wed, 11 Sep 2024 00:17:30 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1726039049; x=1726643849; 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=EWS0zdSRA6QBuHeVPu9p9QQM9sNJybBO18pUWus00es=; b=I1jKX6dzDJ1IkJOFf9/CkjxmVyUN3zxDaZ3uTUTPzaPZbnmmlBwodzXrDwZvGTUKDZ kRGXERDyjQ6mLFXc8i7LChcB8OGgbnhEv4AMlYuouA76SlIZb+ZEz+yLod3gQi9gMFRc L7R6t/QJaAl9tDbyLkONyKIRpPi9k8MYWdG4iDE7taCklX6NbIjqQVTwWdeyI8BGTpr6 Dm0wMhwvzgUbBagYasR/BP85gIqUaLE2rXaYd7u0FkqwPrbSXejLUAUdk5Zmkju4PI/D GUL/yqcxA1GFTXkztHBfhxwJJRlULrBeXC1ZUqGWle+JnII1VsCI9wst53EWEalQC8Ba K1Uw== X-Forwarded-Encrypted: i=1; AJvYcCWQHQ7KMTY8KsPLnVGEGhMbcEJNQ2frH965FVllTsRJEcPyB7u3UXBpEb9YbtVpMLo2vH0UvSN+b0BxKWs=@vger.kernel.org X-Gm-Message-State: AOJu0YzlTE6wBayK4sJkhlKyHjP9q8SVaCnxC2739ySvdBaJ4uxGWzin ZZH+l4SF6N4673Jc0DvVlufiTlB7shwWqJf3koXiO24Dxp1fenRXikBrqIgrT9YQwS59O4Mf3E2 bYemWkUTaEUGOAARKCdyf+DOshlaK+j3+QmPLGsAwITTJss09lUzd1TbmT0zweA== X-Received: by 2002:a05:6a20:d526:b0:1ce:cde2:4458 with SMTP id adf61e73a8af0-1cf5e15710amr5116831637.35.1726039049180; Wed, 11 Sep 2024 00:17:29 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGfPMdzQCpRyvsuzf833UGi7nTs6J1w1LxjL6dvBwUdC2XXOTOBOy5/7LWeNy6FfSCYJeydpA== X-Received: by 2002:a05:6a20:d526:b0:1ce:cde2:4458 with SMTP id adf61e73a8af0-1cf5e15710amr5116788637.35.1726039048478; Wed, 11 Sep 2024 00:17:28 -0700 (PDT) Received: from localhost.localdomain ([2804:1b3:a800:3c59:c8f1:7d33:571a:fde2]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-7db12af2c69sm419278a12.38.2024.09.11.00.17.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Sep 2024 00:17:27 -0700 (PDT) From: Leonardo Bras To: Waiman Long Cc: Leonardo Bras , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Thomas Gleixner , Marcelo Tosatti , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH v1 1/4] Introducing qpw_lock() and per-cpu queue & flush work Date: Wed, 11 Sep 2024 04:17:05 -0300 Message-ID: X-Mailer: git-send-email 2.46.0 In-Reply-To: References: <20240622035815.569665-1-leobras@redhat.com> <20240622035815.569665-2-leobras@redhat.com> 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 Content-Transfer-Encoding: 8bit On Wed, Sep 04, 2024 at 05:39:01PM -0400, Waiman Long wrote: > On 6/21/24 23:58, Leonardo Bras wrote: > > 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 some unrelated task is > > sure to introduce unexpected deadline misses. > > > > It's interesting, though, that local_lock()s in RT kernels become > > spinlock(). We can make use of those to avoid scheduling work on a remote > > cpu by directly updating another cpu's per_cpu structure, while holding > > it's spinlock(). > > > > In order to do that, it's necessary to introduce a new set of functions to > > make it possible to get another cpu's per-cpu "local" lock (qpw_{un,}lock*) > > and also the corresponding queue_percpu_work_on() and flush_percpu_work() > > helpers to run the remote work. > > > > On non-RT kernels, no changes are expected, as every one of the introduced > > helpers work the exactly same as the current implementation: > > qpw_{un,}lock*() -> local_{un,}lock*() (ignores cpu parameter) > > queue_percpu_work_on() -> queue_work_on() > > flush_percpu_work() -> flush_work() > > > > For RT kernels, though, qpw_{un,}lock*() will use the extra cpu parameter > > to select the correct per-cpu structure to work on, and acquire the > > spinlock for that cpu. > > > > queue_percpu_work_on() will just call the requested function in the current > > cpu, which will operate in another cpu's per-cpu object. Since the > > local_locks() become spinlock()s in PREEMPT_RT, we are safe doing that. > > > > flush_percpu_work() then becomes a no-op since no work is actually > > scheduled on a remote cpu. > > > > Some minimal code rework is needed in order to make this mechanism work: > > The calls for local_{un,}lock*() on the functions that are currently > > scheduled on remote cpus need to be replaced by qpw_{un,}lock_n*(), so in > > RT kernels they can reference a different cpu. It's also necessary to use a > > qpw_struct instead of a work_struct, but it just contains a work struct > > and, in PREEMPT_RT, the target cpu. > > > > This should have almost no impact on non-RT kernels: few this_cpu_ptr() > > will become per_cpu_ptr(,smp_processor_id()). > > > > On RT kernels, this should improve performance and reduce latency by > > removing scheduling noise. > > > > Signed-off-by: Leonardo Bras > > --- > > include/linux/qpw.h | 88 +++++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 88 insertions(+) > > create mode 100644 include/linux/qpw.h > > > > diff --git a/include/linux/qpw.h b/include/linux/qpw.h > > new file mode 100644 > > index 000000000000..ea2686a01e5e > > --- /dev/null > > +++ b/include/linux/qpw.h > > @@ -0,0 +1,88 @@ > > +/* SPDX-License-Identifier: GPL-2.0 */ > > +#ifndef _LINUX_QPW_H > > +#define _LINUX_QPW_H > > + > > +#include "linux/local_lock.h" > > +#include "linux/workqueue.h" > > + > > +#ifndef CONFIG_PREEMPT_RT > > + > > +struct qpw_struct { > > + struct work_struct work; > > +}; > > + > > +#define qpw_lock(lock, cpu) \ > > + local_lock(lock) > > + > > +#define qpw_unlock(lock, cpu) \ > > + local_unlock(lock) > > + > > +#define qpw_lock_irqsave(lock, flags, cpu) \ > > + local_lock_irqsave(lock, flags) > > + > > +#define qpw_unlock_irqrestore(lock, flags, cpu) \ > > + local_unlock_irqrestore(lock, flags) > > + > > +#define queue_percpu_work_on(c, wq, qpw) \ > > + queue_work_on(c, wq, &(qpw)->work) > > + > > +#define flush_percpu_work(qpw) \ > > + flush_work(&(qpw)->work) > > + > > +#define qpw_get_cpu(qpw) \ > > + smp_processor_id() > > + > > +#define INIT_QPW(qpw, func, c) \ > > + INIT_WORK(&(qpw)->work, (func)) > > + > > +#else /* !CONFIG_PREEMPT_RT */ > > + > > +struct qpw_struct { > > + struct work_struct work; > > + int cpu; > > +}; > > + > > +#define qpw_lock(__lock, cpu) \ > > + do { \ > > + migrate_disable(); \ > > + spin_lock(per_cpu_ptr((__lock), cpu)); \ > > + } while (0) > > + > > +#define qpw_unlock(__lock, cpu) \ > > + do { \ > > + spin_unlock(per_cpu_ptr((__lock), cpu)); \ > > + migrate_enable(); \ > > + } while (0) > > Why there is a migrate_disable/enable() call in qpw_lock/unlock()? The > rt_spin_lock/unlock() calls have already include a migrate_disable/enable() > pair. This was copied from PREEMPT_RT=y local_locks. In my tree, I see: #define __local_unlock(__lock) \ do { \ spin_unlock(this_cpu_ptr((__lock))); \ migrate_enable(); \ } while (0) But you are right: For PREEMPT_RT=y, spin_{un,}lock() will be defined in spinlock_rt.h as rt_spin{un,}lock(), which already runs migrate_{en,dis}able(). On the other hand, for spin_lock() will run migrate_disable() just before finishing the function, and local_lock() will run it before calling spin_lock() and thus, before spin_acquire(). (local_unlock looks like to have an unnecessary extra migrate_enable(), though). I am not sure if it's actually necessary to run this extra migrate_disable() in local_lock() case, maybe Thomas could help us understand this. But sure, if we can remove this from local_{un,}lock(), I am sure we can also remove this from qpw. > > > + > > +#define qpw_lock_irqsave(lock, flags, cpu) \ > > + do { \ > > + typecheck(unsigned long, flags); \ > > + flags = 0; \ > > + qpw_lock(lock, cpu); \ > > + } while (0) > > + > > +#define qpw_unlock_irqrestore(lock, flags, cpu) \ > > + qpw_unlock(lock, cpu) > > + > > +#define queue_percpu_work_on(c, wq, qpw) \ > > + do { \ > > + struct qpw_struct *__qpw = (qpw); \ > > + WARN_ON((c) != __qpw->cpu); \ > > + __qpw->work.func(&__qpw->work); \ > > + } while (0) > > + > > +#define flush_percpu_work(qpw) \ > > + do {} while (0) > > + > > +#define qpw_get_cpu(w) \ > > + container_of((w), struct qpw_struct, work)->cpu > > + > > +#define INIT_QPW(qpw, func, c) \ > > + do { \ > > + struct qpw_struct *__qpw = (qpw); \ > > + INIT_WORK(&__qpw->work, (func)); \ > > + __qpw->cpu = (c); \ > > + } while (0) > > + > > +#endif /* CONFIG_PREEMPT_RT */ > > +#endif /* LINUX_QPW_H */ > > You may also consider adding a documentation file about the > qpw_lock/unlock() calls. Sure, will do when I send the non-RFC version. Thanks for pointing that out! > > Cheers, > Longman > Thanks! Leo