From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 F23F87FD for ; Thu, 5 Sep 2024 00:08:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725494909; cv=none; b=u5J7WdIMjZ1dKoImZoDmiD6TGeeUqB1xkHkIPR0rq/L7CkuWaFpCPXnYcBEwCfMPN+Vlq2JNGFDDJsdTKYzJaBTxY5nRclSCE+bcWNMFbzDnOYj8edqEtwe3Q1/1puV63S7LJA6o7mcw+GYthexqmT4iFY9uLUcWfUds7vDD6qE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725494909; c=relaxed/simple; bh=xBRl5lCEw9Aaa1Gcw7JKGR18nO9scrn+v/5PHEBHzTM=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=f22YcyJ34vL4gVI2+0UwHxWeqUCxkzXRBcRs0PxJ3onCRVC5/1MtGJsf4WEVa1wBAi/y75054uJ3v/tjJ7OAUvxDKpSHEBhnm/Yax0ODjj0tv9N50oCa+X08EQ7O6GDtlcQ0RlS/Z/7g9g9Yt/CHA7O+vypgqKDTmVMNUVVS1QU= 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=adrF772B; arc=none smtp.client-ip=170.10.133.124 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="adrF772B" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1725494906; 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=UHS4zP9jA0mTvK7JF/ePZcYsAK2+oZaSxTHPolbumnA=; b=adrF772B09M+jZF9ku78jI/t8jkl2MDLbh9TSxv3TNGT1H0XlCsOaAfqWClQffd7EYknsX xKMt+ln9SDp2apVTGJfBu0kADuJqvbfr6kZHj2uFzZ902Oe5cndDGRoHAwnEWzbyXABHCy +sAF8oLuLplqLunhmFJn1XmCOMWYfeQ= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-390-SmZMspabPjaLULqbFRBxhQ-1; Wed, 04 Sep 2024 20:08:20 -0400 X-MC-Unique: SmZMspabPjaLULqbFRBxhQ-1 Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D2AD31955D45; Thu, 5 Sep 2024 00:08:17 +0000 (UTC) Received: from [10.2.16.172] (unknown [10.2.16.172]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 902B21956086; Thu, 5 Sep 2024 00:08:13 +0000 (UTC) Message-ID: Date: Wed, 4 Sep 2024 20:08:12 -0400 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 1/4] Introducing qpw_lock() and per-cpu queue & flush work From: Waiman Long To: 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 Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org References: <20240622035815.569665-1-leobras@redhat.com> <20240622035815.569665-2-leobras@redhat.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 On 9/4/24 17:39, 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 I would suggest adding a comment with a brief description of what qpw_lock/unlock() are for and their use cases. The "qpw" prefix itself isn't intuitive enough for a casual reader to understand what they are for. Cheers, Longman