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Mon, 24 Jun 2024 07:31:52 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id wgcwEeggeWYtfgAAD6G6ig (envelope-from ); Mon, 24 Jun 2024 07:31:52 +0000 Message-ID: <261612b9-e975-4c02-a493-7b83fa17c607@suse.cz> Date: Mon, 24 Jun 2024 09:31:51 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 0/4] Introduce QPW for per-cpu operations To: 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 Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org References: <20240622035815.569665-1-leobras@redhat.com> Content-Language: en-US From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJkBREIBQkRadznAAoJECJPp+fMgqZkNxIQ ALZRqwdUGzqL2aeSavbum/VF/+td+nZfuH0xeWiO2w8mG0+nPd5j9ujYeHcUP1edE7uQrjOC Gs9sm8+W1xYnbClMJTsXiAV88D2btFUdU1mCXURAL9wWZ8Jsmz5ZH2V6AUszvNezsS/VIT87 AmTtj31TLDGwdxaZTSYLwAOOOtyqafOEq+gJB30RxTRE3h3G1zpO7OM9K6ysLdAlwAGYWgJJ V4JqGsQ/lyEtxxFpUCjb5Pztp7cQxhlkil0oBYHkudiG8j1U3DG8iC6rnB4yJaLphKx57NuQ PIY0Bccg+r9gIQ4XeSK2PQhdXdy3UWBr913ZQ9AI2usid3s5vabo4iBvpJNFLgUmxFnr73SJ KsRh/2OBsg1XXF/wRQGBO9vRuJUAbnaIVcmGOUogdBVS9Sun/Sy4GNA++KtFZK95U7J417/J Hub2xV6Ehc7UGW6fIvIQmzJ3zaTEfuriU1P8ayfddrAgZb25JnOW7L1zdYL8rXiezOyYZ8Fm ZyXjzWdO0RpxcUEp6GsJr11Bc4F3aae9OZtwtLL/jxc7y6pUugB00PodgnQ6CMcfR/HjXlae h2VS3zl9+tQWHu6s1R58t5BuMS2FNA58wU/IazImc/ZQA+slDBfhRDGYlExjg19UXWe/gMcl De3P1kxYPgZdGE2eZpRLIbt+rYnqQKy8UxlszsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZAUSmwUJDK5EZgAKCRAiT6fnzIKmZOJGEACOKABgo9wJXsbWhGWYO7mD 8R8mUyJHqbvaz+yTLnvRwfe/VwafFfDMx5GYVYzMY9TWpA8psFTKTUIIQmx2scYsRBUwm5VI EurRWKqENcDRjyo+ol59j0FViYysjQQeobXBDDE31t5SBg++veI6tXfpco/UiKEsDswL1WAr tEAZaruo7254TyH+gydURl2wJuzo/aZ7Y7PpqaODbYv727Dvm5eX64HCyyAH0s6sOCyGF5/p eIhrOn24oBf67KtdAN3H9JoFNUVTYJc1VJU3R1JtVdgwEdr+NEciEfYl0O19VpLE/PZxP4wX PWnhf5WjdoNI1Xec+RcJ5p/pSel0jnvBX8L2cmniYnmI883NhtGZsEWj++wyKiS4NranDFlA HdDM3b4lUth1pTtABKQ1YuTvehj7EfoWD3bv9kuGZGPrAeFNiHPdOT7DaXKeHpW9homgtBxj 8aX/UkSvEGJKUEbFL9cVa5tzyialGkSiZJNkWgeHe+jEcfRT6pJZOJidSCdzvJpbdJmm+eED w9XOLH1IIWh7RURU7G1iOfEfmImFeC3cbbS73LQEFGe1urxvIH5K/7vX+FkNcr9ujwWuPE9b 1C2o4i/yZPLXIVy387EjA6GZMqvQUFuSTs/GeBcv0NjIQi8867H3uLjz+mQy63fAitsDwLmR EP+ylKVEKb0Q2A== In-Reply-To: <20240622035815.569665-1-leobras@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: gwpi3sf7uwfi9khudg8bcomef6177neh X-Rspam-User: X-Rspamd-Queue-Id: 502121C0008 X-Rspamd-Server: rspam02 X-HE-Tag: 1719214314-625619 X-HE-Meta: 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 bH/A4uSl VBxR8Nxk963cNz92nJNeMZ9Rf0fGR5klC5yTl0+hZy6Xnvp1mp1dAvWIKKNz0ne7o9EtLbRsV3M8z8HVMwV5/hjdPkKtXY0+/47lxu8/xRjaHA5T52aCnca6sNU0PmDAkS0TbvROG9nSxQRUgvdfJffp/9+NX20i5xWeNxMz8dY/5FQMDQt8jXFbCkDIn3hmH3ZYce9+TRbPjIIdY4xvRwx60tNnAuoIDFrzwl38PT3G8j3kZOAX96z2rprmiOcogiZsUVkBKMAuA3shT9sEp+QqTajOdniIhqUxKYCTIcodZXcGNCKEQ+jFGDJinQUVP84a30Yx6iYCrzqlVlx40aArY6Imi3b0zINENxiKUeh4boGnbDWOKwukUcRtjTpj3irTUIOZr2a0CO5m1dye1xVYZ8NSVjcnoU1YuSOq5NnxhVKCSFr4T5LcnnJHW2FjCRzvi X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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/ 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