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[70.30.145.63]) by smtp.gmail.com with ESMTPSA id 19-20020ac85713000000b003adc7f652a0sm1420494qtw.66.2023.01.26.12.30.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 26 Jan 2023 12:30:56 -0800 (PST) Date: Thu, 26 Jan 2023 15:30:47 -0500 From: Peter Xu To: James Houghton Cc: Mike Kravetz , David Hildenbrand , Muchun Song , David Rientjes , Axel Rasmussen , Mina Almasry , Zach O'Keefe , Manish Mishra , Naoya Horiguchi , "Dr . David Alan Gilbert" , "Matthew Wilcox (Oracle)" , Vlastimil Babka , Baolin Wang , Miaohe Lin , Yang Shi , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb_range Message-ID: References: <06423461-c543-56fe-cc63-cabda6871104@redhat.com> <6548b3b3-30c9-8f64-7d28-8a434e0a0b80@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Queue-Id: 775391C0011 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: fp3n9papii1jusstj48fwbohx4uwkibd X-HE-Tag: 1674765060-313990 X-HE-Meta: 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 74pcSrbs RcSvEj+uKussHF3jeGnWLDyhtzJ6AWvrw8pljXzMgAUCxXsQZ9l8bcBBjgUXcdbcDSKYumF3Kk6EmiJ7b5gkKrIuJySM4lPi6Pw8oR8kXylWRv4aWqqD5DLZE2KbpTylHhQBNhKfHzmKmnzUw4rwnz17trmPRtJYL1sWsKKCkQ0qE0Wuq3PcXZd9TYRAnLyFB9diVsoYqAiGwIGHZgy0FqHr48N/RNI9LbiBJch4BeRBpI9jzlwAuAImqzbwsDhtGC5B/+omHQXpSG/R54IuIFOM1drJ3/OmFA5gPcLycrxWqHVn1DnZSMvZzztr6DZ2cS+5KzP+aPblXMFtkhuKGQBmw0aLLUYzMMdlWI+XdAqF6Me2irnDwoeTbUnbZ+cMCQS5faGlNfRYQE97Gec5M7KVNJk/+Pz3v2vmER17bAALtL2c= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000016, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: James, On Thu, Jan 26, 2023 at 08:58:51AM -0800, James Houghton wrote: > It turns out that the THP-like scheme significantly slows down > MADV_COLLAPSE: decrementing the mapcounts for the 4K subpages becomes > the vast majority of the time spent in MADV_COLLAPSE when collapsing > 1G mappings. It is doing 262k atomic decrements, so this makes sense. > > This is only really a problem because this is done between > mmu_notifier_invalidate_range_start() and > mmu_notifier_invalidate_range_end(), so KVM won't allow vCPUs to > access any of the 1G page while we're doing this (and it can take like > ~1 second for each 1G, at least on the x86 server I was testing on). Did you try to measure the time, or it's a quick observation from perf? IIRC I used to measure some atomic ops, it is not as drastic as I thought. But maybe it depends on many things. I'm curious how the 1sec is provisioned between the procedures. E.g., I would expect mmu_notifier_invalidate_range_start() to also take some time too as it should walk the smally mapped EPT pgtables. Since we'll still keep the intermediate levels around - from application POV, one other thing to remedy this is further shrink the size of COLLAPSE so potentially for a very large page we can start with building 2M layers. But then collapse will need to be run at least two rounds. -- Peter Xu