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Mon, 27 Feb 2023 14:41:40 -0500 X-MC-Unique: 9Y3So9MEM_SG5dyC6CCJ3Q-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 15529857D07; Mon, 27 Feb 2023 19:41:40 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A5613140EBF4; Mon, 27 Feb 2023 19:41:39 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id F3A0E403BC530; Mon, 27 Feb 2023 16:41:18 -0300 (-03) Date: Mon, 27 Feb 2023 16:41:18 -0300 From: Marcelo Tosatti To: Hillf Danton Cc: Christoph Lameter , Andrew Morton , Aaron Tomlin , Frederic Weisbecker , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 00/11] fold per-CPU vmstats remotely Message-ID: References: <20230209150150.380060673@redhat.com> <20230224023410.2940-1-hdanton@sina.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230224023410.2940-1-hdanton@sina.com> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 X-Rspamd-Queue-Id: 06FC22001D X-Stat-Signature: wxqraa1pk9zk93fkjx8g4p7zu89c8tmd X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1677526904-80283 X-HE-Meta: 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 d5fMblTc Rd9qhXL8vJGId9IEEw++PfhslqyE2uiIw2+2Gydp7GY1FjkpguR52jp1BzliG1VpUt4Yy3xAz8pB/pzydCz38liBbxt3Ljq7/Zh2e1Lc9m6+/QA6bpD6Pt0C/Xenk01yghcydjT0161ALiVLXNdjfErtNn4LxoHRN3F7ELcvhNJm7hgn6/cKNhkEDhSxMTxpdQQtsA+9ICzaZCueIh88biEoTtbLw1c5zOeHDkboEP7Wpe0E= 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: On Fri, Feb 24, 2023 at 10:34:10AM +0800, Hillf Danton wrote: > On Thu, Feb 09, 2023 at 12:01:50PM -0300, Marcelo Tosatti wrote: > > This patch series addresses the following two problems: > > > > 1. A customer provided some evidence which indicates that > > the idle tick was stopped; albeit, CPU-specific vmstat > > counters still remained populated. > > > > Thus one can only assume quiet_vmstat() was not > > invoked on return to the idle loop. If I understand > > correctly, I suspect this divergence might erroneously > > prevent a reclaim attempt by kswapd. If the number of > > zone specific free pages are below their per-cpu drift > > value then zone_page_state_snapshot() is used to > > compute a more accurate view of the aforementioned > > statistic. Thus any task blocked on the NUMA node > > specific pfmemalloc_wait queue will be unable to make > > significant progress via direct reclaim unless it is > > killed after being woken up by kswapd > > (see throttle_direct_reclaim()) > > > > 2. With a SCHED_FIFO task that busy loops on a given CPU, > > and kworker for that CPU at SCHED_OTHER priority, > > queuing work to sync per-vmstats will either cause that > > work to never execute, or stalld (i.e. stall daemon) > > boosts kworker priority which causes a latency > > violation > > > > By having vmstat_shepherd flush the per-CPU counters to the > > global counters from remote CPUs. > > > > This is done using cmpxchg to manipulate the counters, > > both CPU locally (via the account functions), > > and remotely (via cpu_vm_stats_fold). > > Frankly another case of bandaid[1] ? > > [1] https://lore.kernel.org/lkml/20230223150624.GA29739@lst.de/ Only if you disable per-CPU vmstat counters for isolated CPUs (then maintenance of the data structures in isolated CPUs is not necessary). Which would be terrible for performance, however.