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bh=AXIB59EbtVdksLT+wsAse63+38ZSeeP3bCJp0P18k5w=; b=cG1NN0R6Zebj3uVriz/N4yE1Yuu9wbdRQxcb0JvzOYGDCl1iMKZLfLV7w8EHWskwBf+4pc SIVzOjvEHj0dpD3f3+letQqWkxlNRB+PPIEOgyY5fckTI0wKlcVqnucgQV7rX3zj3MX50K gNca5Iq/IrWBLNPkBd9zGepmTp95fHFNKJEACZ2x0nymvYQA8F54VeQg5Zu07sy246MvqL kdE6jn4dephOm20CzWieXCQqauJ6yD7tyr/cf/zYqKFfpNdtGBKGimKHpVhj7vQtTFuHzY /3lGH05/XTeGj3uyc3JoMWydC6ORwc8/jXN3NY5TTRwCnh/G11LQ+KaP8w9CQA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1681196208; 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: in-reply-to:in-reply-to:references:references; bh=AXIB59EbtVdksLT+wsAse63+38ZSeeP3bCJp0P18k5w=; b=qfH70LiJxuNow2J6hVZf9FG47KKDjkCVjGv/McCbLSwvoQvIp+isTn8SNGdZUcID8015c0 jSfws4+E96iGHJDw== To: Ye Bin , dennis@kernel.org, tj@kernel.org, cl@linux.com, linux-mm@kvack.org, yury.norov@gmail.com, andriy.shevchenko@linux.intel.com, linux@rasmusvillemoes.dk Cc: linux-kernel@vger.kernel.org, dchinner@redhat.com, yebin10@huawei.com, yebin@huaweicloud.com, Peter Zijlstra , Valentin Schneider , Linus Torvalds Subject: Re: [PATCH v2 2/2] lib/percpu_counter: fix dying cpu compare race In-Reply-To: <877cuj1mt1.ffs@tglx> References: <20230406015629.1804722-1-yebin@huaweicloud.com> <20230406015629.1804722-3-yebin@huaweicloud.com> <877cuj1mt1.ffs@tglx> Date: Tue, 11 Apr 2023 08:56:48 +0200 Message-ID: <874jpm29fz.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 33C7940009 X-Rspam-User: X-Stat-Signature: str97y4pd661qrztg5wfettw4tjnixyy X-HE-Tag: 1681196210-437435 X-HE-Meta: U2FsdGVkX19wd6S1UoFBXBfLDY7XkJcWLS8+bq7PveA66FkF0lUhIvoGc3/dqMPu9NeCrlKsf1U3keD0LeYyMsBMaemUbAzdg1TUssswVAxMZKKAszR2VxboGRSBxE8fIsyD1pYiu9sa5X4YjQqKC3E6a5Ev2FO5gPKzto6lZIviE4SzOZTo//xDSybYA3O3yQVaanqk4guY3cwA9KKN2S0cyRUg7bkw4f1VMQ8KRbwY0r12+RM4HuGVhI2pWmXumia9SS5ESwpZkiJNM41QDq2O3ylyeFN3mcFli1OuR5cuIWG4Q3+Dp53ezfl1H7O4WD4NttS+soGU9GGzBeR7/UzLakUXo4vkGZywrO+uGr9BXf0ZWdm0SHU/0wTjep9Jw8yS5t0jBQJUgebUlaeXvgEl0OArl6N2+sHXGlAQlCQOj3KY8YiqwurebT2Zc/qpPjQp4pUHZ3DOG6/FtseKgPA0enH6nkUtFvVEelRRuoAy+/n0/mBfxj8z6zMWQXxHIgHCR83XpplWj6iCBquqP8LsRwn+M84dvaD7HD1zvurUP3d/mhx8GiMYYsFq8wT834NTO8556GB1VyHN/eMAJ51mbmG81f5U+AD3ig1VTnsk5rVAr1XIficgxgiPTSkz7A22pxBdfpbJNRRG76z48shIA9dSZv+lR8LTYxgRtFIjeXxt4S3BOOVTJHa6ME8pN4Ab73H6G+p0LMPGcI27RJtTF2EA2JaRjUoo38/WX1LEnW8RloOc2qkGZX2T7MPgl3ZQAmMuiRjHebBHp0oweoiuG2iPedZ3aNKaz/QZsdeZGulg4TMoZJUYnb63h1dW27rfKrlaU7gxykDeoFnu3rpZy7dmOX32crdzzvGx4/ri+BY5S6te+BKF5HEGv2Q2FJTJvjo2rbtvBc9ICtypXJ0cw9uC0extaM5gtAu6sos2aXBjQp1snfOcfkN1ATdi3C+tpdbsXNYqcZmA6zA aALtdZDF cmRZGUzRM1w0b9kZCGfS/1FeJuwU+2Y0Jb33Vj8Xq9Eso9IvHBYY/T9V5vENYCEQEUwdNbe/IFE4ReUqaoSoehqh3TNN21PqBAe6f5iYKyItMFFeoaeZ0zKXISaCzdrZjYVJfyQjOyR6ZhBXm4RiEtifYfx0td0B3tCd71THOMhZWunyO46iwtLx6EQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000003, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Apr 10 2023 at 22:53, Thomas Gleixner wrote: > On Thu, Apr 06 2023 at 09:56, Ye Bin wrote: > cpu_dying_mask is a random number generator w/o cpu_hotplug_lock being > held. And even with that lock held any cpumask operation on it is silly. > The mask is a core detail: > > commit e40f74c535b8 "cpumask: Introduce DYING mask" > > Introduce a cpumask that indicates (for each CPU) what direction the > CPU hotplug is currently going. Notably, it tracks rollbacks. Eg. when > an up fails and we do a roll-back down, it will accurately reflect the > direction. > > It does not tell anything to a user which is not aware of the actual > hotplug state machine state. Even if the mask is most of the time stable, it's a total disaster performance wise. The bits in cpu_dying_mask are sticky until the next online operation. So for a system which has SMT enabled in BIOS, but SMT is disabled on the kernel command line or later via the sysfs knob, this means that the loop in __percpu_counter_sum() will iterate over all shutdown SMT siblings forever. IOW, it will touch nr_of_shutdown_cpus() cachelines for absolutely zero reason. The same applies for the proposed counter. Thanks, tglx