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([2601:188:c102:b180:1f8b:71d0:77b1:1f6e]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8b3295deaf8sm297373485a.44.2025.11.20.21.07.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Nov 2025 21:07:48 -0800 (PST) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <6cd2dc59-e647-411f-ba3e-2a741487abb8@redhat.com> Date: Fri, 21 Nov 2025 00:07:47 -0500 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: [PATCH v2] cgroup: Add preemption protection to css_rstat_updated() To: Jiayuan Chen , cgroups@vger.kernel.org Cc: tj@kernel.org, hannes@cmpxchg.org, mkoutny@suse.com, linux-kernel@vger.kernel.org References: <20251121040655.89584-1-jiayuan.chen@linux.dev> Content-Language: en-US In-Reply-To: <20251121040655.89584-1-jiayuan.chen@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/20/25 11:06 PM, Jiayuan Chen wrote: > BPF programs do not disable preemption, they only disable migration. > Therefore, when running the cgroup_hierarchical_stats selftest, a > warning [1] is generated. > > The css_rstat_updated() function is lockless and reentrant. However, > as Tejun pointed out [2], preemption-related considerations need to > be considered. Since css_rstat_updated() can be called from BPF where > preemption is not disabled by its framework and it has already been > exposed as a kfunc to BPF programs, introducing a new kfunc like bpf_xx > will break existing uses. Thus, we directly make css_rstat_updated() > preempt-safe here. My understand of Tejun's comment is to add bpf_preempt_disable() and bpf_preempt_enable() calls around the css_rstat_updated() call in the bpf program defined in tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c instead of adding that in the css_rstat_updated() function itself. But I may be wrong. Cheers, Longman