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[141.239.159.203]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-1f9eb5df727sm66433865ad.217.2024.06.24.13.17.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 Jun 2024 13:17:44 -0700 (PDT) Sender: Tejun Heo Date: Mon, 24 Jun 2024 10:17:42 -1000 From: Tejun Heo To: Peter Zijlstra Cc: Thomas Gleixner , Linus Torvalds , mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@kernel.org, joshdon@google.com, brho@google.com, pjt@google.com, derkling@google.com, haoluo@google.com, dvernet@meta.com, dschatzberg@meta.com, dskarlat@cs.cmu.edu, riel@surriel.com, changwoo@igalia.com, himadrics@inria.fr, memxor@gmail.com, andrea.righi@canonical.com, joel@joelfernandes.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCHSET v6] sched: Implement BPF extensible scheduler class Message-ID: References: <871q4rpi2s.ffs@tglx> <20240624093426.GH31592@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240624093426.GH31592@noisy.programming.kicks-ass.net> Hello, Peter. On Mon, Jun 24, 2024 at 11:34:26AM +0200, Peter Zijlstra wrote: > I'm confused. Once you've loaded the BPF thing, 'all' tasks you care > about should already be in the bpf class. So any fork() thereafter > should not need to switch classes. > > This means we can have this rwsem be strictly for the bpf tasks as > Thomas suggested. > > What are we missing? Maybe I am confused but let's say the BPF scheduler gets unloaded and reloaded. What would prevent a forking thread which didn't acquire the read lock from racing against the second loading? Also, let's say we can make it conditional but would the extra complication be justifiable? percpu_down_read()'s hot path is one likely() cond test followed by this_cpu_inc() wrapped in preempt_disable(). I'm not really sure eliding that can justify much. That said, as Thomas pointed out, the dl cancel path is silly. Let me clean that up. Thanks. -- tejun