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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4957af6a15bsm78002565e9.4.2026.07.24.06.20.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Jul 2026 06:20:49 -0700 (PDT) Date: Fri, 24 Jul 2026 14:20:48 +0100 From: David Laight To: Zqiang Cc: paulmck@kernel.org, frederic@kernel.org, neeraj.upadhyay@kernel.org, joelagnelf@nvidia.com, urezki@gmail.com, boqun@kernel.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] rcu: Reduce stack usage in show_rcu_gp_kthreads() Message-ID: <20260724142048.53e392a9@pumpkin> In-Reply-To: <20260723100430.17813-1-qiang.zhang@linux.dev> References: <20260723100430.17813-1-qiang.zhang@linux.dev> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 23 Jul 2026 18:04:30 +0800 Zqiang wrote: > When CONFIG_KASAN=y and CONFIG_KASAN_STACK=y builds, the > show_rcu_gp_kthreads() exceeds the 1024-byte frame-size limit: > ... > rcu_for_each_node_breadth_first(rnp) { > if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) && > !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) && > !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks))) > continue; > - pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n", > - rnp->grplo, rnp->grphi, > - (long)data_race(READ_ONCE(rnp->gp_seq)), > - (long)data_race(READ_ONCE(rnp->gp_seq_needed)), > - data_race(READ_ONCE(rnp->qsmask)), > - ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))], > - ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))], > - ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))], > - ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))], > - data_race(READ_ONCE(rnp->n_boosts))); Isn't that code carefully reading all the values twice? Also the "ab"[!!val] generates far worse code than the more obvious (val ? 'a' : 'b'). For the former gcc indexes a constant string, the latter is done using arithmetic. I'm sure there is a good reason for data_race(READ_ONCE(xxx)) ... David