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Fri, 04 Sep 2026 05:24:09 -0700 (PDT) Message-ID: <483ad434-4512-442e-ae6c-4108d5af5e49@gmail.com> Date: Fri, 4 Sep 2026 20:24:05 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] rcu: Align stall warning 'idle=' output with documentation From: Peng Donglin To: paulmck@kernel.org, joelagnelf@nvidia.com Cc: frederic@kernel.org, neeraj.upadhyay@kernel.org, qiang.zhang@linux.dev, rostedt@goodmis.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, Donglin Peng References: <20260105032112.1156359-1-dolinux.peng@gmail.com> Content-Language: en-US In-Reply-To: <20260105032112.1156359-1-dolinux.peng@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/5/26 11:21, Donglin Peng wrote: > From: Donglin Peng > > The RCU stall warning message includes an "idle=" field to indicate > the dyntick-idle state of a CPU. According to Documentation/RCU/stallwarn.rst, > the hexadecimal number before the first '/' represents the low-order 16 > bits of the dynticks counter. An even value denotes that the CPU is in > dyntick-idle mode, while an odd value indicates otherwise. > > This was valid until commit 171476775d32 ("context_tracking: Convert state to atomic_t"), > which merged the context-tracking state and dynticks counter into a > single atomic variable. In the new layout, the dynticks counter occupies > the higher bits starting from CT_RCU_WATCHING_START. > > However, the current stall warning code prints the value from > `ct_rcu_watching_cpu()`, which returns `atomic_read(&ct->state) & > CT_RCU_WATCHING_MASK`. This masks out (clears) the lower state bits, > resulting in a value that is always even. This obscures the CPU's true > idle state and makes the output inconsistent with the documentation. > > To restore consistency between the code's output and the documentation, > shift the atomic value right by CT_RCU_WATCHING_START bits before printing. > This extracts and displays only the relevant dynticks counter portion, > allowing the parity (even/odd) to correctly reflect the CPU's dyntick-idle > state. > > Fixes: 171476775d32 ("context_tracking: Convert state to atomic_t") > Signed-off-by: Donglin Peng > Reviewed-by: Joel Fernandes > --- > v2: > - Use CT_RCU_WATCHING_START to replace ilog2(CT_RCU_WATCHING) to clean > up the code, thanks to Zqiang. > - Clarify the commit message. > --- > kernel/rcu/tree_stall.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h > index b67532cb8770..fd9d7fd6e17c 100644 > --- a/kernel/rcu/tree_stall.h > +++ b/kernel/rcu/tree_stall.h > @@ -555,7 +555,7 @@ static void print_cpu_stall_info(int cpu) > rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' : > "!."[!delta], > ticks_value, ticks_title, > - ct_rcu_watching_cpu(cpu) & 0xffff, > + (ct_rcu_watching_cpu(cpu) >> CT_RCU_WATCHING_START) & 0xffff, > ct_nesting_cpu(cpu), ct_nmi_nesting_cpu(cpu), > rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu), > data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart, Gentle ping on this patch. Thanks!