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Wed, 31 Jul 2024 14:35:44 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEgMw/lq8z11iXy6NClEQGkS+wB2jJkXY1ZzhXNJmlxGdvJTy8QQN/FeK1tXC6AlJ6D1EuiIA== X-Received: by 2002:a05:6214:3908:b0:6b4:fea8:6bfc with SMTP id 6a1803df08f44-6bb8d6845b9mr5535036d6.10.1722461743579; Wed, 31 Jul 2024 14:35:43 -0700 (PDT) Received: from LeoBras.redhat.com ([2804:1b3:a803:da7:cfdf:ab65:d193:5573]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6bb7bf7f7acsm18905066d6.54.2024.07.31.14.35.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 31 Jul 2024 14:35:43 -0700 (PDT) From: Leonardo Bras To: neeraj.upadhyay@kernel.org Cc: Leonardo Bras , linux-kernel@vger.kernel.org, rcu@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, mingo@kernel.org, peterz@infradead.org, paulmck@kernel.org, imran.f.khan@oracle.com, riel@surriel.com, tglx@linutronix.de Subject: Re: [PATCH v2 2/3] locking/csd_lock: Provide an indication of ongoing CSD-lock stall Date: Wed, 31 Jul 2024 18:35:35 -0300 Message-ID: X-Mailer: git-send-email 2.46.0 In-Reply-To: <20240722133735.667161-2-neeraj.upadhyay@kernel.org> References: <20240722133559.GA667117@neeraj.linux> <20240722133735.667161-2-neeraj.upadhyay@kernel.org> 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 Content-Transfer-Encoding: 8bit On Mon, Jul 22, 2024 at 07:07:34PM +0530, neeraj.upadhyay@kernel.org wrote: > From: "Paul E. McKenney" > > If a CSD-lock stall goes on long enough, it will cause an RCU CPU > stall warning. This additional warning provides much additional > console-log traffic and little additional information. Therefore, > provide a new csd_lock_is_stuck() function that returns true if there > is an ongoing CSD-lock stall. This function will be used by the RCU > CPU stall warnings to provide a one-line indication of the stall when > this function returns true. I think it would be nice to also add the RCU usage here, as for the function being declared but not used. > > [ neeraj.upadhyay: Apply Rik van Riel feedback. ] > > Signed-off-by: Paul E. McKenney > Cc: Imran Khan > Cc: Ingo Molnar > Cc: Leonardo Bras > Cc: "Peter Zijlstra (Intel)" > Cc: Rik van Riel > Signed-off-by: Neeraj Upadhyay > --- > include/linux/smp.h | 6 ++++++ > kernel/smp.c | 16 ++++++++++++++++ > 2 files changed, 22 insertions(+) > > diff --git a/include/linux/smp.h b/include/linux/smp.h > index fcd61dfe2af3..3871bd32018f 100644 > --- a/include/linux/smp.h > +++ b/include/linux/smp.h > @@ -294,4 +294,10 @@ int smpcfd_prepare_cpu(unsigned int cpu); > int smpcfd_dead_cpu(unsigned int cpu); > int smpcfd_dying_cpu(unsigned int cpu); > > +#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG > +bool csd_lock_is_stuck(void); > +#else > +static inline bool csd_lock_is_stuck(void) { return false; } > +#endif > + > #endif /* __LINUX_SMP_H */ > diff --git a/kernel/smp.c b/kernel/smp.c > index 81f7083a53e2..9385cc05de53 100644 > --- a/kernel/smp.c > +++ b/kernel/smp.c > @@ -207,6 +207,19 @@ static int csd_lock_wait_getcpu(call_single_data_t *csd) > return -1; > } > > +static atomic_t n_csd_lock_stuck; > + > +/** > + * csd_lock_is_stuck - Has a CSD-lock acquisition been stuck too long? > + * > + * Returns @true if a CSD-lock acquisition is stuck and has been stuck > + * long enough for a "non-responsive CSD lock" message to be printed. > + */ > +bool csd_lock_is_stuck(void) > +{ > + return !!atomic_read(&n_csd_lock_stuck); > +} > + > /* > * Complain if too much time spent waiting. Note that only > * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU, > @@ -228,6 +241,7 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in > cpu = csd_lock_wait_getcpu(csd); > pr_alert("csd: CSD lock (#%d) got unstuck on CPU#%02d, CPU#%02d released the lock.\n", > *bug_id, raw_smp_processor_id(), cpu); > + atomic_dec(&n_csd_lock_stuck); > return true; > } > > @@ -251,6 +265,8 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in > pr_alert("csd: %s non-responsive CSD lock (#%d) on CPU#%d, waiting %lld ns for CPU#%02d %pS(%ps).\n", > firsttime ? "Detected" : "Continued", *bug_id, raw_smp_processor_id(), (s64)ts_delta, > cpu, csd->func, csd->info); > + if (firsttime) > + atomic_inc(&n_csd_lock_stuck); > /* > * If the CSD lock is still stuck after 5 minutes, it is unlikely > * to become unstuck. Use a signed comparison to avoid triggering > -- > 2.40.1 > IIUC we have a single atomic counter for the whole system, which is modified in csd_lock_wait_toolong() and read in RCU stall warning. I think it should not be an issue regarding cache bouncing because in worst case scenario we would have 2 modify / cpu each csd_lock_timeout (which is 5 seconds by default). Thanks! Leo