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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-439f818ff0fsm7048224f8f.4.2026.03.11.05.53.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Mar 2026 05:53:52 -0700 (PDT) Date: Wed, 11 Mar 2026 12:53:50 +0000 From: David Laight To: Peter Zijlstra Cc: Yafang Shao , mingo@redhat.com, will@kernel.org, boqun@kernel.org, longman@redhat.com, rostedt@goodmis.org, mhiramat@kernel.org, mark.rutland@arm.com, mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 0/3] disable optimistic spinning for ftrace_lock Message-ID: <20260311125350.1d89341f@pumpkin> In-Reply-To: <20260311115426.GN606826@noisy.programming.kicks-ass.net> References: <20260311115250.78488-1-laoar.shao@gmail.com> <20260311115426.GN606826@noisy.programming.kicks-ass.net> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-trace-kernel@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 Wed, 11 Mar 2026 12:54:26 +0100 Peter Zijlstra wrote: > On Wed, Mar 11, 2026 at 07:52:47PM +0800, Yafang Shao wrote: > > Recently, we resolved a latency spike issue caused by concurrently running > > bpftrace processes. The root cause was high contention on the ftrace_lock > > due to optimistic spinning. We can optimize this by disabling optimistic > > spinning for ftrace_lock. > > > > While semaphores may present similar challenges, I'm not currently aware of > > specific instances that exhibit this exact issue. Should we encounter > > problematic semaphores in production workloads, we can address them at that > > time. > > > > PATCH #1: introduce slow_mutex_[un]lock to disable optimistic spinning > > PATCH #2: add variant for rtmutex > > PATCH #3: disable optimistic spinning for ftrace_lock > > > > So I really utterly hate this. Yep... Adding the extra parameter is likely to have a measurable impact on everything else. The problematic path is obvious: find_kallsyms_symbol+142 module_address_lookup+104 kallsyms_lookup_buildid+203 kallsyms_lookup+20 print_rec+64 t_show+67 seq_read_iter+709 seq_read+165 vfs_read+165 ksys_read+103 __x64_sys_read+25 do_syscall_64+56 entry_SYSCALL_64_after_hwframe+100 The code needs to drop the ftrace_lock across t_show. Although there is a bigger issue of why on earth the code is reading the list of filter functions at all - never mind all the time. I'll do it by hand when debugging, but I'd have though anything using bpf will know exactly where to add its hooks. David