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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4851883a012sm148720885e9.3.2026.03.04.13.44.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Mar 2026 13:44:49 -0800 (PST) Date: Wed, 4 Mar 2026 21:44:47 +0000 From: David Laight To: Steven Rostedt Cc: Peter Zijlstra , Yafang Shao , mingo@redhat.com, will@kernel.org, boqun@kernel.org, longman@redhat.com, mhiramat@kernel.org, mark.rutland@arm.com, mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [RFC PATCH 1/2] locking: add mutex_lock_nospin() Message-ID: <20260304214447.3e5817ea@pumpkin> In-Reply-To: <20260304155742.7b4de2d1@gandalf.local.home> References: <20260304074650.58165-1-laoar.shao@gmail.com> <20260304074650.58165-2-laoar.shao@gmail.com> <20260304090249.GN606826@noisy.programming.kicks-ass.net> <20260304095415.4d5f2528@pumpkin> <20260304155742.7b4de2d1@gandalf.local.home> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: bpf@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, 4 Mar 2026 15:57:42 -0500 Steven Rostedt wrote: > On Wed, 4 Mar 2026 09:54:15 +0000 > David Laight wrote: > > > That might still be an issue if a high priority process is spinning. > > But a %sys spike doesn't imply a latency spike. > > > > Is this using the osq_lock.c code? > > That will have problems on overprovisioned VMs, it tries to find out > > whether the hypervisor has switched out - but ISTR that is flawed. > > > > In reality a spin lock shouldn't be held for long enough to cause > > any kind latency issue. > > So something in the code that reads the list of filter functions > > needs to be done differently so that the lock isn't held for as long. > > It's not a spinlock, it's an adaptive mutex which spins while the owner of > the mutex is also still running on the CPU. If the spinner CPU triggers a > NEED_RESCHED or the owner goes to sleep, the spinner stops spinning and > goes to sleep too. I think half my brain knew that - otherwise I wouldn't have mentioned the osq_lock.c code. That all reminded me I've a patch that optimises that code a bit. But I do remember thinking it ought to have a 'I been spinning long enough, time to sleep' path. David > > Honestly, this still looks like a non-issue or a corner case that I don't > think requires these changes. > > This looks like one of those "Patient: Doctor it hurts me when I do this. > Doctor: Then don't do that." cases. > > Why is a production system having multiple users cat > avaliable_filter_functions to begin with? > > -- Steve