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Date: Tue, 1 Sep 2026 11:49:43 +0000
From: Dmitry Ilvokhin <d@ilvokhin.com>
To: Alice Ryhl <aliceryhl@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Boqun Feng <boqun@kernel.org>, Dmitry Vyukov <dvyukov@google.com>,
	Thomas Gleixner <tglx@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Randy Dunlap <rdunlap@infradead.org>, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rseq: defer time slice extension yield for sys_futex_wake
Message-ID: <apa71yN7lYR3FKeI@shell.ilvokhin.com>
References: <20260831-sys-futex-wake-time-slice-v1-1-814bb95cc339@google.com>
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In-Reply-To: <20260831-sys-futex-wake-time-slice-v1-1-814bb95cc339@google.com>

On Mon, Aug 31, 2026 at 12:57:26PM +0000, Alice Ryhl wrote:
> When a task is granted an rseq scheduler time slice extension, it is
> expected to finish its critical section and relinquish the CPU via
> rseq_slice_yield(2). If the task issues any other system call while a
> grant is active, rseq_syscall_enter_work() forces an immediate
> reschedule on syscall entry via cond_resched(). This may cause
> significant latency penalty for userspace lock implementations that use
> rseq time slice extensions when unlocking the futex.
> 
> In a userspace mutex unlock sequence:
> 1. The lock is released in userspace.
> 2. If there are waiters, the unlocking thread calls sys_futex_wake()
>    to wake a sleeping waiter.

Just out of curiosity, is there any publicly available implementation of
a mutex that combines rseq time-slice extensions and futexes?

I'm interested in learning more about how these two mechanisms interact
with each other.