From patchwork Wed May 17 15:26:51 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Mathieu Desnoyers X-Patchwork-Id: 69523 Return-Path: X-Original-To: patchwork@sourceware.org Delivered-To: patchwork@sourceware.org Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id 6AF5D3854179 for ; Wed, 17 May 2023 15:28:11 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 6AF5D3854179 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1684337291; bh=I+n62NOzeqoDJaIS0fVUVqcuLGX1IUfTknQBNq/CXhc=; h=To:Cc:Subject:Date:In-Reply-To:References:List-Id: List-Unsubscribe:List-Archive:List-Post:List-Help:List-Subscribe: From:Reply-To:From; b=f7SxcgcqA0fhOJj8OFLkvJ9yekzjbJkBFWqR+UqVzLIV8kV5iKXj9qNcON/96sFVL s3L4Z1vFL5BDjVuex6lZ81Y8SAQti45hNGd3tS0pdKdakNAB6T2yScqB5g1h9al6CX JfVRLtcBb2K7rSXI1AZPzhOFUO8XN0A72tOUK3K8= X-Original-To: libc-alpha@sourceware.org Delivered-To: libc-alpha@sourceware.org Received: from smtpout.efficios.com (unknown [IPv6:2607:5300:203:b2ee::31e5]) by sourceware.org (Postfix) with ESMTPS id 65B1A3858C41 for ; Wed, 17 May 2023 15:27:04 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 65B1A3858C41 Received: from localhost.localdomain (192-222-143-198.qc.cable.ebox.net [192.222.143.198]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4QLxmR53HXz131r; Wed, 17 May 2023 11:27:03 -0400 (EDT) To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Thomas Gleixner , "Paul E . McKenney" , Boqun Feng , "H . Peter Anvin" , Paul Turner , linux-api@vger.kernel.org, Christian Brauner , Florian Weimer , David.Laight@ACULAB.COM, carlos@redhat.com, Peter Oskolkov , Alexander Mikhalitsyn , Chris Kennelly , Ingo Molnar , Darren Hart , Davidlohr Bueso , =?utf-8?q?Andr=C3=A9_Almeida?= , libc-alpha@sourceware.org, Steven Rostedt , Jonathan Corbet , Mathieu Desnoyers , Florian Weimer Subject: [RFC PATCH 1/4] rseq: Add sched_state field to struct rseq Date: Wed, 17 May 2023 11:26:51 -0400 Message-Id: <20230517152654.7193-2-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20230517152654.7193-1-mathieu.desnoyers@efficios.com> References: <20230517152654.7193-1-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 X-Spam-Status: No, score=-11.8 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-Patchwork-Original-From: Mathieu Desnoyers via Libc-alpha From: Mathieu Desnoyers Reply-To: Mathieu Desnoyers Errors-To: libc-alpha-bounces+patchwork=sourceware.org@sourceware.org Sender: "Libc-alpha" Expose the "on-cpu" state for each thread through struct rseq to allow adaptative mutexes to decide more accurately between busy-waiting and calling sys_futex() to release the CPU, based on the on-cpu state of the mutex owner. It is only provided as an optimization hint, because there is no guarantee that the page containing this field is in the page cache, and therefore the scheduler may very well fail to clear the on-cpu state on preemption. This is expected to be rare though, and is resolved as soon as the task returns to user-space. The goal is to improve use-cases where the duration of the critical sections for a given lock follows a multi-modal distribution, preventing statistical guesses from doing a good job at choosing between busy-wait and futex wait behavior. Signed-off-by: Mathieu Desnoyers Cc: Peter Zijlstra (Intel) Cc: Jonathan Corbet Cc: Steven Rostedt (Google) Cc: Carlos O'Donell Cc: Florian Weimer Cc: libc-alpha@sourceware.org --- include/linux/sched.h | 12 ++++++++++++ include/uapi/linux/rseq.h | 17 +++++++++++++++++ kernel/rseq.c | 14 ++++++++++++++ 3 files changed, 43 insertions(+) diff --git a/include/linux/sched.h b/include/linux/sched.h index eed5d65b8d1f..c7e9248134c1 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -2351,11 +2351,20 @@ static inline void rseq_signal_deliver(struct ksignal *ksig, rseq_handle_notify_resume(ksig, regs); } +void __rseq_set_sched_state(struct task_struct *t, unsigned int state); + +static inline void rseq_set_sched_state(struct task_struct *t, unsigned int state) +{ + if (t->rseq) + __rseq_set_sched_state(t, state); +} + /* rseq_preempt() requires preemption to be disabled. */ static inline void rseq_preempt(struct task_struct *t) { __set_bit(RSEQ_EVENT_PREEMPT_BIT, &t->rseq_event_mask); rseq_set_notify_resume(t); + rseq_set_sched_state(t, 0); } /* rseq_migrate() requires preemption to be disabled. */ @@ -2405,6 +2414,9 @@ static inline void rseq_signal_deliver(struct ksignal *ksig, struct pt_regs *regs) { } +static inline void rseq_set_sched_state(struct task_struct *t, unsigned int state) +{ +} static inline void rseq_preempt(struct task_struct *t) { } diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h index c233aae5eac9..c6d8537e23ca 100644 --- a/include/uapi/linux/rseq.h +++ b/include/uapi/linux/rseq.h @@ -37,6 +37,13 @@ enum rseq_cs_flags { (1U << RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT), }; +enum rseq_sched_state { + /* + * Task is currently running on a CPU if bit is set. + */ + RSEQ_SCHED_STATE_ON_CPU = (1U << 0), +}; + /* * struct rseq_cs is aligned on 4 * 8 bytes to ensure it is always * contained within a single cache-line. It is usually declared as @@ -148,6 +155,16 @@ struct rseq { */ __u32 mm_cid; + /* + * Restartable sequences sched_state field. Updated by the kernel. Read + * by user-space with single-copy atomicity semantics. This fields can + * be read by any userspace thread. Aligned on 32-bit. Contains a + * bitmask of enum rseq_sched_state. This field is provided as a hint + * by the scheduler, and requires that the page holding struct rseq is + * faulted-in for the state update to be performed by the scheduler. + */ + __u32 sched_state; + /* * Flexible array member at end of structure, after last feature field. */ diff --git a/kernel/rseq.c b/kernel/rseq.c index 9de6e35fe679..b2eb3bbaa9ef 100644 --- a/kernel/rseq.c +++ b/kernel/rseq.c @@ -91,6 +91,7 @@ static int rseq_update_cpu_node_id(struct task_struct *t) u32 cpu_id = raw_smp_processor_id(); u32 node_id = cpu_to_node(cpu_id); u32 mm_cid = task_mm_cid(t); + u32 sched_state = RSEQ_SCHED_STATE_ON_CPU; WARN_ON_ONCE((int) mm_cid < 0); if (!user_write_access_begin(rseq, t->rseq_len)) @@ -99,6 +100,7 @@ static int rseq_update_cpu_node_id(struct task_struct *t) unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end); unsafe_put_user(node_id, &rseq->node_id, efault_end); unsafe_put_user(mm_cid, &rseq->mm_cid, efault_end); + unsafe_put_user(sched_state, &rseq->sched_state, efault_end); /* * Additional feature fields added after ORIG_RSEQ_SIZE * need to be conditionally updated only if @@ -339,6 +341,18 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs) force_sigsegv(sig); } +/* + * Attempt to update rseq scheduler state. + */ +void __rseq_set_sched_state(struct task_struct *t, unsigned int state) +{ + if (unlikely(t->flags & PF_EXITING)) + return; + pagefault_disable(); + (void) put_user(state, &t->rseq->sched_state); + pagefault_enable(); +} + #ifdef CONFIG_DEBUG_RSEQ /*