From patchwork Thu Jan 30 19:03:10 2025 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Paul E. McKenney" X-Patchwork-Id: 13954901 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 28ED91F1503; Thu, 30 Jan 2025 19:03:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738263800; cv=none; b=smF74UgK0YpxT7WmGS/ekGu0SVpTHkhI8oFFZzGuvuB3USIF8ji4IyT7ShT/DvilYJ4kPI1sf228Qgzi8L65io2a/uAYT3DyRmbs3wmqc2qtvLSoNdWuNKUfK4CPoyw7Q0qCMkVlhMVL7C3SwUi3iwTxtLhYYPcx09mzzQjfT7M= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738263800; c=relaxed/simple; bh=FgQr+mfpSSSft5WT23ngWflMJ8+VHwKTgm4CveuSOKc=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=mWURivt/mfPrqOJZDATBowyjS/PrY52T9WCx2MXzKafVz8LgiEDuXa3tdHR5q7x8KfqBFL0nw4WWckVDSaodL+ye6UOpv3REhuKXQXrLC0ddpdVfPcc/8UrAMHEK2WhHoO6kV5xPxXBcjaWw4iuFWMhHSH7oz+xx0UYTzOhuj7k= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=X5F8PaT9; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="X5F8PaT9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B93AAC116B1; Thu, 30 Jan 2025 19:03:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1738263799; bh=FgQr+mfpSSSft5WT23ngWflMJ8+VHwKTgm4CveuSOKc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=X5F8PaT94MAk6AKePBNQYZaZxbCJsguT59Su7HZK0glvX/l5myeOxqRPWLSYhat+A nTIHAnItZasx7R3O+dB4YCrVYXDk/lJHDhIAQF0ItAKecELlDsfmi0zq3horYqeGRU eqejRMGzAwPCoo/ggMYImJTCbs/cxyL53I/UzEHYmNlv4O4uq1jmgihriVdtDXfixh Z1hyGLOubGDac0KQG7pYQvNNWQN1Zakn5kzGeuPX0mPy+ucQRZMTXY1vY46sBRUtfG FlbBSw0PIxFxVmeI9zfDSAKXuNE+tEHvMQ62Lae0RASO8ymQ/TOQT/ZXw+mZLoPQ+f HDGgbOTrlu6pw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 011DACE3800; Thu, 30 Jan 2025 11:03:19 -0800 (PST) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, "Paul E. McKenney" , Alexei Starovoitov , Andrii Nakryiko , Peter Zijlstra , Kent Overstreet , bpf@vger.kernel.org Subject: [PATCH rcu v2] 13/20] srcu: Add SRCU-fast readers Date: Thu, 30 Jan 2025 11:03:10 -0800 Message-Id: <20250130190317.1652481-13-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: <1034ef54-b6b3-42bb-9bd8-4c37c164950d@paulmck-laptop> References: <1034ef54-b6b3-42bb-9bd8-4c37c164950d@paulmck-laptop> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 This commit adds srcu_read_{,un}lock_fast(), which is similar to srcu_read_{,un}lock_lite(), but avoids the array-indexing and pointer-following overhead. On a microbenchmark featuring tight loops around empty readers, this results in about a 20% speedup compared to RCU Tasks Trace on my x86 laptop. Please note that SRCU-fast has drawbacks compared to RCU Tasks Trace, including: o Lack of CPU stall warnings. o SRCU-fast readers permitted only where rcu_is_watching(). o A pointer-sized return value from srcu_read_lock_fast() must be passed to the corresponding srcu_read_unlock_fast(). o In the absence of readers, a synchronize_srcu() having _fast() readers will incur the latency of at least two normal RCU grace periods. o RCU Tasks Trace priority boosting could be easily added. Boosting SRCU readers is more difficult. SRCU-fast also has a drawback compared to SRCU-lite, namely that the return value from srcu_read_lock_fast()-fast is a 64-bit pointer and that from srcu_read_lock_lite() is only a 32-bit int. [ paulmck: Apply feedback from Akira Yokosawa. ] Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: --- include/linux/srcu.h | 47 ++++++++++++++++++++++++++++++++++++++-- include/linux/srcutiny.h | 22 +++++++++++++++++++ include/linux/srcutree.h | 38 ++++++++++++++++++++++++++++++++ 3 files changed, 105 insertions(+), 2 deletions(-) diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 2bd0e24e9b55..63bddc301423 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -47,9 +47,10 @@ int init_srcu_struct(struct srcu_struct *ssp); #define SRCU_READ_FLAVOR_NORMAL 0x1 // srcu_read_lock(). #define SRCU_READ_FLAVOR_NMI 0x2 // srcu_read_lock_nmisafe(). #define SRCU_READ_FLAVOR_LITE 0x4 // srcu_read_lock_lite(). +#define SRCU_READ_FLAVOR_FAST 0x8 // srcu_read_lock_fast(). #define SRCU_READ_FLAVOR_ALL (SRCU_READ_FLAVOR_NORMAL | SRCU_READ_FLAVOR_NMI | \ - SRCU_READ_FLAVOR_LITE) // All of the above. -#define SRCU_READ_FLAVOR_SLOWGP SRCU_READ_FLAVOR_LITE + SRCU_READ_FLAVOR_LITE | SRCU_READ_FLAVOR_FAST) // All of the above. +#define SRCU_READ_FLAVOR_SLOWGP (SRCU_READ_FLAVOR_LITE | SRCU_READ_FLAVOR_FAST) // Flavors requiring synchronize_rcu() // instead of smp_mb(). void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp); @@ -253,6 +254,33 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp) return retval; } +/** + * srcu_read_lock_fast - register a new reader for an SRCU-protected structure. + * @ssp: srcu_struct in which to register the new reader. + * + * Enter an SRCU read-side critical section, but for a light-weight + * smp_mb()-free reader. See srcu_read_lock() for more information. + * + * If srcu_read_lock_fast() is ever used on an srcu_struct structure, + * then none of the other flavors may be used, whether before, during, + * or after. Note that grace-period auto-expediting is disabled for _fast + * srcu_struct structures because auto-expedited grace periods invoke + * synchronize_rcu_expedited(), IPIs and all. + * + * Note that srcu_read_lock_fast() can be invoked only from those contexts + * where RCU is watching, that is, from contexts where it would be legal + * to invoke rcu_read_lock(). Otherwise, lockdep will complain. + */ +static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *ssp) __acquires(ssp) +{ + struct srcu_ctr __percpu *retval; + + srcu_check_read_flavor_force(ssp, SRCU_READ_FLAVOR_FAST); + retval = __srcu_read_lock_fast(ssp); + rcu_try_lock_acquire(&ssp->dep_map); + return retval; +} + /** * srcu_read_lock_lite - register a new reader for an SRCU-protected structure. * @ssp: srcu_struct in which to register the new reader. @@ -356,6 +384,21 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx) __srcu_read_unlock(ssp, idx); } +/** + * srcu_read_unlock_fast - unregister a old reader from an SRCU-protected structure. + * @ssp: srcu_struct in which to unregister the old reader. + * @scp: return value from corresponding srcu_read_lock_fast(). + * + * Exit a light-weight SRCU read-side critical section. + */ +static inline void srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp) + __releases(ssp) +{ + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST); + srcu_lock_release(&ssp->dep_map); + __srcu_read_unlock_fast(ssp, scp); +} + /** * srcu_read_unlock_lite - unregister a old reader from an SRCU-protected structure. * @ssp: srcu_struct in which to unregister the old reader. diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h index 07a0c4489ea2..380260317d98 100644 --- a/include/linux/srcutiny.h +++ b/include/linux/srcutiny.h @@ -71,6 +71,28 @@ static inline int __srcu_read_lock(struct srcu_struct *ssp) return idx; } +struct srcu_ctr; + +static inline bool __srcu_ptr_to_ctr(struct srcu_struct *ssp, struct srcu_ctr __percpu *scpp) +{ + return (int)(intptr_t)(struct srcu_ctr __force __kernel *)scpp; +} + +static inline struct srcu_ctr __percpu *__srcu_ctr_to_ptr(struct srcu_struct *ssp, int idx) +{ + return (struct srcu_ctr __percpu *)(intptr_t)idx; +} + +static inline struct srcu_ctr __percpu *__srcu_read_lock_fast(struct srcu_struct *ssp) +{ + return __srcu_ctr_to_ptr(ssp, __srcu_read_lock(ssp)); +} + +static inline void __srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp) +{ + __srcu_read_unlock(ssp, __srcu_ptr_to_ctr(ssp, scp)); +} + #define __srcu_read_lock_lite __srcu_read_lock #define __srcu_read_unlock_lite __srcu_read_unlock diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h index ef3065c0cadc..bdc467efce3a 100644 --- a/include/linux/srcutree.h +++ b/include/linux/srcutree.h @@ -226,6 +226,44 @@ static inline struct srcu_ctr __percpu *__srcu_ctr_to_ptr(struct srcu_struct *ss return &ssp->sda->srcu_ctrs[idx]; } +/* + * Counts the new reader in the appropriate per-CPU element of the + * srcu_struct. Returns a pointer that must be passed to the matching + * srcu_read_unlock_fast(). + * + * Note that this_cpu_inc() is an RCU read-side critical section either + * because it disables interrupts, because it is a single instruction, + * or because it is a read-modify-write atomic operation, depending on + * the whims of the architecture. + */ +static inline struct srcu_ctr __percpu *__srcu_read_lock_fast(struct srcu_struct *ssp) +{ + struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp); + + RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_fast()."); + this_cpu_inc(scp->srcu_locks.counter); /* Y */ + barrier(); /* Avoid leaking the critical section. */ + return scp; +} + +/* + * Removes the count for the old reader from the appropriate + * per-CPU element of the srcu_struct. Note that this may well be a + * different CPU than that which was incremented by the corresponding + * srcu_read_lock_fast(), but it must be within the same task. + * + * Note that this_cpu_inc() is an RCU read-side critical section either + * because it disables interrupts, because it is a single instruction, + * or because it is a read-modify-write atomic operation, depending on + * the whims of the architecture. + */ +static inline void __srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp) +{ + barrier(); /* Avoid leaking the critical section. */ + this_cpu_inc(scp->srcu_unlocks.counter); /* Z */ + RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_fast()."); +} + /* * Counts the new reader in the appropriate per-CPU element of the * srcu_struct. Returns an index that must be passed to the matching